Effects of rearing temperature manipulation on oocyte maturation progress in Japanese eel

During the induction of Japanese eel maturation, administering maturation-inducing steroids (MIS) or their precursors at an inappropriate maturational status is a major cause of poor egg quality. In this study, we investigated the feasibility of controlling oocyte maturation progress by rearing in cold and warm water to manipulate the timing of MIS administration. Mature females with oocytes at the migratory nucleus stage were reared for two terms (3 days to 1 day and 1 day to 0 days before MIS administration) at 20/20 °C, 20/15 °C, 15/20 °C, or 15/15 °C, and the maturational status was monitored based on their lipid droplet morphology and oocyte diameter. Oocytes matured faster at 20 °C than at 15 °C in either term. Next, the mature females were reared at 15 or 20 °C depending on the maturational status of each female 3 days and 1 day before MIS administration; the immature females were reared at 20 °C to accelerate their maturation. Consequently, the maturational status of most females was similar at MIS administration. After improvement, this method would lead most females to the optimum maturational status at MIS administration by properly rearing in cold and warm water.