Serum thyroxine, estradiol-17β, and testosterone profiles during the parr-smolt transformation of masu salmon, Oncorhynchus masou.

Changes in serum thyroxine (T4), estradiol-17β (E2) and testosterone (T) levels during the parr-smolt transformation (smoltification) were investigated in the masu salmon (Oncorhynchus masou) in 1985 and 1987. T4 showed a peak in levels at the early stage of smoltification and E2 and T levels peaked almost at the same time. There were no significant differences between the concentrations of serum hormones in female and males. During smoltification, germ cells in the peri-nucleolus and spermatogonia stage were present in the ovary and testis, respectively. These results suggest that E2 and T may be involved in smoltification in the masu salmon.