Eye opacity

A clear, healthy lens is essential for vision, enabling fish to feed, navigate their environment, and respond to threats or social cues. Eye opacity or cataracts can arise from nutritional, genetic, and environmental factors, as well as exposure to infections, parasites, or ultraviolet light. Cataracts cause the lens to cloud, leading to impaired vision. While eye opacity can negatively impact survival, growth, and performance, the welfare impact of eye opacity is context-dependent, influenced by factors such as feeding regime, competition level, stocking density, and environmental complexity. In commercial settings where feed is readily available and predation risk is absent, the functional consequences of impaired vision may be reduced, though the underlying condition still warrants monitoring and management.

Level 0	Level 1	Level 2	Level 3
No visible opacity	Minor opacity, ≤ 10% lens coverage	Clear opacity, > 10 to ≤ 50% lens coverage	Severe opacity, > 50% lens coverage
Rainbow trout eye without any opacity⁴⁹	Rainbow trout eye with minor opacity⁵⁰	Rainbow trout eye with clear opacity⁵¹	Rainbow trout eye with severe opacity⁵²

Protocol application

Assessment requires handling the fish for close visual inspection of the lens. In-water assessment is not reliable for scoring opacity levels.

As it is not always practical to distinguish between opacity to the lens or the cornea, all opacity is included in this indicator.

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

While a level 1 opacity score indicates 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 are scored as level 2 or level 3, the welfare risk is significantly higher than for unilateral cases. If observed, increase monitoring frequency and implement corrective action.
Validation: This scoring system has been adapted from 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 both freshwater and seawater sites over a three-year rearing period. A slit-lamp microscope was used to score cataracts on a scale of 0–5, with a maximum total score of 10 across both eyes. The fish were measured at three different times: after the first growing season (mean weight 50 g, freshwater), after the second growing season (mean weight 734 g, both freshwater and seawater), and after the third growing season (mean weight 2245 g, 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 genetic factors determine variation in tolerance. 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. There was no clear relationship between cataract severity and mortality. The authors explained this could be because there were no predators and fish had unlimited access to feed.
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 for 112 days in cages or in a freshwater laboratory. Cataracts were scored on a scale from 0 to 4.	There was a strong relationship between higher parasite loads and more severe cataracts. In the lake conditions, cataracts developed gradually, with 35% of eyes fully covered or opaque by day 112. In the laboratory conditions, 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.
Remø et al., 2017	Experimental study investigating the formation of cataracts at different temperatures in 80 post-smolt rainbow trout (125 g) over 35 days in laboratory seawater conditions. Two temperatures were tested: 13°C and 19°C. Each lens was scored between 0 and 4. 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-acetyl-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. 2010. “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, 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.
Remø, Sofie Charlotte, Ernst Morten Hevrøy, Olav Breck, Pål Asgeir Olsvik, and Rune Waagbø. 2017. “Lens Metabolomic Profiling as a Tool to Understand Cataractogenesis in Atlantic Salmon and Rainbow Trout Reared at Optimum and High Temperature.” Edited by José L. Soengas. 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.

Nilsson, Jonatan. 2026. Rainbow trout eye without any opacity, score 0. Photograph. Personal communication, Institute of Marine Research.
Nilsson, Jonatan. 2026. Rainbow trout eye with minor opacity, score 1. Photograph. Personal communication, Institute of Marine Research.
Nilsson, Jonatan. 2026. Rainbow trout eye with clear opacity, score 2. Photograph. Personal communication, Institute of Marine Research.
Nilsson, Jonatan. 2026. Rainbow trout eye with severe opacity, score 3. Photograph. Personal communication, Institute of Marine Research.