Changes in Serum Steroid Hormone Levels and in vitro Steroid Hormone Production in the Ovaries of Female Chum Salmon Oncorhynchus keta During Spawning Migration

Abstract

The present paper provides additional data to our previous chum salmon Oncorhynchus keta paper on serum steroid hormone levels with the addition of samples collected in the North Pacific Ocean. Moreover, in the present study in vitro steroid hormone production by ovarian follicles were correlated with serum steroid hormone levels during the spawning migration of female chum salmon. Serum estradiol-17β (E2) levels increased sharply during sea-water migration, but were followed by a sharp drop during upstream migration. Serum 17α, 20β-dihydroxy-4-pregnen-3-one (17α, 20β-diOHprog) levels were very low in the fish collected from the North Pacific Ocean, coastal sea, and river during pre-spawning period, and increased dramatically at the spawning period. There was a good correlation between serum E2 and 17α, 20β-diOHprog levels and in vitro E2and 17a, 20β-diOHprog productions by ovarian follicles in response to chum salmon gonadotropin (SGA). Incubation of ovarian follicles sampled at the coastal sea with SGA resulted in a sharp increase in E2concentrations in the incubation medium, but not in 17a, 20β-diOHprog concentrations. Ovarian follicles from the pre-spawning period did not response to SGA on E2 production, but 17a, 20β-diOHprog levels in medium were highly elevated by SGA in a dose-dependent manner. Postovulatory follicles at the spawning period produced large amounts of 17a, 20β-diOHprog with 1 μg/ml of SGA, but none of E2. These findings suggest that in chum salmon as in other salmonids, E2plays a major role in vitellogenesis, and 17α, 20β-diOHprog has an important role in final oocyte maturation. © 1991, The Japanese Society of Fisheries Science. All rights reserved.