Gonadotropin surge during spawning in male goldfish

Changes in plasma hormone levels during spawning were studied in male goldfish. Blood samples were analyzed for gonadotropin (GtH), testosterone, 11-ketotestosterone (11-KT), and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-diOH-P) in males, and for GtH in females. Spawning was induced by raising water temperature from 12 to 20 degrees in sexually mature goldfish kept in pairs. Plasma GtH levels in males showed a marked increase (GtH surge) which was synchronous with the preovulatory surge in females, and peaked at the onset of spawning. Plasma levels of testosterone and 17 alpha, 20 beta-diOH-P increased almost at the same time as the GtH surge in males, whereas 11-KT levels remained low during the spawning. The small increase in GtH in males could be induced in the absence of females by raising the water temperature, but the levels were significantly enhanced by the presence of ovulatory females. A large amount of milt was observed during spawning, and the amount was correlated with plasma levels of GtH. Increased levels of GtH may be involved in milt production. We can propose that the synchronous GtH surge in both sexes causes ovulation and milt production to occur at the same time, favoring a higher rate of fertilization of the eggs.