Residual concentrations of human chorionic gonadotropin in female whitespotted conger reared under low water temperatures after a single injection

Abstract

To develop methods for inducing maturation of female whitespotted conger, Conger myriaster, a single injection of human chorionic gonadotropin (hCG; 2.5 IU/g body weight, BW) was administered to each of 7 females, which had ovarian oocytes with a larger diameter than 560 μm at a water temperature of 10 °C. Subsequent changes in the serum concentration of hCG and estradiol-17β (E2) and in their BW index [(BW/BW before the injection) × 100] were measured once a week until 9 weeks after the injection was administered. The serum concentration of hCG reached a peak value of 3.68 ± 0.37 IU/ml after one week, followed by a rapid decrease to 4 weeks later and then a gradual decrease thereafter, expressing its lowest value (0.1 ± 0.02 IU/ml) 9 weeks later. A week after administration, the serum E2 concentration rose to 6.1 times that before the administration and remained at that level for 3 weeks. Thereafter, the serum E2 concentration decreased sharply and reached approximately the pre-administration level after 8 weeks. Their BW index increased until 5 weeks after administration at a constant rate to 130% of the weight before injection and then gradually decreased. A booster injection of hCG (0.5 IU/g BW) was administered to each of 4 females exhibiting maximum ovarian oocyte diameter exceeding 850 μm at 9 weeks after the injection, followed by an injection of 17α-hydroxyprogesterone (2 μg/g BW) one day later for induction of ovulation. One of the females ovulated within 67 h of the final injection. These results indicate that a single injection of hCG (2.5 IU/g BW) has effects on promoting the maturation of female whitespotted conger eels for at least one month. In addition, the results suggest the possibility of developing a new method that reduces handling stress for females by increasing the dosing interval by one month or more.