Cold tolerance and homeoviscous adaptation in freshwater alewives (Alosa pseudoharengus)

During cold winters, freshwater populations of alewives (Alosa pseudoharengus) are prone to massive die-offs, the physiological causes of which are poorly understood. We analyzed fatty acid composition before and after a laboratory cold challenge to examine the potential loss of cell membrane fluidity as a factor in winter mortality of these fish. Fish that died during the cold challenge displayed lower levels of polyunsaturated fatty acids, lower indices of unsaturation, and higher levels of monounsaturated fatty acids in membrane lipids compared to controls and survivors. Fatty acids associated with storage lipids were largely unaffected by the cold challenge. Differences in diet (Artemia versus Daphnia) had little effect on the fatty acid composition of these fish. Reduced activity of Δ5 desaturase enzymes may explain the decreased levels of polyunsaturated fatty acids (specifically arachidonic acid and docosahexaenoic acid) we observed in the cold temperature mortalities. Our results indicate that a loss of homeoviscous adaptation may play a role in low temperature mortality of freshwater alewives, and an analysis of alewife diets may improve our ability to predict winter die-offs of these fishes.