Epidemiology of blindness and cataracts in cage reared ongrown Atlantic halibut Hippoglossus hippoglossus

Cataract pathology, intensity and severity, and temporal development were investigated in halibut in the unusual location for halibut of solely the anterior lens. This manifested as an opacity and there was fibre disruption in the anterior cortex with vacuoles. Individual eye blindness was 17–26% and significantly higher in non-migrated eyes, but bilateral blindness was only 0–4%. Cataract prevalence varied greatly in fish between sea pens, from 29 to 83%, and there was a significant increase on time, with a higher prevalence in migrated eyes. The cataract severity score per individual eye increased from 1.24 to 1.69 (score range 0–3) between sampling dates but there was no significant difference in severity in migrated and non-migrated eyes. Neither cataract prevalence nor severity were related to individual fish weight suggesting that growth per se was not an important influence. Physical damage largely affected the protruding non-migrated eye as a consequence of aggression in the hatchery and consequently physical damage of halibut in sea cages was well healed. Damage from UV light and nutritional deficiencies were regarded as possible factors in the formation of these anterior cataracts in ongrown halibut.