Some endocrine factors affecting the development of seawater tolerance during the parr-smolt transformation of the amago salmon, Oncorhynchus rhodurus

Abstract

The development of hypoosmoregulatory abilities assessed by monthly seawater-challenge tests was studied in hatchery-reared amago salmon. At any time from August to May, parr, including precociously mature male and immature small fish, could not regulate serum sodium levels 24 h after transfer to seawater (29‰). The best hypoosmoregulatory performance was obtained from smolts collected in December and January, and this coincided with the peak of smoltification assessed by external appearance. The hypoosmoregulatory capacity of smolts began to decline in February and reached the same levels as parr by May, thus indicating that desmoltification was occurring during the spring. Analysis of weekly plasma samples between 30 August and 14 November demonstrated that the levels of plasma thyroxine of smolts increased significantly just prior to the maximum development of seawater adaptability; parr did not show this increase. The greatest thyroid activity, assessed by changes in thyroid epithelial cell height, was observed in smolts collected in December and January. These results suggest that thyroid hormone is involved in smoltification in the amago salmon. Possible relationships between plasma thyroxine and sex steroid levels in precociously mature male parr and smolts are discussed.