Changes in serum steroid hormone levels and in vitro steroid hormone production in the ovaries of female chum salmon Oncorhynchus keta during spawning migration.

Following the previous study, serum steroid hormone profiles and in vitro steroid hormone production in the testes of male chum salmon Oncorhynchus keta were examined by adding materials sampled at the North Pacific Ocean. The maximum serum level of 11-ketotestos-terone (11-keto T; 56ng/ml at the coastal sea) preceded that of testosterone (T; 48ng/ml at the pre-spawning period). Levels of both androgens in serum decreased in the spawning period when serum 17α, 20β-dihydroxy-4-pregnen-3-one (17α, 20β-diOHprog) level rapidly in-creased. Incubation of testicular fragments from testes of the coastal sea and the pre-spawning period with 17α-hydroxyprogesterone (17α-OHprog; 0.1μg/ml), T (0.1μg/ml) and chum salmon gonadotropin (SGA; 1μg/ml) resulted in elevated 11-keto T concentration in the incubation medium. 17α, 20β-DiOHprog level in medium was highly increased only in the presence of 17α-OHprog. The conversion of 17α-OHprog to 17α, 20β-diOHprog was enhanced by adding SGA to the incubation medium of testicular fragments of individuals collected from the river (pre-spawning period), but not from the coastal sea. These results indicate that 11-lceto T and 17α, 20β-diOHprog respectively are the major steroids responsible for latter phase of sperma-togenesis and spermiation in male chum salmon.