Physiological and behavioral changes occurring during smoltification in the masu salmon, Oncorhynchus masou

Abstract

The development of hypoosmoregulatory ability assessed by monthly seawater challenge tests, changes in serum thyroxine levels, and the timing of downstream migratory behavior was studied during smoltification of yearling hatchery-reared (Hokkaido Fish Hatchery, Mori, Hokkaido) and wild (Oippe River, Aomori Prefecture) masu salmon, Oncorhynchus masou. The best hypoosmoregulatory performance was found in hatchery-reared smolts collected in May, and coincided with the peak of smoltification assessed by external appearance. Analysis of serum samples of wild (1982) masu salmon revealed that the levels of thyroxine were consistently high (7–12 ng/ml) during smoltification with a distinct surge (about 25 ng/ml) on the day of the new moon in late April. These results suggest that thyroid hormones may play an important role in the process of the parr-smolt transformation in the masu salmon. The onset of downstream migratory behavior in hatchery-reared masu salmon kept in an artificial stream was observed around the time of the new moon in May, 1982 and 1983. Moreover, the largest numbers migrated immediately after rainfall following the new moon. Similar observations were obtained from wild masu salmon in the Oippe River (1982). These results implicate the new moon and rainfall as important environmental factors which may initiate downstream migration in the masu salmon.