Maturational steroids and gonadotropin in upstream migratory sockeye salmon

The circulating serum concentrations of various steroid hormones in mature sockeye salmon were measured at four different developmental stages in their upstream migration to spawn at Adams River in British Columbia, Canada. In females, a high level of estradiol-17 beta was found in fish at the first location, and it persisted until immediately before reaching the spawning grounds, 485 km upstream, where it decreased to a minimal level. Free testosterone was extremely high throughout the migration but decreased significantly after spawning while its glucuronide changed reciprocally. Free and conjugated 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17 alpha, 20 beta-P) peaked at the last location before spawning with the glucuronide only 20% of the free steroid in concentration. After spawning, the concentration of free steroid declined but the glucuronide remained constant. 17 alpha-hydroxyprogesterone increased significantly in serum before and after the fish had spawned. During the migration 11-deoxycortisol was present in the serum at all stages but maximal levels were found in postspawned fish. Throughout the migration, the males had high serum levels of 11-ketotestosterone. Lesser amounts of free testosterone were also consistently present but the ratio of free: conjugated decreased from 1.7 at the beginning of the migration to 0.15 on the arrival at the Adams River and 0.10 in postspawned fish. Only low levels of 11 beta-hydroxytestosterone were found except in the postspawned males where the value was equal to one-half that of free testosterone. As in the females, there was a substantial increase in the levels of 17 alpha, 20 beta-P and its glucuronide in the males captured at the spawning grounds. In both sexes gonadotropin levels were low during the migration and high in the postspawned fish.