Effects of photoperiod and temperature on gonadal activity and plasma steroid levels in an autumn-spawning bitterling, Acheilognathus rhombea, during different phases of its annual reproductive cycle.

The kanehira bitterling, Acheilognathus rhombea, which breeds during autumn in the underyearlings and during summer and autumn in the yearlings, was exposed to various photoperiodic and temperature regimes during different phases of its annual reproductive cycle. In early autumn, gonadal development and increase in plasma levels of female estradiol-17 beta (E2) and male testosterone (T) of underyearling fish occurred under the short day length condition (12L12D). In early winter, gonadal regression (GSI reduction in both sexes, termination of vitellogenesis in females, and termination of spermatogonia proliferation in males) and decrease in plasma levels of E2 in females and T in males were induced by the low-temperature treatment (13 degrees), while gonadal maturity was maintained under the moderate temperature (20 degrees). The low temperature did not prevent oocyte maturation and ovulation in females and spermiogenesis and spermiation in males, indicating that temperature effect on gonadal activity is quite different among the maturational stages. The gonadal development of yearling fish did not progress during spring, even under the short day length condition combined with moderate temperature, which suggests that this species is in the refractory period during this season. In early summer, however, the autonomous gonadal maturation of yearling fish kept under moderate temperature progressed even under the long day length condition (15L9D). It is concluded that the causative factor in the initiation of the spawning period of the underyearling fish is the short day length in autumn, and that the causative factor in the termination of the spawning period is the low temperature in winter. An internal factor such as circannual rhythm in the yearlings is probably involved in the refractoriness to the photoperiod during spring and the subsequent earlier maturation during summer.