Changes in plasma insulin-like growth factor-I levels in the precociously maturing amago salmon, Oncorhynchus masou ishikawai

Seasonal. changes in plasma levels of insulin-like growth factor I (IGF-I) in precociously maturing amago salmon (Oncorhynchus masou ishikawai), which matured as 1-year-olds, have been investigated. Profiles of plasma IGF-I levels were compared with changes in growth and maturity, and plasma growth hormone (GH) and thyroxine (T-4) concentrations. The maturity of the fish was determined by calculating the gonadosomatic index; in November, 100% of males and 89% females matured. In both males and females, plasma IGF-I levels increased from March to August, and subsequently, plasma IGF-I levels in the early maturing males and females declined gradually and were maintained at lower levels during the spawning period in November. Plasma GH levels were high in April, and then declined gradually through September. Thereafter, in early maturing fish, a slight increase in plasma GH levels was observed in October and November. No significant changes in plasma T-4 levels were found in the precociously maturing fish. In sharp contrast, plasma IGF-I levels in immature fish remained elevated through September, reaching a peak in October, and then gradually declined in November. In immature females, plasma T-4 and GH levels were elevated in August, reached their maximum in September and then gradually declined until November.