Seasonal variation in the nutritional composition of muscle in masu salmon (Oncorhynchus masou masou) during the freshwater transition and spawning period

Abstract A unique aquaculture practice for masu salmon ( Oncorhynchus masou masou ) has been developed in Miyazaki Prefecture, Japan, in which fish are transferred from seawater (SW) to freshwater (FW) following the SW culture period to extend the shipment season through summer. This study examined changes in the nutritional composition of dorsal muscle during the maturation phase in FW-aquacultured masu salmon by measuring lipid content, fatty acid composition, carotenoid content, free amino acid content, imidazole dipeptide content, and nucleotide-related substances in fish sampled from April (SW period) through October (FW period). Lipid content was maintained at relatively high levels from April through August, but decreased markedly during the spawning period (September to October). In addition, eicosapentaenoic acid decreased markedly during the spawning period, whereas docosahexaenoic acid was relatively preserved, suggesting hierarchical fatty acid mobilization. Astaxanthin, which showed the highest content among the carotenoids examined, reached its maximum level in May and subsequently decreased considerably during the spawning period. Furthermore, anserine, a histidine-containing dipeptide associated with osmoregulatory function, showed the highest level under SW conditions in April and tended to decrease under FW conditions. Inosine 5′-monophosphate, the principal umami-active nucleotide in muscle, was maintained at relatively high levels through September; however, it decreased considerably in October. These results indicate that this FW culture system not only extends the shipment period through summer but may also maintain eating quality and nutritional value comparable to those during the SW aquaculture period until before the spawning season.