Testicular development and serum levels of gonadal steroids during the annual reproductive cycle of captive Japanese sardine

Gonad and blood samples were taken throughout the year from captive males of the Japanese sardine, Sardinops melanostictus, and changes in serum levels of gonadal steroids were examined in relation to the annual gonadal cycle. On the basis of testicular histology, the annual gonadal cycle was divisible into four periods: immature (July-September), spermatogenesis (October-December), spermiation (January-April), and post-spawning (May-June). The pattern of seasonal changes in the gonadosomatic index (GSI) was inversely correlated with that of water temperature, and reflected the degree of testicular maturity. The serum testosterone level was relatively low during spermatogenesis (2.2-2.5 ng/ml), rose markedly around the time of spermiation (7.7-24.6 ng/ml), and became low after spawning and during immature periods (0.6-0.7 ng/ml). The serum 17α,20β-dihydroxy-4-pregnen-3-one level was high in males with spermatogenic or spermiating testes (0.6-1.0 ng/ml), but became low (0.2 ng/ ml during the post-spawning period and was undetectable in immature fish. Although 11-ketotestosterone was detectable in some fish, the values obtained were thought to reflect cross-reactivity of the antiserum employed with testosterone. These findings are discussed in relation to male reproduction of the Japanese sardine and steroidal regulation of spermatogenesis and spermiation in other teleosts.