There has been an increased incidence of caract development in Atlantic salmon reared under modern fish farming conditions during the last 5 years. The development of cataract not only affects the growth of the fish but acute outbreaks in the sea phase often lead to large mortalities. Besides the economic impact, there are serious ethical aspects of producing blind fish. The present project is based upon results from previous field trials, as well as controlled feeding experiments. Increased incidences of cararact have been observed since blood meal has been omitted as a feed ingredient for Atlantic salmon. Results from our studies have indeed shown that both the incidence and the severity of cataract are reduced when feeding a diet containing blood meal or a diet that mimics the nutritional profile of the blood meal diet. It seems therefore likely that the nutritional components of blood meal that are present in either reduced or increased concentrations (e.g. histidine, iron and zinc) when compared t o a non blooded diet may be in part responsible for reducing cataract formation and subsequently increasing growth. The project will examine the metabolism of the essential amino acid histidine using stable isotope labelled 15N histidine and also investigate the metabolism of iron and zinc in relation to eye integrity and health under different rearing conditions. The project will also investigate recent evidence of differences in susceptibility to cataract formation among different breeds of Atlantic salmon. In additon, comparative studies with rainbow trout will be performed to see if Atlantic salmon is more prone to cataract development. Genetically linked differences will be related to the metabolism of histidine, iron and zinc. The project also includes in vitro studies with organ-cultured lenses, focusing both on different growth media and potential stressors. Cataract preventing factors will also be investigated.