Profiles of circulating insulin-like growth factor-I during smoltification of masu salmon reared under different conditions.

We compared profiles of serum insulin-like growth factor (IGF)-I levels during smoltification of masu salmon reared under different environments, hatcheries and growth histories. Masu salmon from the Kenichi River in Hokkaido showed a sharp increase in serum IGF-I from April to May, followed by a peak of gill Na,K-ATPase (NKA) activity. Fish at Kumaishi Hatchery had an IGF-I profile similar to that of the river fish, while the increase in gill NKA was lower. At Shimamaki Hatchery, interval feeding during winter appeared to suppress the spring IGF-I peak. At Kumaishi Hatchery, a difference in size during smoltification affected IGF-I levels at release, but the numbers of adults that returned to the release site were not significantly different. In the following year, three release groups differing in winter size and/or spring growth (Large-High, Large-Low and Small-High) were created. Large-High and Small-High fish showed a higher IGF-I peak than Large-Low fish in April, while smolt-to-adult return of Large-High fish was highest. These results suggest that in smolting masu salmon in freshwater, circulating IGF-I level alone is not a predictor of long-term survival in seawater. However, since growth history in freshwater affected the smolt-to-adult return, optimizing rearing conditions is a critical component of hatchery releases for masu salmon. [ABSTRACT FROM AUTHOR]