Steroid and thyroid hormones during sexual maturation of coho salmon (Oncorhynchus kisutch) in seawater of fresh water.

Osmolality and circulating levels of estradiol-17P, thyroxine, androgens, progesterone, and sodium are measured concomitantly with determination of oocyte maturation in coho salmon (Oncorhynchus kisutch) during the spawning season. Male and female salmon are held in both fresh water (their natural environment) and seawater (an unnatural environment) throughout the season. During the early stages of final gonadal maturation, sodium levels and osmolality of fish in seawater increase substantially, which suggests osmoregulatory difftculties. Also, there is a higher mortality rate of fish held in seawater. The circulating levels of all hormones measured differ between coho salmon in fresh water and those in seawater. We conclude from these differences in hormone levels, the dehydrated eggs, incomplete ovulation, and high adult mortality of fish in seawater that reproductive function is compromised in salmon which are retained in seawater during the spawning season.