Correlations Between Homing , Migration , and Reproduction of Chum Salmon

The onset of gonadal maturation which is regulated by the endocrinological functions is considered to play leading roles in making salmon migrate a long distance from the ocean to their natal streams for spawning. Both cytological profiles of salmon gonadotropin-releasing hormone (sGnRH) neurons and serum gonadal steroid hormone profiles are investigated in chum salmon (Oncorhynchus ketal during the homing migration. Cytophysiological differences were observed between sGnRH neurons in the olfactory nerve and those in the preoptic area before and after upstream migration; the former could possibly be involved in the olfactory functions, and the latter would appear to be involved in gonadal maturation. Changes in serum steroid hormone levels were measured during the homing migration from the North Pacific Ocean to the spawning grounds, and the results revealed good correlations between estradil-17[3 and vitellogenesis, 11-ketotestosterone and spermatogenesis, and 17a,20[3-dihydroxy-4-pregnen-3-one and final gonadal maturation in both sexes, but the roles of testosterone in gonadal development in both sexes were still obscure. These findings are discussed in relation to homing migration and gonadal maturation of chum salmon.

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