Effects of photothermal kinetics and amplitude of photoperiod decrease on the induction of the reproduction cycle in female Eurasian perch Perca fluviatilis

This study was designed to determine the effects of both rates of temperature and photoperiod decreases (further referred as photothermal kinetics) and amplitude of photoperiod decrease on the onset of the ovarian cycle in female Eurasian perch Perca fluviatilis. Two photothermal kinetics were applied on 16 groups of Eurasian perch breeders: one rapid (6weeks) and one slow (16weeks). For each photothermal kinetic, the effects of four amplitudes of photophase decrease (0, 1, 4 and 8h) on gonadosomatic index (GSI), oocyte diameter (OD), oocyte stage and plasma oestradiol level (E2) were studied over a 168day-period. Every combination (kinetic*amplitude) was tested in duplicate. For both photothermal kinetics, females did not initiate oogenesis under a constant 17L:7D photoperiod (0h), (GSI≤1%, OD≤273μm, mainly oocytes at primary growth or cortical alveoli stages and E2≤200pg.mL at day 168). By contrast, oogenesis was initiated when a 4hour- or 8hour photophase decrease was applied (GSI=9-10%, OD=620-665μm, oocytes at exogenous vitellogenesis stage and E2=1000-1300pg.mL at day 168). When a one hour photophase decrease was applied, the female responded differently according to the photothermal kinetics. An inhibition (final GSI=1.3% and OD=268μm) was observed under a rapid decrease (6weeks), while a partial and delayed gonad development (final GSI=4.9% and OD=577μm) was recorded under a slow decrease (16week). This observation suggests that this 1-hour amplitude is close to the threshold of sensitivity below which females do not perceive photoperiod variations. This study shows that a high amplitude of photoperiod decrease (4 or 8h) is required to induce oogenesis efficiently and that a constant photoperiod (17L:7D) can be used to control puberty and delay the onset of the reproductive cycle in female Eurasian perch. Finally, for the first time, a combined effect of both photothermal kinetics and amplitude of photophase decrease was identified in female Eurasian perch.