Contents of Aquaculture, volume 53

Abstract

The present study investigated seasonal changes in plasma 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-diOHprog) levels in precociously mature and fully grown mature male amago salmon (Oncorhynchus rhodurus) and in vitro 17α,20β-diOHprog production by the testes of amago salmon and rainbow trout (Salmo gairdneri) in response to chum salmon gonadotropin. Plasma 17α,20β-diOHprog levels were very low from June to September, and rapidly increased in October at the beginning of the spawning season in both precociously mature and fully grown mature male amago salmon. High levels of 17α,20β-diOHprog were maintained during the period of active spermiation in precociously mature males, and quickly declined in mid-November. Incubation of testicular fragments from spermiating rainbow trout with chum salmon gonadotropin or 17α-hydroxyprogesterone resulted in a highly significant increase in 17α,20β-diOHprog levels in the incubation medium. The chum salmon gonadotropin was also effective in stimulating 17α,20β-diOHprog production by testicular fragments obtained from amago salmon after the breeding season. These findings are discussed in relation to the possible involvement of 17α,20β-diOHprog in spermiation of salmonids.