CATARACT DEVELOPMENT IN ATLANTIC SALMON (SALMO SOLAR L) IN FRESH-WATER

Irreversible bilateral cataracts were diagnosed by slit-lamp biomicros copy in 178 of 200 farm-raised Atlantic salmon (Salmo salar L) fed a s tandard diet over a five-month period. Initial changes were anterior p olar opacities, progressing to involve both the anterior and posterior cortex before changes in the lens nucleus were seen. The lens changes were recorded and given scores according to the severity of the catar acts. At each of 3 samplings, after 2, 4 and 5 months, 200 fish were m easured, weighed and examined by slit-lamp biomicroscopy. At all 3 sam plings, there was a significant correlation between body length and bo th cataract incidence and cataract severity. There was also a signific ant correlation between body weight and cataract incidence and severit y for the 2 last samplings. There was a significant correlation betwee n K-factor as a measure of the shape of the fish, and both cataract in cidence and severity, at all 3 samplings. Evaluation of specific growt h rate in the periods between the examinations showed that the rapidly -growing fish were most susceptible to cataract formation. After catar act developed however, the growth rate slowed. Follow-up examination o f severely affected fish 3 months after transfer to sea water showed a normal cortical zone in the periphery of the lens in 24 out of 28 fis h.