Changes in reproductive parameters during the spawning migration of pink salmon, Oncorhynchus gorbuscha (Walbaum)

The hormones 17β-estradiol, 17α-hydroxy-20β-dihydroprogesterone(17α, 20β-P), 11-ketotestosterone, testosterone, gonadotropin and also vitellogenin, were determined during the spawning migration of wild pink salmon in the Fraser and Thompson Rivers in British Columbia. This stock of pink salmon takes approximately 2 weeks to migrate the 333 km upstream to the spawning grounds. Both sexes were at an advanced stage of sexual development when they entered fresh water. In females both the 17β-estradiol and vitellogenin levels fell precipitously during the migration, to be very low at spawning, whereas the 17α,20β-P level rose rapidly, to be highest at arrival on the spawning grounds. The gonadotropin level also rose rapidly during the migration, and was highest in spent fish. Testosterone was at a high level throughout, although this level decreased steadily during migration. In many respects similar endocrine changes were observed in the male. For example, in the case of androgen levels, both testosterone and 11-ketotestosterone fell steadily during migration but were still relatively high at spawning, whereas both gonadotropin and 17α, 20β-P levels rose markedly as migration progress. However, although the qualitative changes were often similar between the sexes, the levels of 17α, 20β-P, testosterone, and gonadotropin were considerably higher throughout in females than in males. It is concluded that this stock of pink salmon is at an advanced stage of sexual development when it enters fresh water. The endocrine changes observed during this study represent those controlling the final stages of reproduction, specifically final oocyte maturation and ovulation in females, and the final stages of spermatogenesis and spermiation in males.