참조기(Larimichthys polyactis)의 번식생리 특성에 기반한 조기 성숙 유도와 성 분화 연구

Abstract

This research investigated 1) analyzed the characteristics of cultured small yellow croaker, Larimichthys polyactis, reproductive biology, 2) evaluated water temperature and photoperiod conditions effective for inducing sexual maturity, 3) induced early breeding through environmental manipulation, and 4) induced feminization of larvae. Results on the reproductive biology of L. polyactis indicated that gonad development, increased plasma sex steroid hormone (E2, 11-KT) concentrations, and increased GtHs expression (FSHβ, LHβ) in the pituitary gland occurred between February and April when the water temperature and photoperiod increased. Additionally, their gonads degenerated with the decrease in plasma sex steroid hormone (E2, 11-KT) concentrations and GtHs expression (FSHβ, LHβ) in the pituitary gland between June and October when photoperiod decreased. Consequently, it was determined that the sexual maturity of L. polyactis moderated by the hypothalamus-pituitary gland-gonad (HPG) axis, which is affected by environmental factors. For females and males, E2 and 11-KT, respectively, played a major role in gonadal development. L. polyactis that were bred indoors spawned naturally over 118 days from March 28 to July 23. To study the range of environmental factors for artificially inducing sexual maturity of L. polyactis, this research comparatively analyzed photoperiod and water temperature variations and their effects on sexual maturity. The results of the impacts from different photoperiods showed that a long photoperiod (14L/10D), increased plasma sex steroid hormone (E2, 11-KT) concentrations and promoted gonad development. The results of the impacts from varied water temperatures showed that a water temperature of 17℃, increased plasma sex steroid hormone (E2, 11-KT) concentrations and promoted gonadal development, but females did not reach final oocyte maturation, and spawning was not induced. Compositely moderating the water temperature and photoperiod promoted sexual maturity in males and female compared to the initial group due to water temperature exposure of 17℃ or less after day 60 of the experiment. On days 60 and 90 of the experiment, increased plasma sex steroid hormone (E2, 11-KT) concentrations promoted gonadal development in both males and females under long photoperiod and increasing water temperature. Additionally, the gonadosomatic index (GSI) values were higher than when conditions for water temperature and photoperiod were moderated separately. Therefore, when the water temperature drops to 17℃ in both male and female, initiated gonad recrudescence, Once sexual maturity was initiated, the elongation of the photoperiod and rise in water temperature promoted sexual maturity. Based on these results, an environmental moderation program was developed to induce early artificial sexual maturity and spawning for L. polyactis breeding. The results were successful, and shortened the period to 2 months from April’s natural spawning period. Results also provided insightful data on the period when sex determination and sex differentiation of L. polyactis is complete. The primitive gonad was formed on day 20 after hatching, but sex determination and sex differentiation was not initiated until at least day 50. On day 60 after hatching, sex determination and sex differentiation was complete. Thus, the period of sex determination and sex differentiation was between days 50 and 60 after hatching. By varying the water temperature during rearing, the impact of water temperature on sex determination and sex differentiation was evident; the rate of females was 76.7±2.9% in a water temperature of 17℃, 60.8±5.2% in a water temperature of 21℃, and 50.0±5.0% in a water temperature of 25℃. Therefore, the rate of females was greater in low water temperatures. These results support improving the economic efficiency and productivity of L. polyactis aquaculture by increasing the rate of females, reducing rearing period, and enabling planned breeding.