Two-year-old, 500–600 g immature and prepuberal male and female rainbow trout were exposed on June 21 to continuous light (CL group) or, as a control, to the natural photoperiod in the Paris area (C group) and were kept under seasonal variation of temperature. Gametogenesis was studied by periodic sampling of the gonads for quantitative analysis of spermatogenesis and measurement of follicle diameter. Gonadotropin (GTH) profiles were measured in pituitary and plasma by radioimmunoassay. During spawning, the quantity and quality of gametes were measured. Under continuous light, gametogenesis and spawning were delayed by about 2 months as compared with the controls kept under a natural photoperiod. Delayed gametogenesis in the fish under continuous light showed some alterations. In the males, the dynamics of spermatogenesis was modified since the proportion of the germ cells in the testis was changed as compared with the controls; e.g., spermatids were present in the testicular lobules when spermiation started. The yield of spermiation measured in four samplings was lower in males under continuous light as compared with the controls. In the females, follicle diameter and gonadosomatic index tended to remain lower during vitellogenesis in the CL group as compared with the C group, but finally, the diameter of the eggs stripped and the fecundity were not different in both groups. GTH profiles were different only for the plasma in December. The pituitary GTH content was low in December in both groups and sexes, irrespective of the stage of the reproductive cycle and of the light environment. There was no difference in hepatosomatic and viscerosomatic indices between the two groups.
Delay of gametogenesis and spawning by constant illumination of rainbow trout (Salmo gairdneri) during the first reproductive cycle