EFFECTS OF TEMPERATURE AND PHOTOPERIOD ON OVARIAN DEVELOPMENT IN FEMALE AYU, PLECOGLOSSUS-ALTIVELIS

1. Juvenile female ayu (N = 400), Plecoglossus altivelis, were assigne d to each of four different photoperiod/temperature regimes (16 L/18-d egrees-C, 16 L/24-degrees-C, 8 L/18-degrees-C and 8 L/24-degrees-C) fo r 16 weeks. 2. Mean body weight in the 16 L/18-degrees-C treatment gro up was the highest of the four treatment groups. No significant differ ences between body weights of the 16 L/24-degrees-C, 8 L/18-degrees-C and 8 L/24-degrees-C treatment groups were observed. 3. Ovulation and the highest values of gonadosomatic and hepatosomatic indices, plasma testosterone (T) and estradiol-17beta (E2) occurred in the 8 L/18-degr ees-C group. The levels of plasma T or E2 were inconsistent in order a mong the 16 L/18-degrees-C, 8 L/24-degrees-C and 16 L/24-degrees-C tre atment groups. 4. Peak levels of 17alpha-hydroxyprogesterone occurred in the 8 L/18-degrees-C and 8 L/24-degrees-C groups. 5. The data sugge st that gonadal development in ayu is photoperiod and temperature depe ndent: short photoperiod and cool temperature favour gonadal developme nt.