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.

Preliminary findings of ophthalmological abnormalities in farmed halibut.

Ocular abnormalities occurring in farmed halibut at the Sea Fish Industry Authority Marine Farming Unit, Ardtoe, Argyll were investigated clinically and post mortem. A significant number of fish were found to have posterior polar cystic changes either causing scleral ectasia or gross degenerative posterior segment abnormalities with chorioretinal atrophy. Other ocular abnormalities included cataract formation and intraocular inflammation. In a small number of fish gas bubble formation in the anterior chamber was observed during capture and examination. It is possible that the lesions seen in these fish are a form of gas bubble disease although super-saturation, the cause of gas bubble disease previously reported in farmed fish, does not occur in the tanks in which these fish are housed. While this paper provides no answers with regard to the aetiopathogenesis of the lesions, it is hoped that it will stimulate discussion, leading to resolution of these questions through a multidisciplinary approach to the problem.

Occurrence of cataracts in triploid Atlantic salmon (Salmo salar) on four farms in Scotland.

In September 1991 cataracts occurred in two year-classes of triploid Atlantic salmon. The fish showed varying degrees of blindness, were lethargic and became emaciated owing to their inability to feed. The lesions in the lens were mainly in the anterior and posterior cortex and perinuclear areas, the capsule and embryonic nucleus remaining unaffected. Diploid fish of the same year-class did not have cataracts. The origin of the triploid fish, the method of triploidisation, their diet and disease status were investigated, but no predisposing factors could be found to account for the high incidence of cataracts.