Effects of photoperiod and temperature on plasma gonadotropin and spermatogenesis in the rainbow trout Salmo gairdnerii Richardson

Abstract

Two-year old male rainbow trout, Salmo gairdnerii, were exposed to the following photoperiods at 80 or 16 °C from February 24 to June 19 : constant short photoperiod (8 L +16 D), constant long photoperiod (16 L + 8 D), and gradually decreasing photoperiod (from 16 L to 8 L). Control fish were exposed to natural photoperiod and temperature. Plasma and pituitary gonadotropin levels were determined by radioimmunoassay and correlated with spermatogenic response in testes. Control fish did not show advancement of spermatogenesis, whereas those exposed to decreasing photoperiod at 80 or 16 °C showed complete induction of spermatogenesis. While cooler temperature favoured production of spermatogonia and spermatocytes, the warmer temperature stimulated formation of spermatids and spermatozoa. On the other hand, in trout exposed to constant short or constant long photoperiod, spermatogenic response was weak at both temperatures. Plasma GtH levels increased in groups kept under decreasing photoperiod at both 8 °C as well as 16 °C ; the magnitude of rise of GtH was greater at 16 °C than at 8 °C. GtH levels were significantly higher in trout exposed to gradually decreasing photoperiod than in animals kept under constant short or constant long photoperiod at 80 or 16 °C. The data suggest that the optimum photothermal combination for GtH release and induction of spermatogenesis in the rainbow trout is a gradually decreasing photoperiod (from 16 L to 8 L) at 16 °C.