Prolonged Cold Exposure Negatively Impacts Atlantic Salmon (Salmo salar) Liver Metabolism and Function

Large-scale mortality events have occurred during the winter in Atlantic salmon sea cages in Eastern Canada and Iceland. Thus, in salmon held at 3 degrees C that were apparently healthy (i.e., asymptomatic) and that had 'early' and 'advanced' symptoms of 'winter syndrome'/'winter disease' (WS/WD), we measured hepatic lipid classes and fatty acid levels, and the transcript expression of 34 molecular markers of fatty liver disease (FLD; a clinical sign of WS/WD). In addition, we correlated our results with previously reported characteristics associated with this disease's progression in these same individuals. Total lipid and triacylglycerol (TAG) levels increased by similar to 50%, and the expression of 32 of the 34 genes was dysregulated, in fish with symptoms of FLD. TAG was positively correlated with markers of inflammation (5loxa, saa5), hepatosomatic index (HSI), and plasma aspartate aminotransferase levels, but negatively correlated with genes related to lipid metabolism (elovl5b, fabp3a, cd36c), oxidative stress (catc), and growth (igf1). Multivariate analyses clearly showed that the three groups of fish were different, and that saa5 was the largest contributor to differences. Our results provide a number of biomarkers for FLD in salmon, and very strong evidence that prolonged cold exposure can trigger FLD in this ecologically and economically important species.