Optimum temperature of rearing water during artificial induction of ovulation in Japanese eel

To obtain fertilized eggs of the Japanese eel Anguilla japonica, mature females are injected with maturation-inducing steroid (MIS) to induce final oocyte maturation and ovulation and then paired with mature males for spawning (induced spawning method) or stripped to collect eggs for insemination (stripping and insemination method). Progress of final oocyte maturation and ovulation and consequent egg quality are affected by the environmental water temperature. To detect the optimum temperature of the rearing water during artificial induction of ovulation by MIS injection, we investigated the effects of water temperature on ovulation, egg quality, and its deterioration due to over-ripening using the stripping and insemination method. Mature females injected with 17α-hydroxyprogesterone were stocked in tanks maintained at 17.5 °C, 20 °C, 22.5 °C or 25 °C. Earlier ovulation occurred at the higher water temperatures. Viability of the eggs that were collected from females immediately after observation of ovulation was similar at 20 °C and 22.5 °C but inferior at 17.5 °C and 25 °C. After eggs are retained in the body cavity for an hour, their viability was increasingly reduced due to over-ripening at the higher temperatures between 20 °C and 25 °C. Moreover, hatchability of unfertilized eggs placed in tubes and maintained at 15 °C, 17.5 °C, 20 °C, 22.5 °C or 25 °C for 3 h showed a time-dependent decrease due to over-ripening, which progressed slower at the lower temperature band between 20 °C and 25 °C. However, 15 °C and 17.5 °C did not delay the progress of over-ripening compared to 20 °C. We conclude that 17.5 °C and 25 °C are not appropriate for ovulation induction, because egg quality just after ovulation is lower than at 20 °C and 22.5 °C. In the stripping and insemination method, 22.5 °C is disadvantageous compared to 20 °C because of the faster progress of over-ripening, though this does not matter in the induced spawning method as spawning behavior and consequent fertilization are expected to occur at an appropriate timing after ovulation.

► Egg quality at ovulation is superior at 20 °C and 22.5 °C than at 17.5 °C and 25 °C. ► Over-ripening after ovulation progresses slower at 20 °C than at 22.5 °C. ► The optimum temperature during ovulation induction of the Japanese eel is 20 °C.