Changes in serum concentrations of steroid hormones, thyroxine, and vitellogenin during spawning migration the chum...

Changes in serum concentrations of various steroid hormones (estradiol-17 beta, androgens, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-diOHprog], thyroxine, and vitellogenin during the spawning migration (from the coastal sea to the spawning ground) of male and female chum salmon (Oncorhynchus keta) were investigated. In females, estradiol-17 beta levels were high during the early phases of the migration (sea and prespawning period), but significantly decreased in fish at the time of the spawning period (oocyte maturation and ovulation). The changes in estradiol-17 beta levels coincided with those of of serum vitellogenin levels. High levels of androgens were observed throughout the sampling period. Serum 17 alpha, 20 beta-diOHprog levels were extremely low during the early phases of spawning migration, but elevated dramatically in mature or ovulating females. In males, serum concentrations of androgens were high during the early phases of the migration, but sharply declined around the time of the spawning period (spermiation). Similar to females during final maturation, the serum levels of 17 alpha,20 beta-diOHprog in males rapidly increased during spermiation, although the magnitude of the elevation in males was smaller than in females. Serum thyroxine levels were highest both in females and males collected in the coastal sea, and the levels decreased during the spawning migration. These findings are discussed in relation to anadromous migration and sexual maturation of chum salmon.