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Elevation of the Plasma Level of Insulin‐like Growth Factor‐I with Reproductive Maturation prior to Initiation of Spawning Migration of Chum Salmon

When, where, and how oceanic chum salmon initiate spawning migration is unknown although gonadal development and elevation of the activity of the pituitary–gonadal axis (PG‐axis) are essential. Insulin‐like growth factor‐I (IGF‐I) is a somatotropic signal that interacts with the PG‐axis for gametogenesis. We thus examined the plasma level of IGF‐I in immature and maturing chum salmon in the Bering Sea and the Gulf of Alaska. The maturing adults which had maturing gonads left the Bering Sea for the natal river by the end of summer, because almost all fish were immature in September. The plasma level of IGF‐I and corresponding body size in the maturing adults were two‐ to threefold that of immature fish. The plasma IGF‐I level correlated positively with the pituitary contents of follicle‐stimulating hormone and the plasma levels of 11‐ketotestosterone and estradiol‐17β. Therefore, the plasma level of IGF‐I increased with elevation of the PG‐axis activity prior to the initiation of spawning migration from the Bering Sea. Circulatory IGF‐I from visceral organs may inform the status of body growth to the PG‐axis for gonadal development that is inseparable from decision of chum salmon whether to initiate homing behavior from the Bering Sea or not to initiate spawning migration by the coming spawning season.