Thyroid and Sex Steroid Hormone Level in the Pink Salmon Oncorhynchus gorbuscha during Marine and Freshwater Periods of Spawning Migration

— Thyroid and sex steroid hormones have been studied in spawners of the pink salmon Oncorhynchus gorbuscha at the stage of changing the marine section of the spawning migration path to freshwater one . Males of smaller size enter the Keret River earlier than larger specimens; such differences were not found in females. Compared to specimens in the sea, females in the river are characterized by a high level of triiodothyronine and an increased rate of deiodination, and males have a high testosterone level. Females both in the sea and in the river differ from males by a high level of 17ß-estradiol and a low value of the ratio of testosterone concentrations to 17ß-estradiol.