Seasonal changes in the pituitary‐ gonad axis of feral goldfish, Carassius auratus L., from Ontario, Canada

Apparently healthy, feral goldfish, Carassius auratus L., were collected from a pond in Cambridge, Ontario from March until November 1981, and during April 1982. Based on histological examination, oocyte development was divided into five stages and the reproductive cycle into five periods. The origins of oogonia and the atresia process were examined. The gonadosomatic index (GSI) was found to be an accurate indicator of ovarian activity in the feral population. Based on the GSI, proportion of various oocyte stages and egg size, females older than 2 years appeared to spawn during two periods, May to June and late August; 2‐year‐old fish spawned in the first period. There was a reduction in the density, abundance and nuclear diameter of presumptive pituitary gonadotrops in early August, after the completion of ovarian development and before the completion of spawning.