Cataracts in farmed Atlantic salmon (Salmo salar) in Ireland, Norway and Scotland from 1995 to 1997

From 1995 to 1997 cataracts were observed in Atlantic salmon (Salmo salar) in Ireland, Norway and Scotland at around the time of smoltification (when freshwater fish become adapted to the hypertonic seawater) in both fresh- and seawater fish. Over 38,000 fish were screened for the presence of cataracts. Posterior cortical cataracts were the earliest and most consistent change, followed by perinuclear, equatorial and anterior cortical cataracts. On histological examination vacuolation of the lens fibres was consistently present in the posterior cortex. The pattern of the outbreak suggested that a nutritional factor was involved although the variable incidence and severity of the condition indicated that a number of modifying factors may have been involved in the expression of the condition.