Annual development cycle of gonads of Eurasian ruffe (Gymnocephalus cernuus L.) females from lower Odra River sections differing in the influence of cooling water

Abstract

The subject of this study was gonadal development of Eurasian ruffe females collected at three sites: the Regalica River carrying natural waters of the Odra River, Warm Canal to which cooling water from a power plant is discharged, and Lake Dąbie in which the waters from East Odra and Warm Canal are mixed. According to histological analysis, under the natural conditions in the lower Odra River, ruffe's spawning season usually begins in April and lasts till the end of May. The fish from Lake Dąbie started their spawning season in a similar period, while the fish from Warm Canal started their spawning season in March, a month earlier, and also finished their spawning season earlier. The size of oocytes from the ruffe collected in the Warm Canal at the beginning stage of vitellogenesis was smaller (mean diameter of 184.3 μm) and at the end of vitellogenesis larger (755.3 μm) than the size of oocytes from the fish collected at the other two sites (p < 0.05), although their sizes were within the typical range. In the gonads of the fish caught from June to September at all the sites, the presence of degenerating oocytes was confirmed. Folliculi atresia at the beginning of summer was observed in the last not-laid portion of oocytes, while the degeneration of a part of newly produced vitellogenic September oocytes was caused by a high temperature of the lower Odra River water, of over 20°C at that time. Both types of oocyte degeneration are the natural adaptations to the conditions of a changing natural environment.