Seasonal Changes in Plasma Thyroxine Levels in Biwa and Amago Salmon Reared in the Pond

Abstract

Seasonal changes in plasma thyroxine concentrations were examined in biwa Oncorhynchus rhodurus and amago salmon O. rhodurus reared in the pond. About 80% of biwa salmon lost parr marks and became silvery in underyearling June. Thereafter, they kept silvery body color until 2-year-old April, when the experiments were terminated. Plasma thyroxine levels in biwa salmon decreased gradually from underyearling spring (about 4 ng/ml) to autumn, reached a lowest level (0.4 ng/ml) in December, and then increased gradually until yearling October (3-4 ng/ml). The relatively high hormone concentrations during underyearling April and May could be related to the body silvering and downstream migration. On the other hand, about 30% of amago salmon became silvery smolts with full seawater adaptability in underyearling autumn and winter. Plasma thyroxine concentrations in amago salmon showed a clear rise during smoltification and a distinct peak at the time of new moon in October (about 8 ng/ml). Thyroxine concentrations were re-latively high prior to the spermiation of precociously matured males (underyearlings) of both biwa and amago salmon, but decreased to a low level during spermiation, suggesting that thyroid hor-mone may also be involved in gonadal development.