Effects of the thermal threshold and the timing of temperature reduction on the initiation and course of oocyte development in cultured female of Eurasian perch Perca fluviatilis

This study was designed to determine the influence of the thermal threshold and timing of temperature reduction on the initiation of the gonadogenesis under controlled conditions in the females of Eurasian perch. A set of 768 fish was distributed in 16 tanks (48 fish per tank). The photoperiod kinetics was identical in all the treatments (duration = 16 weeks, amplitude of photoperiod decrease = 8 h). The effects of 4 thermal thresholds (22, 18, 14 and 6 °C) in combination with two timing of temperature reduction (0 and 4 weeks between the photoperiodic and thermal reduction, respectively sudden — S or delayed — D), were investigated on the gonadosomatic index (GSI), oocyte diameter (OD), oocyte developmental stage or plasma oestradiol level (E2) during a 140 day period. The results show that all females underwent gonadogenesis whatever the thermal threshold and timing tested. Consequently the photoperiod changes alone allow the induction of reproductive cycle onset even under a high and constant temperature. However, at day 140, the GSI and OD were lower under 22 °C temperature in comparison with other thermal thresholds (GSI = 4%–4.3%) (OD = 577 μm–635 μm) and the oocytes did not exceed stage 3 (late cortical stage). A thermal threshold of 18 °C ensured a good gonadal development (GSI = 7.8%–8% and OD = 673 μm–689 μm) but the oocytes did not exceed stage 4 (early vitellogenesis). The highest GSI were obtained under the temperature of 14 °C whatever the timing of thermal reduction applied (GSI = 8.9%–9.4%; OD = 713 μm–753 μm) and 6 °C with temperature delay (GSI = 8.2%, OD = 727 μm). All the oocytes reached stage 5 (late vitellogenesis) in these treatments. From 22 °C to 14 °C, the thermal timing did not influence gonadogenesis contrary to what was observed at 6 °C, GSI and OD were 6.7% and 637 μm without temperature delay while the GSI and OD were 8.2% and 727 μm with delay. These results show that the temperature only plays a modulator role on the initiation of the gonadal development and suggest that the optimal temperature decrease is in the range of 14–6 °C and that the photoperiod is the signal factor that triggers the initiation of oogenesis.

► We study the effect of temperature reduction on the oogenesis in Eurasian perch.
► The gonadogenesis is induced even with exposure to high and constant temperature.
► The temperature only plays a modulator role on this induction.
► The optimal threshold of temperature decrease is around 6–14 °C.