Changes in activity and transcript level of liver and gill metabolic enzymes during smoltification in wild and hatchery-reared masu salmon (Oncorhynchus masou)

It is important for the success of the masu salmon, Oncorhynchus masou, stock enhancement program in Hokkaido (northern Japan) to demonstrate physiological problems in hatchery-reared (hatchery) smolt for artificial release. The present study examined changes in liver and gill metabolic parameters in wild and hatchery masu salmon during smoltification and elucidated differences in hepatic and gill metabolism between wild and hatchery fish. As reference to freshwater-adapted wild and hatchery smolt in this study, metabolic parameters of coastal smolt were studied. Yearling wild and hatchery smolting fish were collected from the Ken-ichi River and the Donan Research Branch, which used Ken-ichi river water for fish culture, at the same time every month from March through May 2008. Coastal smolts were caught from Nemuro Bay of Hokkaido in June. Decreased hepatic glycogen content during smoltification, which was observed in wild fish and revealed activation of glycogenolysis, was not found in hatchery fish. Hatchery fish demonstrated a positive change in hepatic ATP content during smoltification, while wild fish showed negative change in the content, which reflected activated consumption of hepatic ATP stores during smoltification. Increases in gill pyruvate kinase activity during smoltification, which were found in wild fish and indicated activation of glycolysis, were not detected in hatchery fish. There was a difference in increased timing of hepatic citrate synthase activity during smoltification between hatchery and wild fish. Increased gill citrate synthase activity during smoltification, which was observed in wild fish and reflected enhancement of the citric acid cycle, was not found in hatchery fish. Hatchery smolt revealed lower liver cytochrome c oxidase activity and transcript levels of some respiratory chain enzymes compared to wild smolt in May, which suggested lower respiratory chain capacity in hatchery fish at mid-smolt stage. On the other hand, there were no remarkable differences in hepatic and gill 3-hydroxyacyl-coenzyme A dehydrogenase related to lipolysis and creatine kinase activities, which operate in resolution of creatine phosphate, during smoltification between hatchery and wild fish. These results suggested hatchery masu salmon had some metabolic problems with carbohydrate metabolism, the citric acid cycle, and the respiratory chain. Our study will give valuable information to improve physiological quality of hatchery smolt for artificial release.