LAUSR

Abstract This study investigated the phospholipid fatty acid (PLFA) composition of the dorsal muscle, heart, brain, and spleen and hematology of Atlantic salmon (Salmo salar) (initial weight 72.89 ± 3.12 g) under decreasing water temperature from 16 °C to 12 °C, 8 °C, 6 °C, 4 °C, 2 °C, and 1 °C. Results showed the PLFA composition of the muscle was comparatively similar to that of the heart, whereas the unsaturated index (UI) and ratio of unsaturated to saturated fatty acids (U/S) of the spleen were the lowest. The proportion of monounsaturated fatty acids (MUFAs) of phospholipids was significantly higher in the brain than in the other tissues. The U/S, UI, UFAs (MUFAs and PUFAs) of phospholipids in each tissue increased with the temperature above 8 °C, whereas saturated fatty acids decreased. Moreover, PLFA in the muscle, heart, and spleen were more sensitive to temperature variations. Hematology parameters were unaltered except for triacylglycerol. The results demonstrate that homeostasis could be maintained by compensatory restructuring of the biological membranes in the appropriate temperature range. Homeostasis was broken when tissues responded strongly once the temperature was declined to 6 °C. When the temperature was below 4 °C, aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), and alkaline phosphatase (ALP) increased significantly, and all reached a maximum at 1 °C, especially AST and ALT. When the temperature dropped to below 2 °C, the fish stopped eating, and the compensatory restructuring of PLFA in tissues failed under the combined stressors of low temperature and starvation, causing organ failure, especially in the muscle and spleen.