Effects of long-distance migration on circulating lipids of sockeye salmon (Oncorhynchus nerka)

Lipids circulate as nonesterified fatty acids (NEFAs) or as triacylglycerol (TAG) associated with phospholipids (PLs), the two main components of lipoproteins. Changes in plasma lipid levels of migrating salmon were anticipated in response to the combined demands of exercise and osmoregulation. Circulating NEFAs, TAG, and PLs were measured in sockeye salmon (Oncorhynchus nerka) at different stages of their 500 km migration up the Fraser River (British Columbia, Canada). We found that NEFAs represent < 7% of total plasma fatty acids (FAs) and show only a minor decrease during migration. In contrast, lipoproteins account for > 93% of all the energy of circulating lipids, and concentrations of their main constituents show 27-fold (TAG) and 6-fold (PLs) decreases. The most pronounced change in FA composition of plasma lipids was found in the sample immediately after freshwater transition, with an increase in TAG eicosapentaenoic acid (20:5) and a decrease in TAG palmitic acid (16:0). Lipoproteins are therefore involved in key physiological processes necessary for successful spawning. Previous investigations of lipid metabolism in swimming salmon have focused on plasma NEFAs, but this study shows that shifting attention to lipoproteins should prove more fruitful for future studies of migration energetics.