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.20 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, while noting that the population baseline may shift seasonally.

Level 0 Level 1 Level 2 Level 3

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 are starting to form

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

Assessment requires handling the fish under consistent lighting conditions. Although patchy areas, subtle darkening, and loss of brightness can be distinguished in-water, light refraction at the water surface may alter perceived coloration or reduce photo/video image quality. Furthermore, light refraction and turbidity may affect perceived coloration in automated underwater welfare scoring systems.

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. However, assessing against a baseline can still be misleading if the whole population is stressed or diseased, so the assessor should be familiar with the expected appearance of a healthy trout for that specific site.
  • Consider environmental factors (such as turbidity, lighting, and substrates) that can affect natural coloration.
  • Change in coloration is a nonspecific indicator and must be interpreted alongside other indicators.
  • 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 (6 g) 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 (12 g) rainbow trout in freshwater laboratory conditions over 5 days. The group with higher exposure to the toxin exhibited 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 (5 g) 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.
  • Laique, Talha, Mikkel Gunnes, Ole Folkedal, Jonatan Nilsson, Evelina A. L. Green, Hannah Normann Gundersen, Øyvind Øverli, and Habib Ullah. “Quantification of Opercular Pigmentation Changes in Farmed Atlantic Salmon: A Novel Application for Computer Vision in Fish Welfare Assessment.” Fishes 11, no. 5 (May 2, 2026): 271. https://doi.org/10.3390/fishes11050271.
  • 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.
  • Yi, Mengmeng, Huanda Lu, Yishuai Du, Guoxiang Sun, Ce Shi, Xian Li, Huiqin Tian, and Ying Liu. “The Color Change and Stress Response of Atlantic Salmon (Salmo Salar L.) Infected with Aeromonas Salmonicida.” Aquaculture Reports 20 (March 25, 2021): 100664. https://doi.org/10.1016/j.aqrep.2021.100664.
  • 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.

  1. Øverli, Øyvind, Svante Winberg, Børge Damsgård, and Malcolm Jobling. “Food Intake and Spontaneous Swimming Activity in Arctic Char (Salvelinus alpinus): Role of Brain Serotonergic Activity and Social Interactions.” Canadian Journal of Zoology 76, no. 7 (July 1998): 1366–70. https://doi.org/10.1139/cjz-76-7-1366.