Effects of photoperiod and temperature on gonadal maturation, and plasma steroid and gonadotropin levels in a cyprinid fish, the honmoroko Gnathopogon caerulescens.

The honmoroko (Gnathopogon caerulescens), a small, spring-summer-breeding cyprinid fish, was exposed to various temperature and photoperiod regimes during different phases of its annual reproductive cycle. In autumn, a clear photoperiodism was seen, especially at warm temperatures (18-24 degrees). The gonadosomatic index, plasma gonadotropin (GtH), and gonadal steroids were higher in fish held at long photoperiod (15L/9D) than in those held at short photoperiod (12L/12D); the differences were greater as the temperature increased. In early spring, gonadal recrudescence progressed at temperatures above 7 degrees, regardless of photoperiod. Photoperiod did not affect plasma GtH and gonadal steroid levels at this time of the year. During its spawning season, a long photoperiod (15L/9D) was again required to maintain gonadal activity and high plasma GtH and gonadal steroid levels, especially at high temperature (25 degrees). In early summer, high temperatures (25 and 30 degrees) suppressed gonadal activity; however, gonadal steroid levels were not depressed, and plasma GtH levels actually increased. The data suggest that under natural conditions, high water temperatures in summer may act directly at the gonadal level, and is probably the factor which terminates the breeding season for this species. The results of these four experiments indicate that the honmoroko exhibits a photoperiodic response during the autumn month and its spawning season, and is dependent on the water temperature. The effects of photoperiod on gonadal maturation appear to be GtH mediated, whereas the suppressive effects of high temperature are not.