Changes in fertilization and hatching rates with time after ovulation induced by 17, 20β-dihydroxy-4-pregnen-3-one in the Japanese eel, Anguilla japonica

Abstract

This study examined the time to ovulation in artificially matured Japanese eel following an injection of 17, 20β-dihydroxy-4-pregnen-3-one (DHP), and changes in fertilization and hatching rates at various times after ovulation. Thirteen females, showing body weight indices (body weight/initial body weight × 100) of 119 ± 2.3% with oocytes at the migratory nucleus stage after 9–12 weekly injections of salmon pituitary extract (20 mg pituitary powder /fish/week), received an injection of salmon pituitary extract (20 mg/fish) followed 24 h later by a DHP injection (2μg/g BW) for induction of ovulation. Eleven of 13 females ovulated within 23 h of the final injection; two, seven, and two females ovulated at 14–17 h, 17–20 h and 20–23 h after the final injection, respectively. The remaining two did not ovulate within 26 h of the injection. Eggs from three out of the 11 ovulated females showed relatively high percent fertility (89.6-39.5%) and hatching rates (47.6-15.3%) when they were fertilized after the first detection of ovulation. Both percent fertilization and hatching rates of eggs retained in the body cavity decreased rapidly within 6–9 h after ovulation. These results indicate that ovulation induced by DHP treatment occurs after a short but well-defined period of time, and that artificial fertilization must be carried out immediately after ovulation in order to obtain good quality eggs.