Changes in seawater adaptability and blood thyroxine concentrations during smoltification of the masu salmon, Oncorhynchus masou, and the amago salmon, Oncorhynchus rhodurus

Abstract

The development of hypo-osmoregulatory ability, assessed by monthly seawater-challenge tests and changes in serum thyroxine levels, was studied during smoltification of hatchery-reared masu, Oncorhynchus masou, and amago salmon, Oncorhynchus rhodurus. In masu salmon, the best hypo-osmoregulatory performance was obtained from smolts collected in late May, and coincided with the peak of smoltification assessed by external appearance. Analysis of serum samples from March to August in 1981 showed a distinct surge in serum thyroxine levels; the highest value was obtained in fish undergoing smoltification collected in early May, and it coincided with the new moon. In amago salmon, the maximum development of seawater adaptability occurred in smolts collected during December and January, and this coincided with elevated plasma thyroxine levels. Highest levels were observed in smolts collected in late November, coincident with the new moon. Thus, the elevated blood thyroxine levels observed during smoltification in these two species of salmonids, which smoltify during different periods of the year, may indicate that this hormone plays an important role in the process of parr—smolt transformation. In addition, the peak of blood thyroxine levels, coincident with the new moon in both masu and amago salmon, suggests that the lunar phasing phenomenon is a generalized characteristic of Oncorhynchus species.