Comparison of fatty acids and lipids of smolting hatchery-fed and wild Atlantic salmon Salmo salar.

In Atlantic Canada the Atlantic salmon Salmo salar change from the parr stage to the smolt stage while still in fresh water, preparatory to migration to salt water. In some stocks this takes place during the second overwintering. In several hatcheries where the water temperature drops to 0-0.5 C and the ponds ice over, there is a high incidence of erosion of the dorsal and pectoral fins and sometimes of the caudal fin. No disease organism has been identified, and the lesions heal over in most cases. Dietary fatty acids were thought to be a factor. A detailed study of lipid recoveries and classes has shown that in the skins of abnormal fish the total lipid is 7.8% compared to 4.7% in control fish. Unexpectedly, an analysis of one lot of healthy smoltstage wild fish showed that whole bodies have only a quarter of the lipid of comparable hatchery fish. Comparison of fatty acids showed that wild fish lipids include a higher proportion of arachidonic acid than those of the hatchery fish. In the latter, linoleic acid is provided readily by diet but the elongation to arachidonic acid evidently does not proceed. These results suggest that the smolt lipid is involved intimately with either the cause of the dermal lesion or is a defense mechanism, possibly mediated through oxygenase activity.