Rainbow Trout (Seawater) Operational Welfare Indicators (OWI) Protocol

A standardized, science-based framework for assessing the welfare of farmed rainbow trout (Oncorhynchus mykiss) in seawater ongrowing systems.

Organization:CAP
Species:Rainbow Trout (TRR)
Version:0.1
Last Updated:2025-12-03

Contributions and Acknowledgments

The Centre for Aquaculture Progress would like to extend its sincere thanks to all those who contributed their time, expertise, and feedback to the development of this protocol.

In particular, we wish to thank Dr Marco Cerqueira (CCMAR) for his detailed feedback and close collaboration throughout the development of this document. His thoughtful input greatly strengthened the scientific robustness and practical applicability of the protocol.

We also thank many of our collaborators who reviewed the draft and provided constructive comments.

Copyright and License

© Centre for Aquaculture Progress 2025
Licensed under CC BY 4.0. You may use, share, and adapt this material with attribution.
Full license: https://creativecommons.org/licenses/by/4.0/

Table of Contents

Executive Summary

This protocol provides a standardized, science-based framework for assessing the welfare of farmed rainbow trout (Oncorhynchus mykiss). It serves farm personnel conducting routine welfare monitoring and other industry professionals, including certifiers, auditors, and buyers. This protocol is designed to align with the welfare monitoring and assessment requirements of key certifications, such as RSPCA Assured and Aquaculture Stewardship Council (ASC).

While this protocol was developed primarily for ongrowing rainbow trout in seawater, users can apply it to juveniles or trout reared in other production systems. Application in these contexts requires careful interpretation, as certain thresholds may not directly translate to different life stages and production systems. The Centre for Aquaculture Progress will publish protocols for juvenile rainbow trout in freshwater in 2026.

With global production exceeding 1 million tons in 2023, rainbow trout represent a major commercial aquaculture species.1 At this scale, producers require robust, standardized tools to maintain high welfare standards. Operational Welfare Indicators (OWIs) are scientifically validated practical indicators, including individual morphological indicators, group-based indicators, and environmental indicators, which provide the foundation for systematic on-farm welfare monitoring and assessment.

While practical guidance on OWIs has been developed for other species, such as the Laksvel protocol for Atlantic salmon,2 and valuable documentation on rainbow trout welfare has been published by Nofima3 and APROMAR,4 existing resources lack comprehensive, standardized, and threshold-based assessment across a wide range of indicators for rainbow trout producers. This protocol addresses that gap.

This protocol enables farm personnel to systematically monitor the welfare of their stock, identify potential issues early, and implement corrective actions. This proactive approach helps producers understand the relationship between welfare and production performance, resulting in reduced losses, improved regulatory compliance, strengthened consumer trust, and a stronger and more sustainable aquaculture industry.

Introduction

Rainbow trout, like other teleost fish, are animals capable of experiencing pain and stress. Ensuring their welfare during production requires systematic monitoring through Operational Welfare Indicators (OWIs). The increasing focus on OWI implementation reflects priorities shared across the aquaculture value chain: from producers, certification bodies, and seafood buyers, to consumers, governments, and NGOs. By improving welfare outcomes, producers simultaneously achieve better growth, survival, robustness, and production efficiency, while meeting regulatory requirements and evolving consumer expectations.

This protocol provides a standardized, systematic framework for real-world commercial use, built upon the strongest available scientific evidence to support both on-farm management and compliance with key certification requirements.

These guidelines were primarily developed for ongrowing rainbow trout in seawater. Users can apply this protocol to other life stages or production systems, though application in these contexts requires careful interpretation.

Defining fish welfare and Operational Welfare Indicators (OWIs)

Fish welfare

Animal welfare is a complex and multifaceted topic, with many definitions and conceptual frameworks. Several widely recognized approaches include:

These frameworks are recognized as complementary rather than competing perspectives. Modern integrated approaches, such as the Five Domains Model,10 synthesize these perspectives by assessing welfare across multiple dimensions:

This multidimensional framework acknowledges that welfare encompasses biological functioning, behavioral expression, and subjective experience, with each domain contributing to the animal's overall quality of life.

While acknowledging the complexity of the term 'welfare', this document adopts the same definition used in the Laksvel protocol for Atlantic salmon: "quality of life as perceived by the fish itself."11 This definition aligns with the feeling-based and affective balance perspectives by centering on the fish's subjective experience. However, because subjective experience cannot be directly measured, this protocol uses observable indicators across multiple welfare domains as proxies for internal states.

Operational Welfare Indicators (OWIs)

Operational Welfare Indicators (OWIs) are standardized assessment tools designed for routine implementation in commercial aquaculture. Unlike some other proxy measures such as plasma cortisol concentrations that require laboratory analysis, OWIs enable practical on-farm assessment. These indicators are scientifically validated, practically feasible, and provide actionable data for evidence-based management decisions.12

This document classifies OWIs into three categories:13

Relationship Between OWI Categories and Welfare Domains

The table below shows how the three OWI categories address the Five Domains of welfare:

Welfare Domain Individual-Based Indicators Group-Based Indicators Environmental Indicators
1. Nutrition Emaciation Feeding behavior Feed quality, delivery (not in current protocol)
2. Physical Environment Morphological damage related to environment Swimming behavior Water quality, stocking density
3. Health Morphological damage/disease signs/deformities Mortality Water quality (disease risk factors)
4. Behavioral Interactions Morphological damage from aggression (fin damage) Aberrant/aggressive/feeding/swimming behavior Stocking density, water flow/velocity
5. Mental State Pain-related morphological damage (inferred) Aberrant behavior Environmental stressors (suboptimal water quality or high stocking densities)

Limitations

While OWIs provide valuable welfare assessment capabilities, users should be aware of several important limitations:

Evidence Quality

To address the variability of the evidence base between indicators and to ensure transparency, the table below summarizes the level of supporting evidence for each indicator.

Indicator Welfare significance Rainbow trout-specific evidence Threshold validation
Individual-based indicators
Eye opacity High Moderate Low (extrapolated from salmon)
Eye injury High Very low Low (extrapolated from salmon)
Operculum damage Moderate Very low Low (extrapolated from salmon)
Gill damage High Very low Low (extrapolated from salmon)
Skin hemorrhaging Moderate Very low Low (extrapolated from salmon)
Scale loss Moderate Very low Low (extrapolated from salmon)
Wounds High Low Low (extrapolated from salmon)
Snout damage High Very low Low (extrapolated from salmon)
Fin damage Moderate High Moderate
Spinal deformity High Moderate Low (extrapolated from salmon)
Jaw deformity High Low Low (extrapolated from salmon)
Change of coloration Moderate Low Very low
Condition factor / Emaciation High High Moderate
Sea lice High Low Moderate (extrapolated from salmon)
Group-based indicators
Mortality High Moderate Low
Aberrant behavior High Low Low
Aggressive behavior High Very low Very low
Feeding behavior High Low Low
Swimming behavior High Low Low
Environmental indicators
Stocking density High High Moderate
Dissolved oxygen High High High
Carbon dioxide High Low Very low
Temperature High High Moderate
Turbidity Moderate Low Very low
pH High Moderate Low
Total Suspended Solids Moderate Low Very low
Ammonia High Low Low
Nitrite Low Very low Very low
Nitrate Moderate Very low Very low

Thresholds

Thresholds for each OWI are classified as 'ideal', 'acceptable', 'warning', and 'unacceptable'. The 'acceptable', 'warning', and 'unacceptable' thresholds correspond to traffic light systems (green-yellow-red) commonly used in certification schemes. These levels should not be treated as linear in severity.

Thresholds for individual indicators describe the welfare impact on that individual fish; they do not prescribe target prevalence levels for the population.

Stakeholder feedback

The Centre for Aquaculture Progress will regularly update this protocol based on the latest scientific evidence and evolving operational practices to ensure this protocol remains as relevant and accurate for producers as possible.

The Centre for Aquaculture Progress welcomes feedback and comments from all interested stakeholders to ensure this protocol remains practical, scientifically sound, and commercially relevant. We particularly welcome:

To submit feedback or questions, please contact info@centreforaquacultureprogress.org.

Individual-based indicators

Eye opacity

Eye opacity or cataracts can arise from nutritional, genetic, and environmental factors, as well as exposure to infections, parasites, or ultraviolet light. Cataracts cause lens clouding, leading to impaired vision. This can restrict a fish's ability to feed, avoid aggression or predation, and navigate its environment. As a result, eye opacity can negatively impact survival, growth, and performance.

Ideal Acceptable Warning Unacceptable

No visible opacity

Minor opacity, < 10% lens coverage

Moderate opacity, 10–50% lens coverage

Severe opacity, > 50% lens coverage
Healthy rainbow trout eye
Healthy rainbow trout eye15
Atlantic salmon eye with minor opacity
Atlantic salmon eye with minor opacity16
Atlantic salmon eye with moderate opacity
Atlantic salmon eye with moderate opacity17
Atlantic salmon eye with severe opacity
Atlantic salmon eye with severe opacity18
Protocol Application

Score based on the most affected eye, though recording both eyes is recommended to improve data quality.
Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • For a 'warning' score for opacity, noting whether the opacity is lower (10-25%) or higher (25–50%) supports better management decisions.
  • Lower end opacity means the fish likely retains adequate functional vision.
  • Higher end opacity means vision may be significantly impaired, approaching welfare concerns.
  • While an 'acceptable' opacity score reflects a state of minimal welfare impact, minor cataracts can indicate parasitic infection, metabolic stress, or nutritional problems and should be actively managed to prevent progression.
  • Bilateral cases: Where both eyes would be scored as 'warning' or 'unacceptable', vision is likely significantly impaired, posing greater welfare risk than unilateral opacity. Monitoring should be increased where this is observed.
  • Validation: This scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon. Eye anatomy is similar across salmonids, but species-specific validation is needed.
Key Evidence
Source Summary of source Key result
Kuukka-Anttila et al., 2010 Experimental study investigating cataract growth in 969 rainbow trout with a natural Diplostomum infection in commercial settings across in both freshwater and seawater sites over a 3-year rearing period. A slit-lamp microscope was used to score cataracts from 0–5, for a total score of up to 10 between both eyes. The fish were measured at three different times: after the first growing season (mean weight 50g, freshwater), after the second growing season (mean weight 734g, both freshwater and seawater) and after the third growing season (mean weight 2245g, freshwater only). All 969 individuals had cataracts with a mean individual score of 7.5 and a range from 4 to 10. The authors noted that Diplostomum load is directly related to cataract scores, and that the findings suggest that variation in tolerance is determined by genetic factors. Initial body weight was not correlated with cataract score, but cataract severity measured in the second and third timepoints had a strongly negative relationship with body weight, indicating reduced growth and condition in fish with high cataract scores. In females, higher cataract scores were associated with later maturity, though this was not observed in males. There was no clear relationship between cataract severity and mortality. The authors explained that this could be due to the farming conditions: as there were no predators and fish had unlimited access to feed, meaning reduced vision did not result in starvation.
Karvonen et al., 2004 Experimental study investigating cataract formation in response to eye fluke infection (Diplostomum spathaceum) in 160 rainbow trout (1 year old). Trout were exposed to natural infection in Lake Konnevesi over 112 days in cages or in a freshwater laboratory environment. Cataracts were scored using a slit-lamp microscope on a scale from 0–4. There was a strong relationship between higher parasite loads and more severe cataracts. Under natural lake exposure, cataracts developed gradually, with 35% of eyes fully covered or opaque by day 112. In laboratory exposures, cataracts developed more rapidly; 37% of eyes had a complete cataract and 3% totally opaque by day 29. Cataract intensity remained stable after this time.
Remo et al., 2017 Experimental study investigating the formation of cataracts at different temperatures in 80 post-smolt rainbow trout (125g) over 35 days in laboratory seawater conditions. Two temperatures were tested: 13°C and 19°C. Each lens was scored between 0–4, with a total score between 0–8 per fish. A score of 0 = no opacity, 1 = < 10% opacity, 2 = 10–50% opacity, 3 = 50–75% opacity and 4 = 75–100% opacity. At the start of the study, rainbow trout had a cataract prevalence of 10%, with a mean cataract score of 0.1. In the 13°C group, the prevalence was 53% and the mean score was 0.9, and in the 19°C group the prevalence was 67% and the mean score was 1.3. The authors concluded that the trout's higher concentrations of N-acetyle-histidine and histidine made them less susceptible to cataracts than salmon.
Supporting Evidence
  • Karvonen, A., O. Seppälä, and E. T. Valtonen. 2004. "Eye Fluke-Induced Cataract Formation in Fish: Quantitative Analysis Using an Ophthalmological Microscope." Parasitology 129 (4): 473–78. https://doi.org/10.1017/s0031182004006006.
  • Kuukka-Anttila, H., N. Peuhkuri, I. Kolari, T. Paananen, and A. Kause. 2009. "Quantitative Genetic Architecture of Parasite-Induced Cataract in Rainbow Trout, Oncorhynchus Mykiss." Heredity 104 (1): 20–27. https://doi.org/10.1038/hdy.2009.123.
  • Nilsson, Jonatan, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Remø, Sofie Charlotte, et al. 2017. "Lens Metabolomic Profiling as a Tool to Understand Cataractogenesis in Atlantic Salmon and Rainbow Trout Reared at Optimum and High Temperature." PLOS ONE 12 (4): e0175491. https://doi.org/10.1371/journal.pone.0175491.
  • Wall, T., and E. Bjerkas. 1999. "A Simplified Method of Scoring Cataracts in Fish." Bulletin of the European Association of Fish Pathologists 19 (4): 162–65.
  • Weirup, Lina. "Development of a Fish Welfare Evaluation Index for Rainbow Trout (Oncorhynchus mykiss) in Aquaculture." PhD Thesis, 2022. https://d-nb.info/1262308445/34.

Eye injury

The eyes of farmed fish are sensitive, and injuries can occur due to physical trauma (e.g. handling, crowding, or contact with equipment), poor water quality, disease, parasites or gas supersaturation (which can rupture the blood vessels). Clinical signs of eye injury include bleeding and exophthalmos (eye protrusion). Like eye opacity, eye injuries can reduce visual capacity, compromising a fish's ability to navigate its environment properly. This can lead to increased behavioral and physiological stress.

Ideal Acceptable Warning Unacceptable

No visible eye damage

Healed damage with no active bleeding or inflammation

Any minor fresh injury OR Minor protrusion

Clear fresh injury OR Clear protrusion OR Any visible parasites
Healthy rainbow trout eye
Healthy rainbow trout eye19
Atlantic salmon eye with minor fresh injury
Atlantic salmon eye with minor fresh injury20
Atlantic salmon eye with clear fresh injury
Atlantic salmon eye with clear fresh injury21
Protocol Application

Score based on the most affected eye, though recording both eyes is recommended to improve data quality.
Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • Bilateral cases: Where both eyes would be scored as 'warning' or 'unacceptable', there is a larger welfare risk than unilateral injury. Monitoring should be increased where this is observed.
  • Validation: This scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon. Eye anatomy is similar across salmonids, but species-specific validation is needed.
Key Evidence
Source Summary of source Key result
Pushchina et al., 2020 Experimental study investigating the effects of mechanical eye injury on brain gene expression in 70 rainbow trout (370–455g) in freshwater laboratory conditions. Injury was caused by a sterile needle, and fish were monitored for one-week post-injury. Fish experienced a 1.7 fold increase in cystathionine β-synthase concentration. The injury prompted the formation of specialized repair zones in the brain which were not present in the control. These phenomena were observed 1 week after eye injury, demonstrating an ongoing major response to significant stress.
Noble et al., 2020 Literature review covering a wide range of rainbow trout welfare indicators, including eye injury. Visible eye conditions were associated with significant welfare concerns, with mechanical injuries being the most common cause of damage. Eye damage progresses from minor conditions to severe welfare impacts including blindness, pain, and mortality. As examples, eye flukes can result in blindness, which can impact behavior and feeding ability.
Supporting Evidence
  • Karami, Asma M., Yajiao Duan, Per W. Kania, and Kurt Buchmann. 2022. "Responses towards Eyefluke (Diplostomum Pseudospathaceum) in Different Genetic Lineages of Rainbow Trout." PLOS ONE 17 (10): e0276895–95. https://doi.org/10.1371/journal.pone.0276895.
  • Karvonen, A., O. Seppälä, and E. T. Valtonen. 2004. "Eye Fluke-Induced Cataract Formation in Fish: Quantitative Analysis Using an Ophthalmological Microscope." Parasitology 129 (4): 473–78. https://doi.org/10.1017/s0031182004006006.
  • Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Noble, Chris, Hernán A. Cañon Jones, Børge Damsgård, Matthew J. Flood, Kjell Ø. Midling, Ana Roque, Bjørn-Steinar Sæther, and Stephanie Yue Cottee. "Injuries and Deformities in Fish: Their Potential Impacts upon Aquacultural Production and Welfare." Fish Physiology and Biochemistry 38, no. 1 (September 15, 2011): 61–83. https://doi.org/10.1007/s10695-011-9557-1.
  • Oh, Woo Taek, Jin Woo Jun, Sib Sankar Giri, Saekil Yun, Hyoun Joong Kim, Sang Guen Kim, Sang Wha Kim, Se Jin Han, Jun Kwon, and Se Chang Park. 2019. "Staphylococcus Xylosus Infection in Rainbow Trout (Oncorhynchus mykiss) as a Primary Pathogenic Cause of Eye Protrusion and Mortality." Microorganisms 7 (9): 330. https://doi.org/10.3390/microorganisms7090330.
  • Pushchina, Evgeniya V., Anatoly A. Varaksin, Dmitry K. Obukhov, and Igor M. Prudnikov. 2020. "GFAP Expression in the Optic Nerve and Increased H2S Generation in the Integration Centers of the Rainbow Trout (Oncorhynchus mykiss) Brain after Unilateral Eye Injury." Neural Regeneration Research 15 (10): 1867–67. https://doi.org/10.4103/1673-5374.280320.
  • Thatcher, T. O. 1979. "A Morphological Defect in Shiner Perch Resulting from Chronic Exposure to Chlorinated Sea Water." Bulletin of Environmental Contamination and Toxicology 21 (1): 433–38. https://doi.org/10.1007/bf01685449.
  • Weirup, Lina. "Development of a Fish Welfare Evaluation Index for Rainbow Trout (Oncorhynchus mykiss) in Aquaculture." PhD Thesis, 2022. https://d-nb.info/1262308445/34.

Operculum damage

The operculum (gill cover) protects the gills and maintains efficient water flow for respiration. Opercular damage can reduce a fish's capacity to pump water over the gills, compromising oxygen uptake and leading to respiratory stress. This can cause significant problems, particularly when environmental conditions are poor or when oxygen demand is high. Damaged opercula also leave gill filaments exposed to physical injury, pathogens, and debris, increasing the risk of secondary infections.

Subindicators Ideal Acceptable Warning Unacceptable
Operculum shortening

Operculum fully covers gills

Minor shortening with limited gill tissue visible (< 10%)

Moderate shortening with some gill tissue visible (10–30%)

Severe shortening, with gill tissue clearly visible (> 30%)
Operculum folding

No folding

Minor folding, with gills fully covered

Moderate folding, exposing any part of the gills

Severe folding (cannot close)
Overall

Overall score is determined by the most severe subindicator
Protocol Application

Score based on the most affected operculum, though recording both opercula is recommended to improve data quality.
Where opercular shortening is present, gills MUST be examined (see gill damage indicator).
Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • The operculum should be assessed when fully closed by estimating the percentage of gill tissue visible. This is important for consistent scoring.
  • A folded operculum refers to the gill cover being abnormally pressed, deformed, or collapsed against the fish's head, rather than lying flat and being able to move normally.
  • Bilateral cases: Where both opercula would be scored as 'warning' or 'unacceptable', this signals a larger welfare risk than unilateral damage. Monitoring should be increased where this is observed.
  • A combination of gill damage and operculum damage (shortening or folding) signals a problem requiring urgent intervention.
  • Validation: This scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon.
Key Evidence
Source Summary of source Key result
Peters et al., 1984 Cross-sectional survey investigating environment–induced gill disease in 160 rainbow trout (58–405g) from 8 German freshwater farms after at least 3 months on-site. Opercular shortening was designated as "slight" (< 30%) or "severe" (> 30%). "Slight" gill cover shortening occurred in all fish samples. Prevalence of any gill shortening ranged from ~15% to 80%. Only 3 of the 8 farms had any "severe" cases of operculum shortening.
Noble et al., 2020 Literature review on a number of rainbow trout welfare indicators, including opercular damage. Damage to the opercula is associated with increased mortality rates and susceptibility to diseases. The authors note damage or deformities of the opercula can occur for many potential reasons, including suboptimal rearing conditions, dietary deficiencies, pollution, and traumatic injuries during competitive feeding.
Blaker and Ellis, 2022 Experimental study investigating the prevalence of opercular shortening on one cohort of Atlantic salmon (growing from < 1g to ~300g) in laboratory conditions over 14 months. Over the 14 months there were 12 sampling instances, with at least 30 fish sampled each time. Fish were initially reared in freshwater then transferred to seawater after smoltification. Opercular shortening was first recorded at the parr stage and increased in both prevalence and severity as fish grew. Short opercula were qualitatively associated with gill filament damage, with the extent of gill damage related to the area of operculum missing. Nipping was assessed as the primary cause of short opercula in the experiment.
Supporting Evidence
  • Blaker, E, and T Ellis. "Assessment, Causes and Consequences of Short Opercula in Laboratory-reared Atlantic Salmon (Salmo Salar)." Animal Welfare 31, no. 1 (2022): 79–89. https://doi.org/10.7120/09627286.31.1.007.
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Nilsson, Jonatan, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.

Gill damage

Gills are vital organs responsible for oxygen uptake, carbon dioxide release, and osmoregulation, making them essential for fish survival and performance. Because gills are in continuous contact with surrounding water, they are highly vulnerable to irritants, pathogens, and poor environmental conditions, and serve as an early indicator of suboptimal water quality or disease. Damage to gill tissue can reduce respiratory efficiency and impair the fish's ability to maintain ion and acid–base balance, leading to stress and compromised welfare.

Subindicators Ideal Acceptable Warning Unacceptable
Gill structure

No visible alterations with filaments intact

Trace gill alterations

Minor gill alterations

Clear gill alterations
Gill color

Gill color bright red without any pale areas

Bright red color mostly maintained

Pale spots or brown/yellow discoloration

Extensive pale areas or severe discoloration
Overall

Overall score is determined by the most severe subindicator
Protocol Application

Score based on the most affected gill, though recording both gills is recommended to improve data quality.
Gill assessment requires direct handling to lift the operculum for visual inspection.
Filament structure and color must be assessed to score the gills properly.
Where gill damage is present, opercula MUST also be examined.
Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • When assessing structural alterations, consider irregularities, clubbing, spacing changes, hyperplasia and fusion for accurate scoring.
  • Trace alterations could include slight fraying or shortening affecting only a few filaments on one arch.
  • Mild alterations could include mild hyperplasia, epithelial lifting, or fraying affecting multiple filaments but without functional impairment.
  • Clear alterations could include pronounced hyperplasia, pronounced fraying, hemorrhages or necrosis.
  • A bright red color indicates good blood flow and healthy tissue. Pale areas on the gills can indicate poor perfusion, tissue damage, or necrosis, while brown/yellow discoloration can indicate bacterial infection, necrosis, or chronic inflammation.
  • A combination of gill damage and a shortened or folded operculum signals a problem requiring urgent intervention.
  • Bilateral cases: Where both gills would be scored as 'warning' or 'unacceptable', this signals a larger welfare risk than unilateral damage. Monitoring should be increased where this is observed.
  • Validation: This scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon.
Key Evidence
Source Summary of source Key result
Strzyżewska-Worotyńska et al, 2017 Cross-sectional study investigating environmental impacts on gills of ongrowing rainbow trout (500–850g) in 6 different commercial freshwater farms. Most sampled fish had normal gills. When lesions were present, they were mostly mild and/or reversible. Stage III lesions (fibrosis and necrosis) were about 5 times more common in RAS systems.
Zhan et al., 2022 Experimental study investigating alterations to the mucosal immune system in response to infectious hematopoietic necrosis virus (IHNV) in juvenile rainbow trout. Gill tissue showed significant immune response alterations during viral infection.
Supporting Evidence
  • Nilsson, Jonatan, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

Skin hemorrhaging

The skin is the fish's largest organ and serves as an essential barrier against pathogens, parasites, and environmental stressors. Damage to the skin compromises this barrier, leaving fish more susceptible to infection, osmotic stress, and secondary health problems. Because the skin is in continuous contact with the aquatic environment, hemorrhaging can result from physical trauma during crowding, handling, collisions, or aggressive interactions. Skin hemorrhages can also indicate underlying disease.

Ideal Acceptable Warning Unacceptable

No visible hemorrhaging

Healed hemorrhaging

Fresh minor hemorrhaging

Fresh moderate hemorrhaging
Healthy Atlantic salmon with no injuries
Healthy Atlantic salmon with no injuries33
Atlantic salmon with minor hemorrhaging
Atlantic salmon with minor hemorrhaging34
Atlantic salmon with moderate hemorrhaging
Atlantic salmon with moderate hemorrhaging35
Protocol Application

Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • Distinguish between fresh and healed hemorrhages.
  • Fresh hemorrhages may have active bleeding, red or pink inflamed spots, potential swelling, or edema and indicate recent trauma.
  • Healed hemorrhages show stable discoloration and/or scar tissue and no active bleeding, indicating the welfare issue has been largely resolved.
  • Clear ulcerations, lesions, necrotic tissues, or wounds should be captured under the 'wounds' indicator.
  • When scoring, note the location of any hemorrhaging (e.g. ventral/rib cage).
  • Validation: This scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon.
Key Evidence
Source Summary of source Key result
Schill and Elle, 2000 Experimental study investigating the healing of electroshock-induced hemorrhages in 1210 juvenile rainbow trout (26–29cm in length) over 36–57 days in freshwater conditions. 502 fish were shocked with a continuous direct current of 375V, 7.5A, and then kept in holding pens for 36 days (DC trial). 708 fish were shocked with a pulsed direct current of 325 V, 60 Hz, 25% duty cycle, 7.5A, and then kept in holding pens for 57 days (PDC trial). 50 fish per trial served as controls, handled but not shocked. Hemorrhages were counted and scored on a scale of 0–3. 0 = no hemorrhage; 1 = wound separate from the spine; 2 = wound on spine less than or equal to the width of two vertebrae; and 3 = wound on spine greater than the width of two vertebrae. One day after shocking, the DC trial group had 86.1% of fish with at least one hemorrhage, and a mean of 1.86 hemorrhages per fish, while the PDC trial group had 81.6% of fish with at least one hemorrhage and a mean of 1.45 hemorrhages per fish. In both groups, many hemorrhages were initially scored as level 2. By days 8–15, many of these had progressed to level 3. This was explained as the spreading of blood over time, making the lesions larger. Over time, the color changed from bright red to dull red or brown, and the edges faded over time. In the DC trial, the total number of visible hemorrhages per fish was reduced by 78% by day 36 compared to day 1. In the PDC trial there was a reduction of 92.4% by day 57. Towards the end, many of the remaining injuries were only faint grey areas.
Noble et al., 2020 Literature review on many rainbow trout welfare indicators, including skin damage. Hemorrhages primarily result from physical trauma, including handling, collisions, pumping, or crowding. It is noted that small hemorrhages are typically seen ventrally.
Supporting Evidence
  • Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Pettersen, Jostein M., Marc B.M. Bracke, Paul J. Midtlyng, Ole Folkedal, Lars H. Stien, Håvard Steffenak, and Tore S. Kristiansen. "Salmon Welfare Index Model 2.0: An Extended Model for Overall Welfare Assessment of Caged Atlantic Salmon, Based on a Review of Selected Welfare Indicators and Intended for Fish Health Professionals." Reviews in Aquaculture 6, no. 3 (June 30, 2013): 162–79. https://doi.org/10.1111/raq.12039.
  • Schill, Daniel J., and F. Steven Elle. "Healing of Electroshock-Induced Hemorrhages in Hatchery Rainbow Trout." North American Journal of Fisheries Management 20, no. 3 (August 1, 2000): 730–36. https://doi.org/10.1577/1548-8675(2000)020%3C0730:hoeihi%3E2.3.co;2.
  • Stien, Lars Helge, Chris Noble, Angelico Madaro, Jonatan Nilsson, René Alvestad, Kristine Gismervik, Ewa Harasimczuk, Hogne Bleie, Karoline Skaar Amthor, and Marc B. M. Bracke. "Reviewing the Severity of Handling‐Induced Injuries in the LAKSVEL Protocol for Salmon Welfare." Reviews in Aquaculture 17, no. 4 (August 14, 2025). https://doi.org/10.1111/raq.70076.

Scale loss

Like skin, scales act as a key natural barrier against infections. Scales can be lost during handling events, crowding, pumping, netting, delousing operations, and interactions with other fish. Scale loss can lead to osmoregulatory problems, particularly when extensive.

Ideal Acceptable Warning Unacceptable

No visible scale loss

Individual scales missing only

Small areas of scale loss (< 10% of body surface) OR individual scales missing over the rib cage

Large areas of scale loss (> 10% of body surface) OR small areas of scale loss over the rib cage (< 10% of rib cage area)
Atlantic salmon without scale loss
Atlantic salmon without scale loss36
Example of individual missing scales on Atlantic salmon
Example of individual missing scales on Atlantic salmon37
Example of small area of scale loss on Atlantic salmon
Example of small area of scale loss on Atlantic salmon38
Example of large area of scale loss on Atlantic salmon
Example of large area of scale loss on Atlantic salmon39
Protocol Application

Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • Scales floating in the water is an indication that scale loss is a problem.
  • Scale loss over the rib cage carries a high risk due to proximity to vital organs, and therefore results in at least a warning score.
  • Validation: This scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon.
Key Evidence
Source Summary of source Key result
Bouck and Smith, 1979 Experimental study investigating the effects of the slime and scale removal on smolted Coho salmon (~12cm fork length) in laboratory conditions over 10 days. After scale or slime removal, fish were held in either freshwater or seawater for 10 days, with a minimum of 20 fish per treatment group. An additional experiment allowed fish to recover in freshwater for 1–5 days after 25% scale removal, then transferred to seawater. The control group experienced no scale or slime removal. In the control group, approximately 32% of smolts died after direct transfer to seawater, establishing a baseline mortality. Mortality increased with descaling: 10% scale loss resulted in approximately 50% mortality and 25% scale loss resulted in approximately 75% mortality after 10 days in seawater. Scale-removal treatments did not cause mortality in freshwater. In the recovery groups, smolts with 25% scale removal held in freshwater for 1 day before seawater transfer showed 10% mortality, while those held for 5 days showed 0% mortality. Scales removal over the rib cage caused the highest mortality, attributed to the proximity to internal body organs.
Nilsson and Madaro, 2025 Experimental study investigating the effect of controlled scale loss on physiology and growth in 480 Atlantic salmon (146g) over 6 weeks in laboratory conditions. There were 4 treatment groups of 0%, 5%, 10% and 20% scale loss. Scales were removed from the left dorsal side using a blunt spatula. On the same day as descaling, fish in each treatment group were transferred from freshwater to either brackish water (25 ppt) or seawater (34 ppt) conditions. The temperature was controlled at 10°C. Fish were sampled on days 1, 7, and 42. No negative welfare impacts of descaling were detected. There was zero mortality, and scale loss did not affect specific growth rate at either salinity. There was no relationship between cortisol, glucose, or lactate levels and descaling levels. No wounds were observed at any point, and by day 42, new scales had regrown to nearly original size. The authors noted the same effects may not be observed under the more challenging conditions in commercial settings. For example, there were no temperature fluctuations, pathogen exposure, biological stressors, low oxygen levels, or challenges such as waves or handling.
Kostecki et al., 1987 Experimental study investigating the impacts of scale loss on survival after a turbine passage in Atlantic salmon smolts in commercial conditions and presmolts (45–167g) in laboratory conditions. Smolts were passed through one of two Ossberger turbines (450 or 850kW). The presmolts were divided into 5 groups with varying levels of scale removal: 0% scale removal (control), 10% scale removal in the caudal region, 10% scale removal in the abdominal region, 20% scale removal in the abdominal region, and 30% scale removal in the abdominal and dorsal regions. The fish were then held in freshwater for 8 days, followed by half-strength seawater for 4 days. In the smolt experiment, fish were grouped as controls (never passed through the turbine), fish that died within 48 hours, and survivors beyond 48 hours. Average scale loss was 15.1% in controls, 24.2% in fish that died, and 15.9% in survivors. Fish that died had significantly more scale loss than survivors or controls. From the presmolts, only the group that had lost 30% of scales showed elevated mortality.
Supporting Evidence
  • Bouck, Gerald R., and Stanley D. Smith. "Mortality of Experimentally Descaled Smolts of Coho Salmon (Oncorhynchus Kisutch) in Fresh and Salt Water." Transactions of the American Fisheries Society 108, no. 1 (1979): 67–69. https://doi.org/10.1577/1548-8659(1979)108%3C67:MOEDSO%3E2.0.CO;2.
  • Kostecki, Paul T., Patricia Clifford, Steven P. Gloss, and James C. Carlisle. "Scale Loss and Survival in Smolts of Atlantic Salmon (Salmo salar) after Turbine Passage." Canadian Journal of Fisheries and Aquatic Sciences 44, no. 1 (January 1987): 210–14. https://doi.org/10.1139/f87-028.
  • Moltumyr, Lene, Jonatan Nilsson, Angelico Madaro, Tore Seternes, Fredrik Agerup Winger, Ivar Rønnestad, and Lars Helge Stien. "Long-Term Welfare Effects of Repeated Warm Water Treatments on Atlantic Salmon (Salmo salar)." Aquaculture 548 (February 15, 2022): 737670. https://doi.org/10.1016/j.aquaculture.2021.737670.
  • Nilsson, Jonatan, and Angelico Madaro. "Descaling Did Not Affect Osmoregulation, Growth or Mortality in Atlantic Salmon Smolts after Transfer to Brackish or Sea Water." Aquaculture 605 (July 15, 2025). https://doi.org/10.1016/j.aquaculture.2025.742526.
  • Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Stien, Lars H., Marc B. M. Bracke, Ole Folkedal, Jonatan Nilsson, Frode Oppedal, Thomas Torgersen, Silje Kittilsen, et al. "Salmon Welfare Index Model (SWIM 1.0): A Semantic Model for Overall Welfare Assessment of Caged Atlantic Salmon: Review of the Selected Welfare Indicators and Model Presentation." Reviews in Aquaculture 5, no. 1 (March 2013): 33–57. https://doi.org/10.1111/j.1753-5131.2012.01083.x.
  • Stien, Lars Helge, Chris Noble, Angelico Madaro, Jonatan Nilsson, René Alvestad, Kristine Gismervik, Ewa Harasimczuk, Hogne Bleie, Karoline Skaar Amthor, and Marc B. M. Bracke. "Reviewing the Severity of Handling‐Induced Injuries in the LAKSVEL Protocol for Salmon Welfare." Reviews in Aquaculture 17, no. 4 (August 14, 2025). https://doi.org/10.1111/raq.70076.

Wounds

Like skin hemorrhaging, wounds represent breaks in the skin, a key barrier against external dangers such as pathogens. Beyond causing pain and stress, open wounds in direct contact with water can quickly become infected, posing a welfare risk. Wounds can also interfere with the osmoregulatory functions of fish, causing further stress.

Ideal Acceptable Warning Unacceptable

No wounds

Healed wounds only

1-2 fresh small wounds

Any fresh large wounds OR 3+ fresh small wounds OR any muscle-exposed wounds OR any infected wounds
Healthy Atlantic salmon with no injuries
Healthy Atlantic salmon with no injuries40
Rainbow trout with healed wound
Rainbow trout with healed wound41
Rainbow trout with small wound
Rainbow trout with small wound42
Atlantic salmon with large wound
Atlantic salmon with large wound43
Protocol Application

Scoring this indicator involves counting total wounds, determining wound size (small or large), and checking the depth and infection status.
Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • The status of injuries (fresh, healed, or infected) is critical to assess because fresh tissue damage is automatically more severe.
  • Fresh wounds may have active bleeding, red or pink inflamed spots, potential swelling, or edema and indicate recent trauma.
  • Healed wounds have a stable size with scar tissue formed and no active bleeding, indicating the welfare issue has been largely resolved.
  • Infected wounds may have white or grey fungal growth or excessive debris or mucus.
  • Wound size classification is based on the recommendations of Noble et al., who used a 10 pence piece (~2.5cm diameter) as the threshold. Therefore:
  • Small wounds are < 2.5cm diameter, and large wounds are > 2.5cm diameter.
  • For practical field reference, a 10 pence piece, Euro 50 cent coin, or US quarter can be used to approximate this threshold.
  • When scoring, note the wound location to further identify trends (e.g. dorsal surface, lateral sides, ventral surface, head).
  • Validation: This scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon.
Key Evidence
Source Summary of source Key result
Schmidt et al., 2016 Experimental study investigating the wound healing in 154 rainbow trout (112g) in freshwater laboratory conditions over 1 year. Four circular wounds were made between the lateral line and dorsal fin on the left side of each fish. The wounds were spaced 1cm apart, 3mm deep, and penetrated into the muscle. Fish were divided between two tanks, with half of the fish in each tank wounded. In one of the tanks, the fish were exposed to an immunomodulatory β-glucan product. Inflammatory and matrix-remodeling genes were strongly upregulated from day 1 to at least day 14, and remained elevated until day 100. Wound areas enlarged until day 14 due to swelling and then slowly contracted. By day 100, the epidermis had closed, but the dermis remained thickened and disorganized, and only 20% of wounds had fully contracted. No proper muscle regeneration was identified. After one year, scales had not fully regenerated, with some small, irregular scales around wound edges. At this point, the wound appeared cosmetically healed. The immunomodulatory β-glucan treatment had no effect on wound closure speed.
Ingerslev et al., 2010 Experimental study investigating the impacts of standardized skin and muscle injury on 80 rainbow trout (~43g at time of injury, growing to ~84g at the end of the experiment) in freshwater systems over 42 days. Each fish received approximately 50 punctures using a 25 needle array pressed twice in a 6mm x 6mm pattern to a 6 mm depth. The injury site was above the lateral line, on the left side, behind the dorsal fin. On days 7, 14, and 21, clear bleeding was visible, with swollen skin on day 7 and signs of inflammation still present but nearly resolved by day 14. By day 28, bleeding was no longer visible and external swelling had essentially resolved. IL-1β and IL-8 (pro-inflammatory cytokines) expression increased post-injury, but were localized to the site of the injury. Upregulation decreased as the injury healed and was about normal at day 28.
Supporting Evidence
  • Ingerslev, H. C., T. Lunder, and M. E. Nielsen. "Inflammatory and Regenerative Responses in Salmonids Following Mechanical Tissue Damage and Natural Infection." Fish & Shellfish Immunology 29, no. 3 (September 2010): 440–50. https://doi.org/10.1016/j.fsi.2010.05.002.
  • Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, Chris, Hernán A. Cañon Jones, Børge Damsgård, Matthew J. Flood, Kjell Ø. Midling, Ana Roque, Bjørn-Steinar Sæther, and Stephanie Yue Cottee. "Injuries and Deformities in Fish: Their Potential Impacts upon Aquacultural Production and Welfare." Fish Physiology and Biochemistry 38, no. 1 (September 15, 2011): 61–83. https://doi.org/10.1007/s10695-011-9557-1.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Pettersen, Jostein M., Marc B.M. Bracke, Paul J. Midtlyng, Ole Folkedal, Lars H. Stien, Håvard Steffenak, and Tore S. Kristiansen. "Salmon Welfare Index Model 2.0: An Extended Model for Overall Welfare Assessment of Caged Atlantic Salmon, Based on a Review of Selected Welfare Indicators and Intended for Fish Health Professionals." Reviews in Aquaculture 6, no. 3 (June 30, 2013): 162–79. https://doi.org/10.1111/raq.12039.
  • Schill, Daniel J, and F. Steven Elle. "Healing of Electroshock-Induced Hemorrhages in Hatchery Rainbow Trout." North American Journal of Fisheries Management 20, no. 3 (August 1, 2000): 730–36. https://doi.org/10.1577/1548-8675(2000)020%3C0730:hoeihi%3E2.3.co;2.
  • Schmidt, J.G., E.W. Andersen, B.K. Ersbøll, and M.E. Nielsen. "Muscle Wound Healing in Rainbow Trout (Oncorhynchus mykiss)." Fish & Shellfish Immunology 48 (January 2016): 273–84. https://doi.org/10.1016/j.fsi.2015.12.010.
  • Stien, Lars H., Marc B. M. Bracke, Ole Folkedal, Jonatan Nilsson, Frode Oppedal, Thomas Torgersen, Silje Kittilsen, et al. "Salmon Welfare Index Model (SWIM 1.0): A Semantic Model for Overall Welfare Assessment of Caged Atlantic Salmon: Review of the Selected Welfare Indicators and Model Presentation." Reviews in Aquaculture 5, no. 1 (March 2013): 33–57. https://doi.org/10.1111/j.1753-5131.2012.01083.x.
  • Stien, Lars Helge, Chris Noble, Angelico Madaro, Jonatan Nilsson, René Alvestad, Kristine Gismervik, Ewa Harasimczuk, Hogne Bleie, Karoline Skaar Amthor, and Marc B. M. Bracke. "Reviewing the Severity of Handling‐Induced Injuries in the LAKSVEL Protocol for Salmon Welfare." Reviews in Aquaculture 17, no. 4 (August 14, 2025). https://doi.org/10.1111/raq.70076.
  • Lene Rydal Sveen, "Aquaculture Relevant Stressors and Their Impacts on Skin and Wound Healing in Post-Smolt Atlantic Salmon (Salmo salar L.)" (PhD thesis, 2018), https://bora.uib.no/bora-xmlui/bitstream/handle/1956/19024/Dr.thesis_2018_Lene_Rydal_Sveen.pdf.
  • Virtanen, Miiro Ilmari, Monica F. Brinchmann, Deepti Manjari Patel, and Martin Haugmo Iversen. "Chronic Stress Negatively Impacts Wound Healing, Welfare, and Stress Regulation in Internally Tagged Atlantic Salmon (Salmo salar)." Frontiers in Physiology 14 (April 3, 2023). https://doi.org/10.3389/fphys.2023.1147235.

Snout damage

Snout and mouth damage can impede feeding ability, leading to reduced growth. In extreme cases, fish may even starve due to inability to feed. Snout damage can also cause difficulties in breathing, disrupting use of the buccal-opercular pump.

Ideal Acceptable Warning Unacceptable

No visible snout damage

Healed snout damage

Fresh minor snout damage

Fresh moderate snout damage OR any infected wounds
Atlantic salmon with no snout damage
Atlantic salmon with no snout damage44
Atlantic salmon with minor snout damage
Atlantic salmon with minor snout damage45
Atlantic salmon with moderate snout damage
Atlantic salmon with moderate snout damage46
Protocol Application

Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • The status of injuries (fresh, healed, or infected) is critical to assess because fresh tissue damage is automatically more severe.
  • Fresh damage may have active bleeding, red or pink inflamed spots, potential swelling, or edema and indicate recent trauma.
  • Healed damage will have a stable size with scar tissue formed and no active bleeding, indicating the welfare issue has been largely resolved.
  • Infected wounds can have white or grey fungal growth and excessive debris or mucus.
  • Where snout damage is identified, observe feeding behavior to confirm whether the fish can open and close its mouth properly. If feeding is impaired, monitoring should be increased.
  • Note whether the damage is bilateral (affecting both the upper and lower jaws) and the specific location (e.g. tip damage could impair precision feeding, rostral damage could affect the mouth opening or closing).
  • Validation: This scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon.
Key Evidence
Source Summary of source Key result
Sneddon, 2003 Experimental study investigating behavioral and physiological responses consistent with pain perception in 25 rainbow trout (~61g) in freshwater laboratory conditions. There were 5 treatment groups: control (only handled), saline-injected, acid-injected, acid-and-morphine-injected, and morphine-injected. The injection was in the trout's lips. Acid-injected fish rested on the substrate and rocked from side to side on their pectoral fins. The authors interpreted this as possible stereotypic or comfort-seeking behavior. Fish also rubbed their lips against gravel or tank walls, which was interpreted by the authors as an attempt to relieve pain. These behaviors were nearly absent in controls and were significantly reduced by morphine.
Moltumyr et al., 2022 Experimental study investigating external injury outcomes after two warm water exposures in 332 seawater-accimatized Atlantic salmon post-smolts (1380g). Treatments were conducted at approximately 34°C (and 9°C for the control) for 30 seconds, administered twice 23–24 days apart. For the treatment, fish were dip-netted and placed in a plastic chamber for 30 seconds. Various external injuries were scored from 0 to 3 (increasing in severity). Final scoring was conducted 17–18 days after the second exposure. Fish in the warm water group displayed vigorous behavior, frantically trying to escape. Some fish displayed such vigorous escape behavior that they fainted after colliding with the walls. The fish in the control group behaved more calmly. Compared to the control group, treated fish showed higher prevalence of scores 2 and 3 for scale loss, snout wounds, eye problems, and active fin injuries. The baseline prevalence for scale loss scores between 1–3 was very high for both the control and treatment groups, at 99–100%. The baseline prevalence for skin bleeding scores between 1–3 was 73–79%. Snout wounds had a prevalence of 7–12% before the treatment. At the end of the experiment, there was a 99% prevalence, with the severity of wounds significantly higher in the warm water group.
Stien et al., 2025 Review of injury-based welfare indicators of farmed salmon in the Laksvel protocol. Snout wounds can affect either the upper or lower jaw, and do not need to be restricted only to the tip of the snout. When salmon are transported (e.g., through pumps or delousing systems), they move headfirst, increasing risk of snout collision into equipment or other fish. Snout wounds can become infected, e.g. leading to mouth rot or yellow mouth.
Supporting Evidence
  • Moltumyr, Lene, Jonatan Nilsson, Angelico Madaro, Tore Seternes, Fredrik Agerup Winger, Ivar Rønnestad, and Lars Helge Stien. "Long-Term Welfare Effects of Repeated Warm Water Treatments on Atlantic Salmon (Salmo salar)." Aquaculture 548 (February 15, 2022): 737670. https://doi.org/10.1016/j.aquaculture.2021.737670.
  • Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Noble, Chris, Hernán A. Cañon Jones, Børge Damsgård, Matthew J. Flood, Kjell Ø. Midling, Ana Roque, Bjørn-Steinar Sæther, and Stephanie Yue Cottee. "Injuries and Deformities in Fish: Their Potential Impacts upon Aquacultural Production and Welfare." Fish Physiology and Biochemistry 38, no. 1 (September 15, 2011): 61–83. https://doi.org/10.1007/s10695-011-9557-1.
  • Sneddon, Lynne U. "The Evidence for Pain in Fish: The Use of Morphine as an Analgesic." Applied Animal Behaviour Science 83, no. 2 (September 2003): 153–62. https://doi.org/10.1016/s0168-1591(03)00113-8.
  • Stien, Lars Helge, Chris Noble, Angelico Madaro, Jonatan Nilsson, René Alvestad, Kristine Gismervik, Ewa Harasimczuk, Hogne Bleie, Karoline Skaar Amthor, and Marc B. M. Bracke. "Reviewing the Severity of Handling‐Induced Injuries in the LAKSVEL Protocol for Salmon Welfare." Reviews in Aquaculture 17, no. 4 (August 14, 2025). https://doi.org/10.1111/raq.70076.

Fin damage

Fins are critical for swimming efficiency, maneuverability, balance, maintaining position in water currents, and natural behaviors. Fin damage can reduce swimming performance, feeding efficiency, and the ability to avoid aggression or predators, impacting fish welfare and survival. Fin damage may arise from aggressive fin-nipping, poor water quality, overcrowding, rough handling, or contact with abrasive surfaces, making it a useful welfare indicator.

Ideal Acceptable Warning Unacceptable

No visible damage to any fin

Minor alteration of fin

Moderate alteration of fin

Severe alteration of fin OR any necrotic lesions or ulcerations
Intact rainbow trout anal fin
Intact rainbow trout anal fin47
Rainbow trout dorsal fin with minor alteration
Rainbow trout dorsal fin with minor alteration48
Rainbow trout pelvic fin with moderate alteration
Rainbow trout pelvic fin with moderate alteration49
Rainbow trout caudal fin with severe alteration
Rainbow trout caudal fin with severe alteration50
Protocol Application

Score based on the most damaged fin, though recording all fins is recommended to improve data quality.
Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • Damage to a single fin damage indicates isolated trauma (e.g., from handling or aggression).
  • Different fins serve different purposes, and the impact of damage varies, for example:
  • Caudal fin: damage can reduce swimming efficiency.
  • Dorsal and anal fins: damage can affect balance and maneuverability.
  • Pectoral fins: damage can impair steering, braking, and precise movements.
  • Pelvic fins: damage affects stability, but is least critical for survival.
  • Bilateral cases: Where both fins in a pair (i.e. pectoral or pelvic fins) would be scored as 'warning' or 'unacceptable', this signals a greater welfare risk than unilateral damage. Monitoring should be increased where this is observed.
  • Damage across multiple fins can indicate systemic issues (chronic aggression, poor water quality, overcrowding, etc).

fin_scoring_protocol.png
Figure: Protocol for scoring fin damage in juvenile rainbow trout. Reproduced from Hoyle et al., 2007. 0 corresponds to 'ideal', 1 to 'acceptable', 2 to 'warning', and 3+ to 'unacceptable'.

Key Evidence
Source Summary of source Key result
Weirup 2022 Cross-sectional study investigating chronic stress in ongrowing rainbow trout across 8 commercial freshwater systems in Germany. 10 adult female rainbow trout (~18 months old) were sampled per system. Fish were examined for external morphological damage, including fin damage, and sampled for scale cortisol using advanced laboratory techniques. Chronic stress (estimated through scale cortisol) correlated strongly with total fin injury across all farms. Fin injuries were common and varied significantly between farms, while other types of morphological damage were rare. Limited water supply and exchange, poor water quality, high stocking densities, and high feeding frequencies increased stress and fin damage.
Cvetkovikj et al., 2015 Questionnaire-based survey investigating the relationship between husbandry practices and fin damage conducted across 7 commercial trout farms in Macedonia. Lower levels of daily feed ration, increased number of meals, and grading frequency were the strongest predictors of fin damage across different fins. Fin damage reflects aggression and competition during feeding and suboptimal husbandry practices.
Person-Le Ruyet et al., 2008 Experimental study investigating the impacts of different water quality levels and stocking densities on ongrowing rainbow trout (111.5g) in freshwater conditions over 84 days. Three stocking densities were tested: 25, 75, and 120kg/m³. Two water quality levels were tested: high (with oxygen concentration of 12mg/L and total ammonia nitrogen of 0.3mg/L) and a low (with oxygen concentration of 6mg/L and total ammonia nitrogen of 0.6mg/L). Higher stocking density and better water quality both correlated with increased fin damage over time. The authors hypothesized higher fin damage in better water quality conditions resulted from higher metabolic activity of the fish and more activity during feeding compared to the low water quality group, which led to more physical contact. Pectoral fins deteriorated faster than dorsal fins under poorer conditions.
Supporting Evidence
  • Andrews, M., M. Stormoen, H. Schmidt‐Posthaus, T. Wahli, and P. J. Midtlyng. "Rapid Temperature‐Dependent Wound Closure Following Adipose Fin Clipping of Atlantic Salmon Salmo salar L." Journal of Fish Diseases 38, no. 6 (June 11, 2014): 523–31. https://doi.org/10.1111/jfd.12261.
  • Arnold, G. P., Paul W. Webb, and B. H. Holford. "The Role of the Pectoral Fins in Station-Holding of Atlantic Salmon Parr (Salmo salar L.)." Journal of Experimental Biology 156, no. 1 (March 1, 1991): 625–29. https://doi.org/10.1242/jeb.156.1.625.
  • Cvetkovikj, Aleksandar, Miroslav Radeski, Dijana Blazhekovikj-Dimovska, Vasil Kostov, and Vangjel Stevanovski. "Factors Affecting Fin Damage of Farmed Rainbow Trout." Macedonian Veterinary Review 38, no. 1 (November 24, 2014): 61–71. https://doi.org/10.14432/j.macvetrev.2014.11.032.
  • Cvetkovikj, Aleksandar, Miroslav Radeski, Dijana Blazhekovikj-Dimovska, Vasil Kostov, and Stevanovski Vangjel. "Fin Damage of Farmed Rainbow Trout in the Republic of Macedonia." Macedonian Veterinary Review 36, no. 2 (2013): 73–83.
  • Hoyle, I., B. Oidtmann, T. Ellis, J. Turnbull, B. North, J. Nikolaidis, and T.G. Knowles. "A Validated Macroscopic Key to Assess Fin Damage in Farmed Rainbow Trout (Oncorhynchus mykiss)." Aquaculture 270, no. 1-4 (September 2007): 142–48. https://doi.org/10.1016/j.aquaculture.2007.03.037.
  • Koll, Raphael, Ronald M. Brunner, Alexander Rebl, Marieke Verleih, Frieder Hadlich, Joan Martorell-Ribera, and Tom Goldammer. "Resection of the Adipose Fin from Rainbow Trout Acutely Alters the Cerebral Transcriptome and Respiratory Frequency." Aquaculture 594 (August 10, 2024): 741472. https://doi.org/10.1016/j.aquaculture.2024.741472.
  • Latremouille, David N. "Fin Erosion in Aquaculture and Natural Environments." Reviews in Fisheries Science 11, no. 4 (October 1, 2003): 315–35. https://doi.org/10.1080/10641260390255745.
  • Laursen, Danielle Caroline, Patricia I. M. Silva, Bodil K. Larsen, and Erik Höglund. "High Oxygen Consumption Rates and Scale Loss Indicate Elevated Aggressive Behaviour at Low Rearing Density, While Elevated Brain Serotonergic Activity Suggests Chronic Stress at High Rearing Densities in Farmed Rainbow Trout." Physiology & Behavior 122 (October 2013): 147–54. https://doi.org/10.1016/j.physbeh.2013.08.026.
  • MacIntyre, Craig. "Water Quality and Welfare Assessment on United Kingdom Trout Farms." PhD Thesis, 2008. https://dspace.stir.ac.uk/bitstream/1893/434/1/FINAL%20THESIS.pdf.
  • Moutou, K. A., I. D. McCarthy, and D. F. Houlihan. "The Effect of Ration Level and Social Rank on the Development of Fin Damage in Juvenile Rainbow Trout." Journal of Fish Biology 52, no. 4 (April 1, 1998): 756–70. https://doi.org/10.1111/j.1095-8649.1998.tb00818.x.
  • Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Person-Le Ruyet, Jeannine, Laurent Labbé, Nicola Le Bayon, Armelle Sévère, Annick Le Roux, Hervé Le Delliou, and Loic Quéméner. "Combined Effects of Water Quality and Stocking Density on Welfare and Growth of Rainbow Trout (Oncorhynchus mykiss)." Aquatic Living Resources 21, no. 2 (April 1, 2008): 185–95. https://doi.org/10.1051/alr:2008024.
  • Person-Le Ruyet, Jeannine, Nicolas Le Bayon, and Sylvie Gros. "How to Assess Fin Damage in Rainbow Trout, Oncorhynchus mykiss?" Aquatic Living Resources 20, no. 2 (April 2007): 191–95. https://doi.org/10.1051/alr:2007031.
  • Weirup, Lina. "Development of a Fish Welfare Evaluation Index for Rainbow Trout (Oncorhynchus mykiss) in Aquaculture." PhD Thesis, 2022. https://d-nb.info/1262308445/34.

Spinal deformity

Deformities, including spinal abnormalities, compromise essential biological functions and cause ongoing stress. Spinal deformities can reduce capacity to maintain proper body position and swim efficiently. Deformity prevalence is a valuable indicator of prior and ongoing management practices, as deformities can arise from genetic factors, nutritional deficiencies, environmental stressors, or inappropriate rearing conditions.

Ideal Acceptable Warning Unacceptable

No visible spinal deformity

Suspected spinal deformity

Minor spinal deformity

Any visible kinks in spine
Healthy Atlantic salmon with no deformity
Healthy Atlantic salmon with no deformity51
Atlantic salmon with suspected spinal deformity
Atlantic salmon with suspected spinal deformity52
Atlantic salmon with minor spinal deformity
Atlantic salmon with minor spinal deformity53
Atlantic salmon with visible kink near the tail
Atlantic salmon with visible kink near the tail54
Protocol Application

Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • Fish with a straight dorsal profile and smooth body curvature will score 'ideal'.
  • A suspected spinal deformity ('acceptable') will not have any clear, visible kinks, but will show some subtle irregularities or deviations.
  • A minor deformity ('warning') will include visible deviations.
  • A clear deformity ('unacceptable') will include visible kinks or clear deviations or clear deviations or curves.
  • Studies show that many vertebral deformities are not visible externally. Due to the nature of OWIs, this protocol only enables assessment of externally visible deformities. This may underestimate total deformity prevalence, but identifies fish with more severe welfare impacts.
  • Validation: This scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon.
Key Evidence
Source Summary of source Key result
Fjelldal et al., 2025 Experimental study investigating the progression of spinal deformities and growth in 12,000 female rainbow trout as they grew from 36g to 5.5kg in laboratory conditions over one year. Fish were initially raised in freshwater tanks and then transferred to sea cages. Between 80–100 fish were sampled at 4 different sampling points. Other validation groups were also assessed (different strains or fish raised in different conditions). Note: Deformities were detected using X-ray imaging, which identified internal vertebral deformities not visible by external inspection. There was a dramatic increase in deformities, from 20% of fish at 36g to 93% at 5.5kg. The average number of deformed vertebrae per affected fish also increased, from 4 to 12. Deformity severity strongly correlated negatively and fish length and weight. Tail deformities correlated with caudal fin degradation, with the authors suggesting a bidirectional relationship. All validation groups showed a 74–83% deformity rate.
Davidson et al., 2011 Two experimental studies investigating the impacts of different freshwater RAS conditions on rainbow trout health and welfare. Experiment 1 compared very low water exchange and high water exchange in rainbow trout (151g) over six months. Experiment 2 compared low water exchange to near-zero water exchange in rainbow trout (18g). Skeletal deformities increased substantially in zero-exchange systems, reaching 38% in the most restricted system versus 0% in higher exchange systems. Increased deformities correlated with increased mortality, but causation was not established. Poor water quality may have been the cause of both increased deformities and mortality.
Aunsmo et al., 2007 Experimental study investigating how clinically scored spinal deformity relates to growth and carcass downgrading in ongrowing Atlantic salmon raised in commercial environments, using a visual scale from 0–3 at slaughter. Outcomes were not separated by score, so the below results refers to all fish scoring between 1–3. All fish with spinal deformity scores above 0 were all downgraded from superior production quality, demonstrating the negative economic impact of detectable deformities. Deformed fish had a mean slaughter weight of approximately 2.8kg (about 62% of the mean weight of 4.5kg of superior grade fish), indicating a large and sustained growth deficit associated with visible deformities. Radiography showed compressed or fused vertebrae, mainly in the tail, and in approximately one third of cases extending cranially. Increasing severity of vaccine-related adhesions and abdominal melanisation increased the odds of spinal deformity, suggesting fish with severe internal lesions and deformity experience combined burdens of chronic pathology and reduced growth.
Supporting Evidence
  • Aunsmo, A., A. Guttvik, P. J. Midtlyng, R. B. Larssen, Ø. Evensen, and E. Skjerve. "Association of Spinal Deformity and Vaccine‐Induced Abdominal Lesions in Harvest‐Sized Atlantic Salmon, Salmo salar L." Journal of Fish Diseases 31, no. 7 (June 28, 2008): 515–24. https://doi.org/10.1111/j.1365-2761.2007.00899.x.
  • Davidson, John, Christopher Good, Carla Welsh, and Steven T. Summerfelt. "Abnormal Swimming Behavior and Increased Deformities in Rainbow Trout Oncorhynchus mykiss Cultured in Low Exchange Water Recirculating Aquaculture Systems." Aquacultural Engineering 45, no. 3 (November 2011): 109–17. https://doi.org/10.1016/j.aquaeng.2011.08.005.
  • Fjelldal, Per Gunnar, Sankar Murugesan, Tone Vågseth, Audun Østby Pedersen, Angelico Madaro, Samantha Bui, Harald Kryvi, Lars Helge Stien, and Jonatan Nilsson. "Vertebral Deformities in Cultured Big Size Rainbow Trout: Radiological Analysis from Juvenile to Harvest Size." Aquaculture 596 (October 5, 2024): 741729. https://doi.org/10.1016/j.aquaculture.2024.741729.
  • Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Özcan, Filiz. "Prevalence and Causes of Skeletal Deformities in Cultured Juvenile Oncorhynchus mykiss. Skeletal Deformities in Cultured Juvenile Oncorhynchus mykiss." Revista Colombiana de Ciencias Pecuarias 38 (2025). https://doi.org/10.17533/udea.rccp.e359311.
  • Powell, Mark D., Matthew A. Jones, and Maite Lijalad. "Effects of Skeletal Deformities on Swimming Performance and Recovery from Exhaustive Exercise in Triploid Atlantic Salmon." Diseases of Aquatic Organisms 85 (May 27, 2009): 59–66. https://doi.org/10.3354/dao02056.
  • Witten, P. Eckhard, Laura Gil-Martens, Ann Huysseune, Harald Takle, and Kirsti Hjelde. "Towards a Classification and an Understanding of Developmental Relationships of Vertebral Body Malformations in Atlantic Salmon (Salmo salar L.)." Aquaculture 295, no. 1-2 (October 2009): 6–14. https://doi.org/10.1016/j.aquaculture.2009.06.037.

Jaw deformity

Jaw deformities, such as jaw length discrepancies or curved jaws, can prevent normal feeding and breathing. Most jaw deformities tend to occur during early life stages and, like spinal deformities, are a valuable indicator of prior and ongoing management practices.

Subindicators Ideal Acceptable Warning Unacceptable
Jaw Deformity

Normal jaw length and alignment

Suspected jaw deformity or misalignment

Minor jaw deformity or misalignment

Moderate jaw deformity or misalignment
Pughead

No pughead

Suspected pughead

Minor pughead

Moderate pughead
Overall

Overall score is determined by the most severe subindicator
Protocol Application

Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • Assess jaw shortening and pugheadedness separately for most accurate results by checking for jaw length and alignment, and then checking for any compressed skull shape.
  • Bilateral cases: Bilateral deformities (where both the upper and lower jaw are affected) are assessed as worse because if only one jaw is affected, the other jaw is able to compensate. Monitoring should be increased where this is observed.
  • Validation: This scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon.
Key Evidence
Source Summary of source Key result
Godoy et al., 2010 Experimental study assessing jaw bone structure and osteogenic markers in normal and mandibular-deformed rainbow trout smolts (< 400g) in freshwater commercial farms. 5 normal and 5 mandibular fish were compared. Deformed jaws showed lower calcium and phosphorus content, indicating mechanically weak and/or painful jaws, limiting function during feeding. Deformed fish showed poorer nutritional status, with lower plasma total protein and globulins and markedly reduced visceral fat, indicating chronic energy deficit due to impaired feed intake. Deformed fish also had higher blood glucose and an altered mineral profile, suggesting physiological stress.
Näslund and Jawad, 2021 Literature review assessing the causes and impacts of pugheadedness in multiple species of fish, including rainbow trout. Pugheadedness is linked with exophthalmos and is more likely in more deformed individuals. In severe cases, this can expose the anterior of the eyeball. Since the bones in the jaw may be severely deformed, this can restrict complete opening or closure of the mouth, potentially reducing feeding efficiency and therefore growth.
Supporting Evidence
  • Cano, Irene, John Worswick, Brian Mulhearn, Matt Green, Stephen W. Feist, and Morag Clinton. "Cranial Mandibular Fibrosis Syndrome in Adult Farmed Rainbow Trout Oncorhynchus mykiss." Pathogens 10, no. 5 (April 30, 2021): 542–42. https://doi.org/10.3390/pathogens10050542.
  • Godoy, Karina, Cristian Sandoval, Claudio Vásquez, Carlos Manterola-Barroso, Barbara Toledo, Joel Calfuleo, Carolina Beltrán, et al. "Osteogenic and Microstructural Characterization in Normal versus Deformed Jaws of Rainbow Trout (Oncorhynchus mykiss) from Freshwater." Frontiers in Marine Science 10 (November 24, 2023). https://doi.org/10.3389/fmars.2023.1301449.
  • Näslund, Joacim, and Laith A. Jawad. "Pugheadedness in Fishes." Reviews in Fisheries Science & Aquaculture 30, no. 3 (August 11, 2021): 306–29. https://doi.org/10.1080/23308249.2021.1957772.
  • Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

Change of coloration

Changes in coloration are a key visual indicator of stress, disease, and compromised welfare in farmed fish. In some species, subordinate fish develop darker coloration. Changes in coloration can result from environmental challenges or physiological disturbances. Pale coloration can indicate anemia or severe stress, and patchy coloration can indicate skin disease or parasites. However, rainbow trout can have diverse natural colorations depending on genetics, environment and age, so producers must first establish the normal population baseline before assessing deviations.

Ideal Acceptable Warning Unacceptable

Appropriate coloration for fish strain and environment (e.g. bright, normal patterns and color intensity)

Very mild color variations that are stable and not associated with signs of stressful conditions

Moderate darkening OR Loss of normal brightness OR Patchy areas starting to form

Severe whole body darkening OR Marked pale coloration OR Extensive patchy/mottled appearance
Protocol Application

Calculate the distribution of scores across the thresholds within the sample.

Scoring Notes
  • The population baseline should be used to assess this indicator rather than prescribed colors.
  • Consider environmental factors (such as turbidity, lighting, and substrates) that can affect natural coloration.
  • Validation: This scoring system has not been validated for rainbow trout specifically and is based on expert opinion.
Key Evidence
Source Summary of source Key result
Zhan et al., 2022 Experimental study investigating alterations to mucosal immune response following infectious hematopoietic necrosis virus (IHNV) in juvenile rainbow trout (6g) in freshwater laboratory conditions over 30 days. The study found darkening of the skin as one of the key clinical symptoms appearing 4–7 days post infection.
Clinton et al., 2021 Experimental study investigating gill responses to toxins in 128 juvenile (12g) rainbow trout in freshwater laboratory conditions over 5 days. The group with high exposure to the toxin was accompanied by darker skin coloration. Only the rainbow trout with the most severe cases had a darker coloration.
Hofer and Gatumu 1994 Experimental study measuring retinal damage associated with nitrite exposure in 75 juvenile rainbow trout (5g) in freshwater laboratory conditions over 20 days. The study used three treatment groups: one control group and two treatment groups with lower and higher nitrite levels. In both nitrite treatment groups, approximately 15% of fish developed dark coloration. Fish that developed this were determined to be intolerant, as they also accumulated high nitrite in their blood, developed methemoglobinemia, showed retinal damage, and exhibited quiet behavior and reduced feeding.
Supporting Evidence
  • Berejikian, Barry A., Stephen B. Mathews, and Thomas P. Quinn. "Effects of Hatchery and Wild Ancestry and Rearing Environments on the Development of Agonistic Behavior in Steelhead Trout (Oncorhynchus mykiss) Fry." Canadian Journal of Fisheries and Aquatic Sciences 53, no. 9 (1996): 2004–14. https://doi.org/10.1139/cjfas-53-9-2004.
  • Clinton, Morag, Elżbieta Król, Dagoberto Sepúlveda, Nikolaj R. Andersen, Andrew S. Brierley, David E. K. Ferrier, Per Juel Hansen, Niels Lorenzen, and Samuel A. M. Martin. "Gill Transcriptomic Responses to Toxin-Producing Alga Prymnesium Parvum in Rainbow Trout." Frontiers in Immunology 12 (December 8, 2021). https://doi.org/10.3389/fimmu.2021.794593.
  • González-Garoz, Roberto, Almudena Cabezas, Montserrat Fernández-Muela, Andrea Martínez Villalba, Elisabeth González de Chávarri, Morris Villarroel, Álvaro De la Llave-Propín, Jesús De la Fuente, Rubén Bermejo-Poza, and María Teresa Díaz. "Rainbow Trout Welfare: Comparing Stunning Methods in Winter and Summer." Fish Physiology and Biochemistry 51, no. 3 (June 9, 2025). https://doi.org/10.1007/s10695-025-01526-7.
  • Hofer, R., and E. Gatumu. "Necrosis of Trout Retina (Oncorhynchus mykiss) after Sublethal Exposure to Nitrite." Archives of Environmental Contamination and Toxicology 26, no. 1 (January 1, 1994): 119–23. https://doi.org/10.1007/bf00212803.
  • Kittilsen, S., J. Schjolden, I. Beitnes-Johansen, J. C. Shaw, T. G. Pottinger, C. Sørensen, B. O. Braastad, M. Bakken, and O. Overli. "Melanin-Based Skin Spots Reflect Stress Responsiveness in Salmonid Fish." Hormones and Behavior 56, no. 3 (September 2009): 292–98. https://doi.org/10.1016/j.yhbeh.2009.06.006.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Peters, Gabriele, Mohamed Faisal, Tom Lang, and Iman Ahmed. "Stress Caused by Social Interaction and Its Effect on Susceptibility to Aeromonas Hydrophila Infection in Rainbow Trout Salmo Gairdneri." Diseases of Aquatic Organisms 4 (1988): 83–89. https://doi.org/10.3354/dao004083.
  • Zhan, Mengting, Zhenyu Huang, Gaofeng Cheng, Yongyao Yu, Jianguo Su, and Zhen Xu. "Alterations of the Mucosal Immune Response and Microbial Community of the Skin upon Viral Infection in Rainbow Trout (Oncorhynchus mykiss)." International Journal of Molecular Sciences 23, no. 22 (November 14, 2022): 14037–37. https://doi.org/10.3390/ijms232214037.

Condition factor / Emaciation

Condition factor measures the relationship between a fish's weight and length, providing a quantitative outcome, although it is not always practical to measure in all scenarios. Poor condition factor (appearing thin or emaciated) can indicate inadequate nutrition, poor feed management, chronic stress, disease, or suboptimal environmental conditions that impair growth and energy storage.

Subindicators Ideal Acceptable Warning Unacceptable
Condition Factor
1.1–1.4
1–1.1 , 1.4–1.6
0.9–1 , 1.6–2
≤ 0.9 , > 2
Emaciation

No clear signs of emaciation

Slightly lean OR slightly robust

Lean appearance OR excessive robustness

Signs of emaciation OR signs of severe obesity
Overall

Overall score is determined by the most severe subindicator
Protocol Application

If weighing and measuring individual fish is practical, use the condition factor scoring system. Whether condition factor is measured or not, a always conduct a visual assessment.
Condition factor (often denoted as 'K') is calculated as follows: K = 100 × W / L³, where W is the total body weight of the individual fish and L is the fork length. For adult rainbow trout, W can be measured in grams and L in cm.
In most cases, condition factor will align with the visual appearance of the fish. Where they do not match, visual assessment will take priority for scoring. For example, a fish might be calculated to have an 'ideal' K value of 1.2, but still appear emaciated due to body shortening or deformity. In such cases, score the fish according to its visual category.

Scoring Notes
  • Decreasing condition factor over time is a sign of poor welfare, even if in absolute terms the condition factor remains in an 'ideal' or 'acceptable' threshold. For example, condition factor decreasing from 1.3 to 1.1 is a warning sign.
  • Signs of emaciation include visible spine or ribs and reduced muscle mass.
  • Very high condition factors can indicate problems such as obesity or deformity (e.g., where a spinal deformity has caused the body length to be compressed).
  • Validation: The condition factor thresholds are based on the studies set out below. The visual scoring system has not been validated for rainbow trout specifically, but is based on the Laksvel scoring system for Atlantic salmon.
Key Evidence
Source Summary of source Key result
Macintyre 2008 Large-scale survey investigating welfare in rainbow trout conducted across 44 freshwater rainbow trout farms in the British Isles from July 2005 to April 2007. Each farm was visited twice, with up to four different systems (cages, ponds, raceways, tanks) sampled per visit. Both small (< 100g) and large (> 100g) fish were sampled, with 12 or 24 fish taken from each system, totaling 3699 fish sampled from 189 systems. Condition factor ranged from a low of 0.84 to a high of 2.2, with a mean of 1.31. The author considered this mean condition factor as a normal, healthy range. Increasing condition factor was associated with improved welfare up to 1.3, after which improvements plateaued, and large deviations were associated with worse welfare. Condition factors below 1.0 were associated with increased cortisol.
Pottinger et al., 2003 Experimental study investigating biomarkers associated with fasting in 900 2-year old ongrowing rainbow trout (282g) in freshwater laboratory conditions over 17 weeks. Fish were divided into three groups: continuously fed (control), fasted and then refed (fasted for 9 weeks then fed for the remainder of the study period), and continuously fasted (fasted for the entire 17 weeks). At various intervals, 10 fish per tank were sampled. Both fasted groups showed rapid, significant declines in body weight and condition factor. Refed fish quickly recovered weight and condition factor after feeding resumed, becoming indistinguishable from the control by the end of the study. Fasting did not consistently elevate cortisol levels. The baseline condition factor for the unfasted group was approximately 1.2. When fasted fish reached a condition factor of < 1.1 at week 11, growth hormone rose extremely high and somatolactin levels showed clear, significant differences, with persistently high free fatty acids. After week 11, the level of free fatty acid levels dropped, indicating severe starvation.
Salem et al., 2007 Experimental study investigating the effect of starvation on global gene expression in 12 ongrowing rainbow trout (193g) in laboratory conditions over 3 weeks. The control group was continuously fed. Starved fish lost significant weight (172g vs. 280g in fed fish). In starved fish, 202 genes were down-regulated compared to 27 up-regulated, indicating the fish were going into a critical survival mode.
Gao, 2024 Fact sheet discussing the importance of measuring and calculating condition factor in farmed salmonid species. Emaciation was defined as a condition factor < 0.9, while positive performance was defined as condition factor > 1.1, and possible deformity defined as > 1.6.
Supporting Evidence
  • Barnham, Charles, and Alan Baxter. "Condition Factor, K, for Salmonid Fish." State of Victoria, Department of Primary Industries, March 1998.
  • Dürrani, Ömerhan. "Do the Length-Weight Relationships and Condition Factors of Farmed Rainbow Trout, Brook, and Brown Trout Differ from Their Wild Counterparts?" Aquatic Research 6, no. 4 (2023): 253–59. https://doi.org/10.3153/ar23024.
  • Furné, M., M. García-Gallego, M. C. Hidalgo, A. E. Morales, A. Domezain, J. Domezain, and A. Sanz. "Oxidative Stress Parameters during Starvation and Refeeding Periods in Adriatic Sturgeon (Acipenser Naccarii) and Rainbow Trout (Oncorhynchus mykiss)." Aquaculture Nutrition 15, no. 6 (November 28, 2008): 587–95. https://doi.org/10.1111/j.1365-2095.2008.00626.x.
  • Gao, N. "Condition Factor for Salmonid Aquaculture." Ministry of Agriculture, Food and Agribusiness, October 2024. https://www.ontario.ca/files/2024-11/omafa-condition-factor-for-salmonids-en-2024-11-26.pdf
  • MacIntyre, Craig. "Water Quality and Welfare Assessment on United Kingdom Trout Farms." PhD Thesis, 2008. https://dspace.stir.ac.uk/bitstream/1893/434/1/FINAL%20THESIS.pdf.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • North, B. P., J. F. Turnbull, T. Ellis, M. J. Porter, H. Migaud, J. Bron, and N. R. Bromage. "The Impact of Stocking Density on the Welfare of Rainbow Trout (Oncorhynchus mykiss)." Aquaculture 255, no. 1-4 (May 2006): 466–79. https://doi.org/10.1016/j.aquaculture.2006.01.004.
  • Pottinger, T. G., M. Rand-Weaver, and J. P. Sumpter. "Overwinter Fasting and Re-Feeding in Rainbow Trout: Plasma Growth Hormone and Cortisol Levels in Relation to Energy Mobilisation." Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology 136, no. 3 (November 2003): 403–17. https://doi.org/10.1016/s1096-4959(03)00212-4.
  • Salem, Mohamed, Jeff Silverstein, Caird E. Rexroad, and Jianbo Yao. "Effect of Starvation on Global Gene Expression and Proteolysis in Rainbow Trout (Oncorhynchus mykiss)." BMC Genomics 8, no. 1 (September 19, 2007). https://doi.org/10.1186/1471-2164-8-328.
  • Sumpter, J. P., P. Y. Le Bail, A. D. Pickering, T. G. Pottinger, and J. F. Carragher. "The Effect of Starvation on Growth and Plasma Growth Hormone Concentrations of Rainbow Trout, Oncorhynchus mykiss." General and Comparative Endocrinology 83, no. 1 (July 1991): 94–102. https://doi.org/10.1016/0016-6480(91)90109-j.

Group-based indicators

Mortality

Mortality serves as a key but crude welfare indicator. While high mortality can reasonably be assumed to indicate poor welfare, fish in aquaculture systems can have poor welfare even with low mortality levels.

The Norwegian Food Safety Authority has stated that the average mortality rate in Norway is too high and is unacceptable.1 In 2024, 2.4 million rainbow trout died during the marine phase in Norway, representing an estimated annual cumulative mortality rate of 15.0%.2 In 2021, approximately 5% of all Norwegian locations for Atlantic salmon and rainbow trout achieved mortality rates below 5% in the sea phase.3 Individual farmers, scientific institutions, and others have put forward that this is a feasible mortality rate to achieve.

Beyond recording mortality rates, producers should also record causes of mortality to build understanding of trends over time. This is crucial for enabling appropriate actions to avoid and prevent further mortalities.

Ideal Acceptable Warning Unacceptable
< 1 % per day
1–2 % per day
2–3 % per day
≥ 3 % per day
Protocol Application

The figures above refer to both annualized and daily mortality rates (with daily mortality rates extrapolated from annual rates, rounded for simplicity). Annual rates should be the primary metric for overall assessment, with daily rates important for operational tracking and management decisions.

Scoring Notes
  • If daily mortality > 0.03%, initiate immediate investigation.
  • Along with mortality rates, it is critical to track and record causes of mortality. Understanding why fish are dying is essential for identifying patterns, implementing preventative measures, and distinguishing between normal background mortality and emerging problems. Where many mortalities cannot be explained, monitoring should increase and management practices reassessed.
  • Raw mortality numbers are not the only important metric. Increasing mortality rates, even if absolute rates remain in the 'ideal' or 'acceptable' categories, are still cause for concern.
  • If mortality rates increase or spike, management practices should be reassessed.
  • Being aware of seasonal trends is also important for making improved management decisions.
  • Validation: These thresholds are based on Norwegian industry aspirations and expert opinion.
Supporting Evidence
  • Davidson, John, Christopher Good, Carla Welsh, and Steven T. Summerfelt. "Abnormal Swimming Behavior and Increased Deformities in Rainbow Trout Oncorhynchus mykiss Cultured in Low Exchange Water Recirculating Aquaculture Systems." Aquacultural Engineering 45, no. 3 (November 2011): 109–17. https://doi.org/10.1016/j.aquaeng.2011.08.005.
  • Fjelldal, Per Gunnar, Sankar Murugesan, Tone Vågseth, Audun Østby Pedersen, Angelico Madaro, Samantha Bui, Harald Kryvi, Lars Helge Stien, and Jonatan Nilsson. "Vertebral Deformities in Cultured Big Size Rainbow Trout: Radiological Analysis from Juvenile to Harvest Size." Aquaculture 596 (October 5, 2024): 741729. https://doi.org/10.1016/j.aquaculture.2024.741729.
  • Landbruks- og matdepartementet. "Melding Til Stortinget: Dyrevelferd. Meld. St. 8 (2024–2025)." Landbruks- og matdepartementet, 2024.
  • Liu, Qun, Zhishuai Hou, Haishen Wen, Jifang Li, Feng He, Jinhuan Wang, Biao Guan, and Qinglong Wang. "Effect of Stocking Density on Water Quality and (Growth, Body Composition and Plasma Cortisol Content) Performance of Pen-Reared Rainbow Trout (Oncorhynchus mykiss)." Journal of Ocean University of China 15, no. 4 (May 23, 2016): 667–75. https://doi.org/10.1007/s11802-016-2956-2.
  • MacIntyre, Craig. "Water Quality and Welfare Assessment on United Kingdom Trout Farms." PhD Thesis, 2008. https://dspace.stir.ac.uk/bitstream/1893/434/1/FINAL%20THESIS.pdf.
  • Moldal, T., J. Wiik-Nielsen, V. H. S. Oliveira, J. C. Svendsen, and I. Sommerset. "Norwegian Fish Health Report 2024." Norwegian Veterinary Institute, 2025. https://www.vetinst.no/rapporter-og-publikasjoner/rapporter/2025/norwegian-fish-health-report-2024/_/attachment/inline/6b11b72c-ee8f-4529-921f-1a3d85dc419e:2d59843d7c1e34e9200669ae47f2974d8ee51b6a/Fish%20Health%20Report%202024.pdf.
  • Moutou, K. A., I. D. McCarthy, and D. F. Houlihan. "The Effect of Ration Level and Social Rank on the Development of Fin Damage in Juvenile Rainbow Trout." Journal of Fish Biology 52, no. 4 (April 1, 1998): 756–70. https://doi.org/10.1111/j.1095-8649.1998.tb00818.x.
  • Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • North, B. P., J. F. Turnbull, T. Ellis, M. J. Porter, H. Migaud, J. Bron, and N. R. Bromage. "The Impact of Stocking Density on the Welfare of Rainbow Trout (Oncorhynchus mykiss)." Aquaculture 255, no. 1-4 (May 31, 2006): 466–79. https://doi.org/10.1016/j.aquaculture.2006.01.004.
  • Park, Kunhong, Jinseo Choi, Younghun Lee, and Jeonghwan Park. "Evaluation of the Optimal Dissolved Oxygen Level for Rainbow Trout (Oncorhynchus mykiss) in the Recirculating Aquaculture System." Susan Haeyang Gisul Yeongu 59, no. 4 (November 30, 2023): 387–98. https://doi.org/10.3796/ksfot.2023.59.4.387.
  • Person‐Le Ruyet, Jeannine, Laurent Labbé, Nicolas Le Bayon, Armelle Sévère, Annicke Le Roux, Hervé Le Delliou, and Loïc Quéméner. "Combined Effects of Water Quality and Stocking Density on Welfare and Growth of Rainbow Trout (Oncorhynchus mykiss)." Aquatic Living Resources 21, no. 2 (2008): 185–95. https://doi.org/10.1051/alr:2008024.
  • Roque d'Orbcastel, Emmanuelle, Jeanine Person-Le Ruyet, Nicolas Le Bayon, and Jean-Paul Blancheton. "Comparative Growth and Welfare in Rainbow Trout Reared in Recirculating and Flow through Rearing Systems." Aquacultural Engineering 40, no. 2 (March 2009): 79–86. https://doi.org/10.1016/j.aquaeng.2008.11.005.
  • Zahedi, Saeed, Arash Akbarzadeh, Jalil Mehrzad, Ahmad Noori, and Mohammad Harsij. "Effect of Stocking Density on Growth Performance, Plasma Biochemistry and Muscle Gene Expression in Rainbow Trout (Oncorhynchus mykiss)." Aquaculture 498 (January 1, 2019): 271–78. https://doi.org/10.1016/j.aquaculture.2018.07.044.

Aberrant behavior

Aberrant behavior includes fish that are unusually lethargic, frantic, deep, side swimming, body rocking, surface gasping, burrowing, or clumping. This includes stereotypic behavior: repetitive behavior that lacks a clear purpose, often observed in captive animals with limited ability to engage in natural behaviors. It is generally accepted that these types of behaviors are a response to a poor environment.1 These behaviors may also indicate the influence of stressors, disease, or deformities.

Producers should understand normal swimming behaviors in their production to better identify abnormal behaviors.

Subindicators Ideal Acceptable Warning Unacceptable
Minor aberrant behavior

No minor aberrant behaviors

Isolated individuals (< 0.1%) showing minor aberrant behaviors

Noticeable prevalence (0.1–2%) of minor aberrant behaviors

High prevalence (> 2%) of minor aberrant behaviors
Major aberrant behavior

No major aberrant behaviors

 –

Isolated individuals (< 0.1%) expressing major aberrant behaviors

Noticeable prevalence (> 0.1%) of major aberrant behaviors
Protocol Application

Use a consistent method of observing and assessing behavior to ensure data is standardized, including location and time of observation. An example protocol could be: observe behavior for 5 minutes before feeding, 5 minutes during feeding, and then 5 minutes at another time in the day. For consistency, conduct observations at the same time each day.

When in doubt, assess conservatively and use a more severe threshold level.

Scoring Notes
  • Fish can behave aberrantly in many ways, and not all aberrant behaviors indicate the same severity of welfare issues. For example:
  • Minor aberrant behaviors can include:
    • Slight lethargy (reduced, but activity is still present).
    • Minor positioning changes (slightly deeper or higher than usual).
    • Reduced schooling participation.
  • Major aberrant behaviors can include:
    • Surface gasping (indicating respiratory distress).
    • Body rocking or stereotypies.
    • Side swimming.
    • Severe lethargy (minimal movement, poor responsiveness).
    • Frantic swimming.
  • What behaviors are considered aberrant also depends on the baseline trends in the production system.
  • Acute vs chronic aberrant behavior has different welfare implications. Acute behavior is linked to a specific stressor (e.g., a handling event) over a short period of time. Chronic behavior persists over weeks, indicating a systemic welfare problem.
  • Where aberrant behaviors can be linked to an acute stressor, note this for future reference.
  • Validation: The prevalence thresholds are derived from Pettersen et al. (2014). The distinction between minor and major aberrant behaviors is based on expert opinion and should be adapted to the baseline behavior patterns of each production system.
Key Evidence
Source Summary of source Key result
Davidson et al., 2011 Two experimental studies investigating the impacts of different freshwater RAS conditions on rainbow trout health and welfare. Experiment 1 compared very low water exchange and high water exchange in rainbow trout (151g) over six months. Experiment 2 compared low water exchange to near-zero water exchange in rainbow trout (18g). In both studies, trout in lower-exchange systems swam significantly faster than those in higher exchange systems. In study 1, high-exchange fish swam at approximately 0.7 BL/s, and low-exchange fish swam at approximately 1.4 BL/s. In study 2, the very low-exchange trout swam at approximately 2.1 BL/s, and the near-zero exchange trout swam at approximately 3.4BL/s. The authors hypothesized this was a physiological flight response caused by the chronically stressful water quality conditions. Approximately 4 times more side swimming was observed in the low-exchange system in study 1. The authors hypothesized this could be due to musculature imbalance, skeletal deformities, or swim bladder deviation. In study 2, the near-zero exchange group displayed more abnormal behaviors including erratic swimming, swimming at oblique angles, surface swimming, and yawning/gulping. These behaviors became more severe over time. In the near-zero system, skeletal deformity prevalence reached up to 38%. The authors hypothesized the abnormal swimming may contribute to more deformities. Mortality was also higher in this group.
Sneddon, 2003 Experimental study investigating behavioral and physiological responses consistent with pain perception in 25 rainbow trout (~61g) in freshwater laboratory conditions. There were 5 treatment groups: control (only handled), saline-injected, acid-injected, acid-and-morphine-injected, and morphine-injected. The injection was in the trout's lips. Acid-injected fish rested on the substrate and rocked from side to side on their pectoral fins. The authors interpreted this as possible stereotypic or comfort-seeking behavior. Fish also rubbed their lips against gravel or tank walls, which was interpreted by the authors as an attempt to relieve pain. These behaviors were nearly absent in controls and were significantly reduced by morphine.
Pettersen et al., 2014 Review paper presenting a welfare index to assess welfare of Atlantic salmon in sea cages. Behavior was included as an indicator. The authors concluded that aberrant behavior was a sign of severe welfare issues. They suggested that 0% was ideal, < 0.1% was acceptable, 0.1–2% was poor and > 2% was critical for welfare. They categorized evidence of aggression as potentially serious and similar in severity to abnormal behavior. They suggested that surface gasping and lethargy could indicate severe gill disease.
Supporting Evidence
  • Latremouille, David N. "Fin Erosion in Aquaculture and Natural Environments." Reviews in Fisheries Science 11, no. 4 (2003): 315–35. https://doi.org/10.1080/10641260390255745.
  • Martins, Catarina I. M., Leonor Galhardo, Chris Noble, Børge Damsgård, Maria T. Spedicato, Walter Zupa, Marilyn Beauchaud, et al. "Behavioural Indicators of Welfare in Farmed Fish." Fish Physiology and Biochemistry 38, no. 1 (July 28, 2011): 17–41. https://doi.org/10.1007/s10695-011-9518-8.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Oppedal, Frode, Tim Dempster, and Lars H. Stien. "Environmental Drivers of Atlantic Salmon Behaviour in Sea-Cages: A Review." Aquaculture 311, no. 1-4 (February 2011): 1–18. https://doi.org/10.1016/j.aquaculture.2010.11.020.
  • Özcan, Filiz. "Prevalence and Causes of Skeletal Deformities in Cultured Juvenile Oncorhynchus mykiss. Skeletal Deformities in Cultured Juvenile Oncorhynchus mykiss." Revista Colombiana de Ciencias Pecuarias 38 (March 2025). https://doi.org/10.17533/udea.rccp.e359311.
  • Papandroulakis, Nikos, Konstandia Lika, Tore S. Kristiansen, Frode Oppedal, Pascal Divanach, and Michael Pavlidis. "Behaviour of European Sea Bass,Dicentrarchus Labrax L., in Cages - Impact of Early Life Rearing Conditions and Management." Aquaculture Research 45, no. 9 (January 2013): 1545–58. https://doi.org/10.1111/are.12103.
  • Pettersen, Jostein M., Marc B.M. Bracke, Paul J. Midtlyng, Ole Folkedal, Lars H. Stien, Håvard Steffenak, and Tore S. Kristiansen. "Salmon Welfare Index Model 2.0: An Extended Model for Overall Welfare Assessment of Caged Atlantic Salmon, Based on a Review of Selected Welfare Indicators and Intended for Fish Health Professionals." Reviews in Aquaculture 6, no. 3 (June 30, 2013): 162–79. https://doi.org/10.1111/raq.12039.
  • Sneddon, Lynne U. "The Evidence for Pain in Fish: The Use of Morphine as an Analgesic." Applied Animal Behaviour Science 83, no. 2 (September 2003): 153–62. https://doi.org/10.1016/s0168-1591(03)00113-8.
  • Wagner, Glenn N., Mark D. Fast, and Stewart C. Johnson. "Physiology and Immunology of Lepeophtheirus Salmonis Infections of Salmonids." Trends in Parasitology 24, no. 4 (April 2008): 176–83. https://doi.org/10.1016/j.pt.2007.12.010.

Aggressive behavior

Aggressive behavior can cause harm to other fish and is therefore a useful welfare indicator for the population. Aggressive behaviors, including nipping and increased chasing, can be evidenced by physical injuries such as bite marks. Aggression can lead to hierarchy formation, where subordinate fish are at higher risk of stress, e.g. due to an inability to compete for feed. Aggressive behavior can also indicate insufficient resources or space. Beyond impacts on social stability, aggressive behavior can result in direct physical harm to attacked fish, including causing fin damage or wounds.

Subindicators Ideal Acceptable Warning Unacceptable
Feeding

No aggressive behavior OR Normal aggressive behavior with no harmful outcomes

Moderate aggressive behavior with minimal harm during feeding

Elevated aggressive behavior with visible harm during feeding

Severe aggressive behavior causing significant harm during feeding
Non-Feeding

No aggressive behavior OR Normal aggressive behavior with no harmful outcomes

 –

Moderate aggressive behavior with minimal harm outside of feeding

Elevated aggressive behavior with visible harm outside of feeding
Protocol Application

Use a consistent method of observing and assessing behavior to ensure data is standardized, including location and time of observation. An example protocol could be: observe behavior for 5 minutes before feeding, 5 minutes during feeding, and then 5 minutes at another time in the day. For consistency, conduct observations at the same time each day.

Count and note aggressive acts during the observation period, and then use a combination of prevalence and severity of aggressive acts to determine the score.

When in doubt, assess conservatively and use a more severe threshold level.

Scoring Notes
  • Normal aggressive behavior can involve brief chasing, establishment of a feeding hierarchy, threat postures, brief confrontations, or positioning disputes during feeding.
  • Moderate aggressive behavior can involve minor fin nipping, extensive chasing, or positioning disputes outside of feeding.
  • Elevated aggressive behavior can involve aggressive acts occurring at higher frequencies and resulting in visible harm (such as bite marks or moderate fin damage clearly attributable to aggressive behavior).
  • Severe aggressive behavior can involve persistent harassment causing severe harm such as open wounds or severe fin damage.
  • Visible harms resulting from aggressive behavior include wounds or tissue punctures from biting, fin damage as a result of nipping, and chasing wounds (e.g., scrapes or scale loss).
  • When aggressive behavior is observed, note the specific behaviors observed to provide more context.
  • Validation: The thresholds are based on expert opinion.
Key Evidence
Source Summary of source Key result
Noble et al., 2007 Experimental study investigating the effects of different self-feeding regimes on growth, behavior, and fin damage in 100 juvenile rainbow trout (90g) in freshwater laboratory conditions over 48 days. Fish were offered either a single 2 hour access period to self-feeders, three 2 hour access periods to self-feeders or unlimited access to self-feeders. The authors found a significant increase in aggressive behavior in the "one access period" group, which was also associated with increased caudal fin damage.
Noble et al., 2020 Literature review on many rainbow trout welfare indicators, including aggressive behavior. The review highlighted aggression during certain life stages or handling routines as a sign of social stress. Heightened current speed can reduce aggression in salmonids, as fish spend more energy on maintaining position.
Supporting Evidence
  • Batzina, Alkisti, Christina Dalla, Zeta Papadopoulou-Daifoti, and Nafsika Karakatsouli. "Effects of Environmental Enrichment on Growth, Aggressive Behaviour and Brain Monoamines of Gilthead Seabream Sparus Aurata Reared under Different Social Conditions." Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 169 (March 2014): 25–32. https://doi.org/10.1016/j.cbpa.2013.12.001.
  • Martins, Catarina I. M., Leonor Galhardo, Chris Noble, Børge Damsgård, Maria T. Spedicato, Walter Zupa, Marilyn Beauchaud, et al. "Behavioural Indicators of Welfare in Farmed Fish." Fish Physiology and Biochemistry 38, no. 1 (July 28, 2011): 17–41. https://doi.org/10.1007/s10695-011-9518-8.
  • Montero, D., G. Lalumera, M. S. Izquierdo, M. J. Caballero, M. Saroglia, and L. Tort. "Establishment of Dominance Relationships in Gilthead Sea BreamSparus Aurata Juveniles during Feeding: Effects on Feeding Behaviour, Feed Utilization and Fish Health." Journal of Fish Biology 74, no. 4 (March 9, 2009): 790–805. https://doi.org/10.1111/j.1095-8649.2008.02161.x.
  • Noble, C., K. Mizusawa, K. Suzuki, and M. Tabata. "The Effect of Differing Self-Feeding Regimes on the Growth, Behaviour and Fin Damage of Rainbow Trout Held in Groups." Aquaculture 264, no. 1-4 (April 6, 2007): 214–22. https://doi.org/10.1016/j.aquaculture.2006.12.028.
  • Oppedal, Frode, Tim Dempster, and Lars H. Stien. "Environmental Drivers of Atlantic Salmon Behaviour in Sea-Cages: A Review." Aquaculture 311, no. 1-4 (February 2011): 1–18. https://doi.org/10.1016/j.aquaculture.2010.11.020.
  • Peters, G., Mohamed Faisal, Tom Lang, and Iman Ahmed. "Stress Caused by Social Interaction and Its Effect on Susceptibility to Aeromonas Hydrophila Infection in Rainbow Trout Salmo Gairdneri." Diseases of Aquatic Organisms 4 (May 31, 1988): 83–89. https://doi.org/10.3354/dao004083.
  • Pettersen, Jostein M., Marc B.M. Bracke, Paul J. Midtlyng, Ole Folkedal, Lars H. Stien, Håvard Steffenak, and Tore S. Kristiansen. "Salmon Welfare Index Model 2.0: An Extended Model for Overall Welfare Assessment of Caged Atlantic Salmon, Based on a Review of Selected Welfare Indicators and Intended for Fish Health Professionals." Reviews in Aquaculture 6, no. 3 (June 30, 2013): 162–79. https://doi.org/10.1111/raq.12039.
  • Wagner, Glenn N., Mark D. Fast, and Stewart C. Johnson. "Physiology and Immunology of Lepeophtheirus Salmonis Infections of Salmonids." Trends in Parasitology 24, no. 4 (April 2008): 176–83. https://doi.org/10.1016/j.pt.2007.12.010.

Feeding behavior

Stressors can significantly reduce the motivation to feed and feed intake in fish, which can lead to reduced growth.1 Reduced feeding can also indicate disease or poor water quality.2 Fish often have a reduced appetite after experiencing handling procedures such as crowding, pumping, or transport. The added danger of uneaten feed is that it can contribute to deteriorating water quality. The amount of time taken to recover to normal feeding behaviors after such an event is also a useful indicator of welfare.3

Subindicators Ideal Acceptable Warning Unacceptable
Behavior

Strong anticipatory behavior before feeding

Anticipatory behavior before feeding

Reduced anticipatory behavior before feeding OR Slow, reluctant, or declining feeding

Poor anticipatory behavior before feeding OR Minimal activity during feeding
Waste – – – –
Protocol Application

Use a consistent method of observing and assessing behavior to ensure data is standardized, including location and time of observation. An example protocol could be: observe behavior for 5 minutes before feeding, 5 minutes during feeding, and then 5 minutes at another time in the day. For consistency, conduct observations at the same time each day.

For consistency in measuring feed percentage, measure at the same time each day (e.g. 30 minutes after feeding). Calculate the waste percentage by dividing uneaten feed by total feed offered.

Two scoring systems are provided. It is recommended to score using both, and then take the worst score. When in doubt, assess conservatively and use a more severe threshold level.

Scoring Notes
  • Anticipatory behavior before feeding includes behaviors such as:
  • Increased swimming activity in the minutes before feeding.
  • Movement towards the feeding area when staff approach.
  • Schooling behavior near the expected feeding location.
  • Contrasting activities with baseline activity during non-feeding periods.
  • Feed waste percentage is a more objective and evidence-based measure than anticipatory behavior.
  • Feeding behavior may be impacted by one-off events such as handling, which should also be noted by the assessor.
  • Validation: The thresholds are based on expert opinion.
Key Evidence
Source Summary of source Key result
Øverli et al., 2006 Experimental study investigating how two selectively bred lines of 300 rainbow trout (bred to be either highly responsive or lowly responsive to stress) (456–493g) relate to feed waste in laboratory conditions over 12 days. Fish were hand-fed once daily over 20 minutes with graduated feeding regimes: 0.5% body mass/day (days 1–3), 1.0% body mass/day (days 4–6), and 2.0% body mass/day (days 7–12). Uneaten pellets were collected from PVC grid pellet traps 15 minutes after feeding ended. Highly-responsive to stress fish wasted approximately 11% of feed while the lowly-responsive to stress fish wasted approximately 4% of feed. The difference in feed waste increased progressively as rations increased. The highly-responsive fish also demonstrated slower growth rates and poorer feed conversion efficiency. There was also a positive correlation between initial size variation within tanks and total feed waste.
McKay and Gjerde 1985 Experimental study investigating the effects of different salinities on growth in 90 rainbow trout (50–150g) over 12 weeks. Fish had been reared in freshwater and then transferred to salinities of 0, 10, 20, 24, 28, and 32 ppt. Uneaten food was associated with poor welfare: mortality rates increased with more uneaten food, particularly when 35% or more of feed was not eaten. Appetite significantly decreased with increased salinity, with the best appetite at 0 ppt.
Noble et al., 2020 Literature review on many rainbow trout welfare indicators, including feeding behavior. A poor or absent feed response can indicate disease or stress. While increased swimming speed before or at the start of feeding can indicate positive anticipation, persistence during meals or over several days may reflect competition or resource scarcity and signal reduced welfare.
Supporting Evidence
  • Andrew, J. E., J. Holm, S. Kadri, and F. A. Huntingford. "The Effect of Competition on the Feeding Efficiency and Feed Handling Behaviour in Gilthead Sea Bream (Sparus Aurata L.) Held in Tanks." Aquaculture 232, no. 1-4 (April 5, 2004): 317–31. https://doi.org/10.1016/s0044-8486(03)00528-3.
  • Arechavala-Lopez, P., J. C. Caballero-Froilán, M. Jiménez-García, X. Capó, S. Tejada, J. L. Saraiva, A. Sureda, and D. Moranta. "Enriched Environments Enhance Cognition, Exploratory Behaviour and Brain Physiological Functions of Sparus Aurata." Scientific Reports 10, no. 1 (July 9, 2020). https://doi.org/10.1038/s41598-020-68306-6.
  • Chandroo, Kris P., Steven J. Cooke, R. Scott McKinley, and Richard D. Moccia. "Use of Electromyogram Telemetry to Assess the Behavioural and Energetic Responses of Rainbow Trout, Oncorhynchus mykiss (Walbaum) to Transportation Stress." Aquaculture Research 36, no. 12 (2005): 1226–38. https://doi.org/10.1111/j.1365-2109.2005.01347.x.
  • Gregory, T. Ryan, and Chris M. Wood. "The Effects of Chronic Plasma Cortisol Elevation on the Feeding Behaviour, Growth, Competitive Ability, and Swimming Performance of Juvenile Rainbow Trout." Physiological and Biochemical Zoology 72, no. 3 (May 1999): 286–95. https://doi.org/10.1086/316673.
  • Hofer, R., and E. Gatumu. "Necrosis of Trout Retina (Oncorhynchus mykiss) after Sublethal Exposure to Nitrite." Archives of Environmental Contamination and Toxicology 26, no. 1 (January 1994): 119–23. https://doi.org/10.1007/bf00212803.
  • Latremouille, David N. "Fin Erosion in Aquaculture and Natural Environments." Reviews in Fisheries Science 11, no. 4 (October 1, 2003): 315–35. https://doi.org/10.1080/10641260390255745.
  • Martins, Catarina I. M., Leonor Galhardo, Chris Noble, Børge Damsgård, Maria T. Spedicato, Walter Zupa, Marilyn Beauchaud, et al. "Behavioural Indicators of Welfare in Farmed Fish." Fish Physiology and Biochemistry 38, no. 1 (July 28, 2011): 17–41. https://doi.org/10.1007/s10695-011-9518-8.
  • McKay, L. R., and B. Gjerde. "The Effect of Salinity on Growth of Rainbow Trout." Aquaculture 49, no. 3-4 (November 1, 1985): 325–31. https://doi.org/10.1016/0044-8486(85)90089-4.
  • Munday, B. L., C. K. Foster, F. R. Roubal, and R. J. G. Lester. "Paramoebic Gill Infection and Associated Pathology of Atlantic Salmon, Salmo salar and Rainbow Trout, Salmo Gairdneri in Tasmania." In Pathology in Marine Science. Proceedings of the Third International Colloquium on Pathology in Marine Aquaculture, 1990.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Noble, C., K. Mizusawa, K. Suzuki, and M. Tabata. "The Effect of Differing Self-Feeding Regimes on the Growth, Behaviour and Fin Damage of Rainbow Trout Held in Groups." Aquaculture 264, no. 1-4 (April 6, 2007): 214–22. https://doi.org/10.1016/j.aquaculture.2006.12.028.
  • Oppedal, Frode, Tim Dempster, and Lars H. Stien. "Environmental Drivers of Atlantic Salmon Behaviour in Sea-Cages: A Review." Aquaculture 311, no. 1-4 (February 2011): 1–18. https://doi.org/10.1016/j.aquaculture.2010.11.020.
  • Özcan, Filiz. "Prevalence and Causes of Skeletal Deformities in Cultured Juvenile Oncorhynchus mykiss. Skeletal Deformities in Cultured Juvenile Oncorhynchus mykiss." Revista Colombiana de Ciencias Pecuarias 38 (2025). https://doi.org/10.17533/udea.rccp.e359311.
  • Papandroulakis, Nikos, Konstandia Lika, Tore S. Kristiansen, Frode Oppedal, Pascal Divanach, and Michael Pavlidis. "Behaviour of European Sea Bass,Dicentrarchus Labrax L., in Cages - Impact of Early Life Rearing Conditions and Management." Aquaculture Research 45, no. 9 (January 2013): 1545–58. https://doi.org/10.1111/are.12103.
  • Peters, G., Mohamed Faisal, Tom Lang, and Iman Ahmed. "Stress Caused by Social Interaction and Its Effect on Susceptibility to Aeromonas Hydrophila Infection in Rainbow Trout Salmo Gairdneri." Diseases of Aquatic Organisms 4 (May 31, 1988): 83–89. https://doi.org/10.3354/dao004083.
  • Pettersen, Jostein M., Marc B.M. Bracke, Paul J. Midtlyng, Ole Folkedal, Lars H. Stien, Håvard Steffenak, and Tore S. Kristiansen. "Salmon Welfare Index Model 2.0: An Extended Model for Overall Welfare Assessment of Caged Atlantic Salmon, Based on a Review of Selected Welfare Indicators and Intended for Fish Health Professionals." Reviews in Aquaculture 6, no. 3 (June 30, 2013): 162–79. https://doi.org/10.1111/raq.12039.
  • Powell, M. D., M. A. Jones, and M. Lijalad. "Effects of Skeletal Deformities on Swimming Performance and Recovery from Exhaustive Exercise in Triploid Atlantic Salmon." Diseases of Aquatic Organisms 85 (2009): 59–66. https://doi.org/10.3354/dao02056.
  • Roque, Ana, M. Filipa Castanheira, Anna Toffan, Pablo Arechavala-Lopez, Edgar Brun, Morris Vilarroel, Enric Gisbert, et al. "Report on Fish Welfare and List of Operational Welfare Indicators in Sea Bream. Deliverable 4.5 of the Horizon 2020 Project MedAID (GA Number 727315)," 2020. http://www.medaid-h2020.eu/index.php/deliverables/.
  • Wagner, Glenn N., Mark D. Fast, and Stewart C. Johnson. "Physiology and Immunology of Lepeophtheirus Salmonis Infections of Salmonids." Trends in Parasitology 24, no. 4 (April 2008): 176–83. https://doi.org/10.1016/j.pt.2007.12.010.

Swimming behavior

Swimming behavior of fish reflects their physiological condition and interaction with their environment. Disease, injury, or deformity can severely affect swimming behavior due to physiological changes the fish must cope with.

Ideal Acceptable Warning Unacceptable

Normal active swimming with appropriate cage use

Mostly normal swimming with minor variations

Clear swimming abnormalities

Severe swimming dysfunction
Protocol Application

Use a consistent method of observing and assessing behavior to ensure data is standardized, including location and time of observation. An example protocol could be: observe behavior for 5 minutes before feeding, 5 minutes during feeding, and then 5 minutes at another time in the day. For consistency, conduct observations at the same time each day.

When variations to normal swimming are observed, these variations should be documented in notes.

When in doubt, assess conservatively and use a more severe threshold level.

Scoring Notes
  • When assessing swimming behavior, focus on organization and schooling rather than specific geometric patterns.
  • Activity level changes are more welfare-relevant than specific swimming patterns.
  • Normal variation is expected with feeding and changing environmental conditions.
  • Abnormal hyperactivity may indicate a chronic stressor, poor water quality, or disease.
  • Lethargy may also indicate disease, severe stress, or poor nutrition.
  • Ideal: Normal active swimming includes organized schooling and responsiveness to stimuli.
  • Acceptable: Minor variations from normal swimming includes unusual avoidance of enclosure areas, some disruption to schooling, and occasional increased activity.
  • Warning: Clear swimming abnormalities include fish clustering at the bottom or in the corners of the enclosure, disorganized movement, and episodes of hyperactivity or lethargy.
  • Unacceptable: Severe swimming dysfunction includes fish avoiding open or feeding areas, constant bottom dwelling, no schooling structure, extreme lethargy, and frantic swimming
  • Good use of the space in the enclosure indicates comfort and environmental adequacy.
  • Use of the full water column indicates comfort throughout the cage.
  • Bottom-dwelling may indicate low oxygen, poor water quality, or disease.
  • Corner-clustering may indicate water flow problems, aggression, or a fear response.
  • Validation: The thresholds are based on expert opinion.
Key Evidence
Source Summary of source Key result
Noble et al., 2020 Literature review on many rainbow trout welfare indicators, including swimming behavior. Unstructured swimming at the bottom of the tank indicates acute stress, deviating from the normal circular schooling pattern typically seen in healthy fish.
Oppedal et al., 2011 Review article on the behaviors of Atlantic salmon. Salmonids typically form a circular swimming pattern at daytime and avoid both the innermost part of the cage and corners and bottom (though this structure breaks down during feeding). Fish that remain at the bottom of the tank may be experiencing environmental stress, such as low dissolved oxygen. It is normal for swimming speed to be reduced at night. A lack of reduced swimming speed at night may indicate disrupted circadian rhythms due to artificial lighting.
Supporting Evidence
  • Arechavala-Lopez, P., J. C. Caballero-Froilán, M. Jiménez-García, X. Capó, S. Tejada, J. L. Saraiva, A. Sureda, and D. Moranta. "Enriched Environments Enhance Cognition, Exploratory Behaviour and Brain Physiological Functions of Sparus Aurata." Scientific Reports 10, no. 1 (July 9, 2020). https://doi.org/10.1038/s41598-020-68306-6.
  • Arechavala-López., Pablo. "A Guide on Fish Welfare in Spanish Aquaculture - Volume 3: Welfare of Gilthead Sea Bream." Spanish Aquaculture Business Association, 2024. https://drive.google.com/file/d/1Xp7Z-H53U_ZRmLf-cTFjCvs3dnHA-vOg/view.
  • García Palencia, H. "Monitorización Automatizada Del Comportamiento de Lubinas En Situaciones de Estrés Mediante Técnicas de Deep Learning." Grado en Ingeniería Electrónica de Comunicaciones, Universidad Politecnica Madrid, 2025. https://oa.upm.es/89484/1/TFG_GARCIA_PALENCIA_HECTOR.pdf.
  • Herbert, N. A., and J. F. Steffensen. "The Response of Atlantic Cod, Gadus Morhua, to Progressive Hypoxia: Fish Swimming Speed and Physiological Stress." Marine Biology 147, no. 6 (April 2005): 1403–12. https://doi.org/10.1007/s00227-005-0003-8.
  • Hofer, R., and E. Gatumu. "Necrosis of Trout Retina (Oncorhynchus mykiss) after Sublethal Exposure to Nitrite." Archives of Environmental Contamination and Toxicology 26, no. 1 (January 1994): 119–23. https://doi.org/10.1007/bf00212803.
  • Latremouille, David N. "Fin Erosion in Aquaculture and Natural Environments." Reviews in Fisheries Science 11, no. 4 (October 1, 2003): 315–35. https://doi.org/10.1080/10641260390255745.
  • Martins, Catarina I. M., Leonor Galhardo, Chris Noble, Børge Damsgård, Maria T. Spedicato, Walter Zupa, Marilyn Beauchaud, et al. "Behavioural Indicators of Welfare in Farmed Fish." Fish Physiology and Biochemistry 38, no. 1 (July 28, 2011): 17–41. https://doi.org/10.1007/s10695-011-9518-8.
  • Munday, B. L., C. K. Foster, F. R. Roubal, and R. J. G. Lester. "Paramoebic Gill Infection and Associated Pathology of Atlantic Salmon, Salmo salar and Rainbow Trout, Salmo Gairdneri in Tasmania." In Pathology in Marine Science. Proceedings of the Third International Colloquium on Pathology in Marine Aquaculture, 1990.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Oppedal, Frode, Tim Dempster, and Lars H. Stien. "Environmental Drivers of Atlantic Salmon Behaviour in Sea-Cages: A Review." Aquaculture 311, no. 1-4 (February 2011): 1–18. https://doi.org/10.1016/j.aquaculture.2010.11.020.
  • Özcan, Filiz. "Prevalence and Causes of Skeletal Deformities in Cultured Juvenile Oncorhynchus mykiss. Skeletal Deformities in Cultured Juvenile Oncorhynchus mykiss." Revista Colombiana de Ciencias Pecuarias 38 (2025). https://doi.org/10.17533/udea.rccp.e359311.
  • Papandroulakis, Nikos, Konstandia Lika, Tore S. Kristiansen, Frode Oppedal, Pascal Divanach, and Michael Pavlidis. "Behaviour of European Sea Bass, Dicentrarchus Labrax L., in Cages - Impact of Early Life Rearing Conditions and Management." Aquaculture Research 45, no. 9 (January 2013): 1545–58. https://doi.org/10.1111/are.12103.
  • Pettersen, Jostein M., Marc B.M. Bracke, Paul J. Midtlyng, Ole Folkedal, Lars H. Stien, Håvard Steffenak, and Tore S. Kristiansen. "Salmon Welfare Index Model 2.0: An Extended Model for Overall Welfare Assessment of Caged Atlantic Salmon, Based on a Review of Selected Welfare Indicators and Intended for Fish Health Professionals." Reviews in Aquaculture 6, no. 3 (June 30, 2013): 162–79. https://doi.org/10.1111/raq.12039.
  • Powell, M. D., M. A. Jones, and M. Lijalad. "Effects of Skeletal Deformities on Swimming Performance and Recovery from Exhaustive Exercise in Triploid Atlantic Salmon." Diseases of Aquatic Organisms 85 (2009): 59–66. https://doi.org/10.3354/dao02056.
  • Remen, M., M. A. J. Nederlof, O. Folkedal, G. Thorsheim, A. Sitjà-Bobadilla, J. Pérez-Sánchez, F. Oppedal, and R. E. Olsen. "Effect of Temperature on the Metabolism, Behaviour and Oxygen Requirements of Sparus Aurata." Aquaculture Environment Interactions 7, no. 2 (2015): 115–23. https://doi.org/10.3354/aei00141.
  • Wagner, Glenn N., Mark D. Fast, and Stewart C. Johnson. "Physiology and Immunology of Lepeophtheirus Salmonis Infections of Salmonids." Trends in Parasitology 24, no. 4 (April 2008): 176–83. https://doi.org/10.1016/j.pt.2007.12.010.

Environmental indicators

Stocking density

Stocking density affects fish welfare through competition for space and resources, water quality impacts, and social stress. Both very high and very low densities can be problematic. High densities increase competition and stress, while very low densities can disrupt normal schooling behavior and increase aggression. Optimal stocking density depends on the production system, life stage, and environmental conditions.

Ideal Acceptable Warning Unacceptable
5–15 kg/m³
15–20 kg/m³
OR
2–5 kg/m³
20–25 kg/m³
OR
< 2 kg/m³
> 25 kg/m³
Protocol Application

Calculate stocking density as total fish biomass (kg) divided by enclosure volume (m³).
Monitor stocking density regularly as fish grow.

Scoring Notes
  • At very low densities for commercial settings (e.g. < 5 kg/m³) and at 'warning' and 'unacceptable' levels, it is recommended to increase monitoring frequencies for other welfare indicators such as aggressive behavior, water quality impacts, and mortality.
  • When other important welfare indicators (such as dissolved oxygen, mortality, aggressive behavior) score poorly, consider adjusting stocking density.
  • Validation: The prevalence thresholds are derived from considering the results in the studies listed below.
Key Evidence
Source Summary of source Key result
Ellis et al., 2002 Study on relationships between stocking density and welfare in farmed rainbow trout. Found complex relationships between density and welfare indicators. Optimal density depends on multiple factors including water quality management.
Saraiva et al., 2022 Review on finding the optimal stocking density for fish welfare. Discusses the balance between fish welfare and production efficiency at different stocking densities.
Supporting Evidence
  • Ellis, T., et al. "The Relationships between Stocking Density and Welfare in Farmed Rainbow Trout." Journal of Fish Biology 61, no. 3 (September 2002): 493–531. https://doi.org/10.1111/j.1095-8649.2002.tb00893.x.
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Saraiva, J. L., et al. "Finding the 'Golden Stocking Density': A Balance between Fish Welfare and Farmers' Perspectives." Frontiers in Veterinary Science 9 (July 25, 2022). https://doi.org/10.3389/fvets.2022.930221.

Dissolved oxygen

Dissolved oxygen is critical for fish respiration and is one of the most important environmental parameters for welfare. Low oxygen (hypoxia) causes respiratory stress, reduced feeding, and in severe cases, mortality. Rainbow trout have relatively high oxygen requirements compared to some other species. Oxygen levels can fluctuate significantly with temperature, time of day, and feeding activity.

Ideal Acceptable Warning Unacceptable
≥ 80 %
60–80 %
40–60 %
< 40 %
Protocol Application

Monitor dissolved oxygen continuously or at regular intervals throughout the day.
Measure at multiple depths if depth stratification may occur.
Record temperature alongside DO as they are linked.

Scoring Notes
  • If the oxygen monitoring device measures the absolute level of oxygen and does not provide a saturation reading, temperature compensation must be applied. Online calculators can be used.
  • Due to diurnal variation, expect daily fluctuations of 20–30%, with morning readings typically being the lowest.
  • Depth stratification can occur, with oxygen depletion possible in deeper layers.
  • Feeding may have an impact on available oxygen, with temporary oxygen depletion occurring post-feeding in intensive systems.
  • At 'warning' and 'unacceptable' oxygen levels, monitor fish behavioral changes, such as an increase in surface swimming.
  • Acute oxygen depletion or oversaturation (e.g. for less than 4 hours) is less concerning than sustained exposure.
  • Oxygen interacts with temperature. Higher temperatures reduce oxygen capacity in the water while increasing fish oxygen demand.
  • Validation: The above oxygen levels have only been validated for temperatures between 13–21°C based on available rainbow trout studies. The impacts of different oxygen levels outside this range is not clear. If using these thresholds for temperatures outside of this range, proceed with caution.
Key Evidence
Source Summary of source Key result
Noble et al., 2020 Literature review on rainbow trout welfare indicators. Dissolved oxygen is critical for rainbow trout welfare. Levels below 60% saturation cause stress, and below 40% cause severe distress.
Supporting Evidence
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

Carbon dioxide

Carbon dioxide is a metabolic waste product that accumulates in aquaculture systems. High CO₂ levels cause respiratory acidosis, affect blood oxygen transport, and can impair growth and welfare. CO₂ levels are particularly important in intensive systems and recirculating aquaculture systems (RAS) where accumulation can occur.

Ideal Acceptable Warning Unacceptable
< 10 mg/L
10–15 mg/L
15–25 mg/L
≥ 25 mg/L
Protocol Application

Monitor CO₂ levels, particularly in intensive systems.
Account for carbonate buffering in seawater measurements.
CO₂ and pH are closely linked and both should be measured.

Scoring Notes
  • Account for carbonate buffering in seawater measurements.
  • Reduced appetite is an early indicator of CO₂ stress.
  • Monitor for reduced growth rates and blood pH changes in moderate CO₂ exposure, indicating physiological impacts.
  • Diurnal variation results in CO₂ typically highest in the early morning.
  • CO₂ and pH are closely linked and both should be measured.
  • Post-feeding spikes of CO₂ may result in temporary increases, but should resolve within 2–4 hours.
  • Acute CO₂ increases (e.g. for less than 4 hours) is less concerning than sustained exposure.
  • Monitor for behavioral changes including reduced feeding and surface gasping during elevated CO₂ periods.
  • Validation: Limited rainbow trout-specific studies exist, with evidence from Atlantic salmon also relied upon. Threshold validation is also limited.
Key Evidence
Source Summary of source Key result
Noble et al., 2020 Literature review on rainbow trout welfare indicators. CO₂ accumulation affects respiratory function and can impair welfare, particularly in intensive systems.
Supporting Evidence
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

Temperature

Temperature is a fundamental environmental parameter that affects virtually all aspects of fish physiology, including metabolism, growth, oxygen demand, and immune function. Rainbow trout are cold-water fish with an optimal temperature range typically between 10-18°C. Temperatures outside this range cause physiological stress and can be lethal at extremes.

Ideal Acceptable Warning Unacceptable
10–18 °C
6–10 °C
OR
18–20 °C
4–6 °C
OR
20–22 °C
< 4 °C
OR
> 22 °C
Protocol Application

Monitor temperature continuously or at regular intervals.
Consider depth stratification and measure at multiple depths if needed.
Record alongside dissolved oxygen as they are closely linked.

Scoring Notes
  • Larger fish are generally more tolerant to a wider variety of temperatures than smaller fish.
  • Natural thermal tolerance varies seasonally. Anticipate temperature challenges during extreme weather.
  • Gradual temperature changes are better tolerated than rapid fluctuations.
  • Poor water movement can create temperature gradients.
  • Low temperatures can cause reduced feeding, slower growth, and increased susceptibility to disease, while high temperatures can cause increased respiratory rates, reduced oxygen carrying capacity, stress, and mortality risk.
  • Validation: The thresholds are largely based on evidence from rainbow trout in freshwater systems and have not been validated for rainbow trout in seawater conditions.
Key Evidence
Source Summary of source Key result
Noble et al., 2020 Literature review on rainbow trout welfare indicators. Rainbow trout have optimal growth and welfare at 10-18°C. Temperatures above 20°C cause significant stress.
Supporting Evidence
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

Turbidity

Turbidity refers to the cloudiness of water caused by suspended particles. High turbidity can irritate gills, reduce visibility for feeding, and may indicate poor water quality or disease conditions. The source of turbidity matters - suspended sediments have different effects than organic particles or algae.

Ideal Acceptable Warning Unacceptable
< 5 NTU
5–10 NTU
10–20 NTU
≥ 20 NTU
Protocol Application

Monitor turbidity levels using a nephelometer.
Calibrate using a consistent methodology with the same measurement angle and depth.

Scoring Notes
  • It is difficult to give recommendations that accurately reflect welfare in all cases, as turbidity can increase from the suspension of different kinds of materials, all of which may have varied impacts on welfare. The above thresholds should be applied with caution.
  • Nephelometers should be calibrated using a consistent methodology, with the same measurement angle and depth.
  • Acute exposure to elevated turbidity levels (e.g. < 24 hours) has less severe welfare impacts than chronic exposure (e.g. > 7 days).
  • Where there is chronic exposure, consider temporary fish relocation if possible.
  • Exposure to > 20 NTU can have detrimental effects on welfare and health.
  • Validation: Threshold validation is very limited for rainbow trout.
Key Evidence
Source Summary of source Key result
Sorenson et al., 1977 Review of suspended and dissolved solids effects on freshwater biota. Established relationships between turbidity levels and impacts on fish welfare and health.
Supporting Evidence
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Sorenson, D.L., et al. 1977. Suspended and dissolved solids effects on freshwater biota: a review. United States Environmental Protection Agency, Report 600/3-77-042.

pH

pH affects fish physiology through impacts on gill function, blood chemistry, and enzyme activity. Rainbow trout tolerate a pH range of approximately 6.5-8.5, with an optimal range of 7.0-8.0. Extreme pH values cause direct physiological stress, while suboptimal values can impair gill function and osmoregulation.

Ideal Acceptable Warning Unacceptable
7–8
6.5–7
OR
8–8.3
6–6.5
OR
8.3–8.5
< 6
OR
> 8.5
Protocol Application

Monitor pH regularly, particularly in systems where it may fluctuate.
pH and CO₂ are closely linked and both should be measured.

Scoring Notes
  • Acute (e.g. < 24 hours) and infrequent (e.g. one time per month) exposure to suboptimal pH levels will have less severe welfare impacts than chronic (e.g. > 7 days) or frequent (e.g. > 30 days per year) exposure.
  • If 'unacceptable' pH occurs frequently at the site, the site is not suitable as welfare cannot be maintained at the location.
  • Validation: The upper 'unacceptable' threshold (> 8.5) is set conservatively due to evidence gaps between pH 8 and pH 9.5. The lower 'acceptable' boundary is set at 6.5 based on limited evidence.
Key Evidence
Source Summary of source Key result
Noble et al., 2020 Literature review on rainbow trout welfare indicators. pH affects osmoregulation and gill function. Extreme values cause physiological stress.
Supporting Evidence
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

Total Suspended Solids

Total Suspended Solids (TSS) measures the concentration of particles in the water. High TSS can irritate gills, reduce water clarity, and indicate poor waste management. TSS is particularly important in intensive systems and RAS where solid waste accumulates. The composition of suspended solids (organic vs inorganic) affects their impact on fish welfare.

Ideal Acceptable Warning Unacceptable
< 15 mg/L
15–25 mg/L
25–40 mg/L
≥ 40 mg/L
Protocol Application

Monitor TSS levels, particularly in systems where accumulation may occur.
Consider the source of suspended solids when interpreting results.

Scoring Notes
  • TSS levels interact with other water quality parameters - high TSS often accompanies poor oxygen and elevated ammonia.
  • Chronic exposure to elevated TSS is more harmful than acute exposure.
  • The source of TSS matters: uneaten feed and feces have different impacts than inorganic particles.
  • Validation: Threshold validation is very limited for rainbow trout.
Key Evidence
Source Summary of source Key result
Sorenson et al., 1977 Review of suspended and dissolved solids effects on freshwater biota. Established relationships between suspended solids and impacts on fish.
Supporting Evidence
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Sorenson, D.L., et al. 1977. Suspended and dissolved solids effects on freshwater biota: a review. United States Environmental Protection Agency, Report 600/3-77-042.

Ammonia (NH₃)

Ammonia is a metabolic waste product excreted by fish. In water, ammonia exists in two forms: ionized (NH₄⁺) and unionized (NH₃). Unionized ammonia is highly toxic to fish, causing gill damage, neurological effects, and mortality at elevated concentrations. The proportion of unionized ammonia increases with pH and temperature, making these parameters important to consider together.

Ideal Acceptable Warning Unacceptable
< 0.02 mg/L
0.02–0.05 mg/L
0.05–0.1 mg/L
≥ 0.1 mg/L
Protocol Application

Monitor ammonia levels regularly, particularly in intensive systems.
Note that total ammonia nitrogen (TAN) readings need pH and temperature correction to determine unionized ammonia (NH₃).
The above thresholds are for unionized ammonia (NH₃), which is the toxic form.

Scoring Notes
  • The toxic form is unionized ammonia (NH₃), not total ammonia nitrogen (TAN) or ammonium (NH₄⁺).
  • At higher pH and temperature, a greater proportion of ammonia is in the toxic unionized form.
  • Chronic exposure to sublethal levels causes gill damage and stress before acute effects become apparent.
  • Ammonia levels typically spike after feeding and can show diurnal variation.
  • Validation: Limited specific validation for rainbow trout in seawater conditions.
Key Evidence
Source Summary of source Key result
Noble et al., 2020 Literature review on rainbow trout welfare indicators. Ammonia toxicity is a major concern in intensive aquaculture. Chronic exposure causes gill damage and stress.
Supporting Evidence
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

Nitrite (NO₂⁻)

Nitrite is an intermediate product in the nitrification process, where ammonia is converted to nitrate by bacteria. Nitrite is toxic to fish because it binds to hemoglobin and reduces oxygen-carrying capacity (methemoglobinemia or "brown blood disease"). Nitrite toxicity is significantly lower in seawater than freshwater because chloride ions compete with nitrite uptake at the gills.

Ideal Acceptable Warning Unacceptable
< 0.1 mg/L
0.1–0.5 mg/L
0.5–1 mg/L
≥ 1 mg/L
Protocol Application

Monitor nitrite levels, particularly in systems with biofilters or during new system startup.
Nitrite toxicity is reduced in seawater compared to freshwater due to chloride competition.

Scoring Notes
  • Nitrite toxicity is significantly reduced in seawater compared to freshwater due to chloride competition.
  • Spikes in nitrite often indicate biofilter problems or system imbalance.
  • Rainbow trout are relatively sensitive to nitrite compared to some other species.
  • Validation: Very limited specific validation for rainbow trout. Thresholds are conservative.
Key Evidence
Source Summary of source Key result
Lewis and Morris, 1986 Review of nitrite toxicity to fish. Established that nitrite causes methemoglobinemia. Toxicity varies by species and environmental conditions.
Supporting Evidence
  • Lewis, William M., and Donald P. Morris. "Toxicity of Nitrite to Fish: A Review." Transactions of the American Fisheries Society 115, no. 2 (March 1986): 183–95.
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

Nitrate (NO₃⁻)

Nitrate is the end product of nitrification and is much less toxic than ammonia or nitrite. However, nitrate can accumulate in closed or recirculating systems to levels that affect fish welfare. High nitrate levels can impair osmoregulation, growth, and immune function over time. Regular water exchange or denitrification is needed to prevent accumulation.

Ideal Acceptable Warning Unacceptable
< 50 mg/L
50–100 mg/L
100–200 mg/L
≥ 200 mg/L
Protocol Application

Monitor nitrate levels, particularly in RAS or other systems where accumulation may occur.
Nitrate is less toxic than ammonia or nitrite but can accumulate to problematic levels.

Scoring Notes
  • Nitrate is less acutely toxic than ammonia or nitrite but can cause chronic stress at elevated levels.
  • Accumulation is primarily a concern in RAS or other low-exchange systems.
  • Very high nitrate levels (> 200 mg/L) can affect immune function and growth.
  • Validation: Very limited specific validation for rainbow trout.
Key Evidence
Source Summary of source Key result
Noble et al., 2020 Literature review on rainbow trout welfare indicators. Nitrate is relatively non-toxic but can accumulate to problematic levels in closed systems.
Supporting Evidence
  • Noble, C., et al. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

References

  1. FAO. "Global Aquaculture Production Quantity (1950 - 2023)." FAO, n.d. https://www.fao.org/fishery/statistics-query/en/aquaculture/aquaculture_quantity.
  2. Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  3. Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  4. APROMAR. "A Guide on Fish Welfare in Spanish Aquaculture – Volume 4: Welfare of Rainbow Trout." Spanish Aquaculture Business Association, 2025. https://drive.google.com/file/d/1axrO7Z6CNXBZdFsOE0vC2ZqiYKIhuugZ/view.
  5. Fraser, D., D. M. Weary, E. A. Pajor, and B. N. Milligan. "A Scientific Conception of Animal Welfare That Reflects Ethical Concerns." Animal Welfare 6, no. 3 (August 1997): 187–205. https://doi.org/10.1017/s0962728600019795.
  6. Fraser, D., D. M. Weary, E. A. Pajor, and B. N. Milligan. "A Scientific Conception of Animal Welfare That Reflects Ethical Concerns." Animal Welfare 6, no. 3 (August 1997): 187–205. https://doi.org/10.1017/s0962728600019795.
  7. Fraser, D., D. M. Weary, E. A. Pajor, and B. N. Milligan. "A Scientific Conception of Animal Welfare That Reflects Ethical Concerns." Animal Welfare 6, no. 3 (August 1997): 187–205. https://doi.org/10.1017/s0962728600019795.
  8. Broom, D. M. "Animal Welfare: Concepts and Measurement." Journal of Animal Science 69, no. 10 (October 1, 1991): 4167–75. https://doi.org/10.2527/1991.69104167x.
  9. Reimert, I., L. E. Webb, M. A. van Marwijk, and J. E. Bolhuis. "Review: Towards an Integrated Concept of Animal Welfare." Animal 17, no. 4 (August 2023). https://doi.org/10.1016/j.animal.2023.100838.
  10. Mellor, David J., Ngaio J. Beausoleil, Katherine E. Littlewood, Andrew N. McLean, Paul D. McGreevy, Bidda Jones, and Cristina Wilkins. "The 2020 Five Domains Model: Including Human–Animal Interactions in Assessments of Animal Welfare." Animals 10, no. 10 (October 14, 2020): 1870. https://doi.org/10.3390/ani10101870.
  11. Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  12. Martins, Catarina I. M., Leonor Galhardo, Chris Noble, Børge Damsgård, Maria T. Spedicato, Walter Zupa, Marilyn Beauchaud, et al. "Behavioural Indicators of Welfare in Farmed Fish." Fish Physiology and Biochemistry 38, no. 1 (July 28, 2011): 17–41. https://doi.org/10.1007/s10695-011-9518-8.
  13. Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. "Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages." Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  14. Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. "Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare." Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  15. Pacific Southwest Region USFWS from Sacramento, US, Public domain, via Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Rainbow_trout_(26809663537).jpg (modified).
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