Effects of thermal acclimation on tissue fatty acid composition of freshwater alewives (Alosa pseudoharengus).

In this study, we examine changes in fatty acid composition of polar and neutral lipids in gill, liver, and muscle of freshwater alewives (Alosa pseudoharengus) after temperature challenges in the laboratory. Alewives experienced either a warm or cold challenge in which temperatures were raised or lowered 0.5°C day(-1) over 4-6 weeks. In alewives experiencing the cold challenge, gill polar lipids showed evidence of significant remodeling, including decreases in palmitic acid and saturated fatty acids and increases in n-3 and n-6 highly unsaturated fatty acids including docosahexaenoic acid (DHA) and arachidonic acid. In alewives experiencing the warm challenge, we observed significant increases in saturated fatty acids (due mainly to increased palmitic acid) and decreases in polyunsaturated fatty acids in polar lipids of muscle and liver tissue. Fish that died during the cold challenge had significantly higher levels of palmitic acid in muscle polar lipids compared to fish that survived; fish that died during the warm challenge displayed complex changes in fatty acid composition. Based on theoretical considerations, the changes in polar lipids we observed during thermal acclimation are likely to promote appropriate membrane fluidity under each thermal regime. The increased incorporation of highly unsaturated fatty acids during cold acclimation could have significant physiological and ecological implications. In particular, since highly unsaturated fatty acids are typically scarce in freshwater food webs, dietary deficiencies in these essential fatty acids may be a significant factor in winter mortality of freshwater alewives.