The reproductive cycle of male and female brown trout (Salmo trutta fario): a quantitative study

International audience; Gonadal development and gametogenesis have been studied in the brown trout ( Salmo trutta) beginning at the establishment of the first reproductive cycle. Monthly samples were taken from 14-month old fishes beginning in July and extending over a 20-month period. Complete gametogenesis was observed in males from the first year of sampling and from the following year in females. Spermatogenesis was analysed over the first two cycles and a quantitative study showed that the volume of spermatozoa produced in relation to body weight was slightly higher at the end of the second cycle. At the end of the second reproductive cycle, the process of spermiation ceased while there were still spermatozoa in the testes, although most of them had been evacuated at the end of the first cycle. Spermatogenesis seemed to be more efficient during the second cycle but spermiation yield was higher during the first cycle. Oogonia were always present in the female ovaries and there was mitotic activity in July of the second year of life and in October-November of the third year. Previtellogenetic stages (accumulation of RNA and lipid) were already present at 14 months of age when sampling started. During the second year of life there was limited accumulation of yolk in the oocytes but it did not modify total ovary weight. In this population, vitellogenesis was complete and most of the females began to ovulate only in the third year. Before the end of the cycle in October-November, the number of previtellogenetic oocytes (stages 1, 2 and 3) increased markedly. This was followed in December by an increase in the number of oocytes containing yolk bodies, representing no doubt the class of oocytes undertaking vitellogenesis in the next cycle. the patterns of initiation of the first and second cycles thus appeared to be very different.