Studies On The Out-of-Season And In-Season Artificial Spawning And Sexual Size Dimorphism In Largemouth Bass Micropterus Salmoides

The annual price of largemouth bass（Micropterus salmoides）fluctuates strongly,and the price from June to September can be twice as high as that in other months.Out-of-season spawning can improve huge economic benefits,but artificial controllable out-of-season spawning technology has not been reported.Largemouth bass is a kind of fish with asynchronous oocyte development,and its production and scientific research work are based on natural spawning.The immature artificial spawning technology hinders the application of hybrid breeding and polyploid breeding.In addition,the sexual size dimorphism accompanying by the gonad development of female fish brings a series of thorny problems to the industry.Therefore,this study focuses on artificial controllable out-of-season spawning,artificial induced-spawning technology,and the relationship between sexual size dimorphism and gonadal development.The main research findings are as follows:（1）The largemouth bass treated by 18NP（18℃overwintering water temperature,natural photoperiod）,18CP（18℃overwintering water temperature,controlled photoperiod）,and 16CP（16℃overwintering water temperature,controlled photoperiod）could successfully spawn from November to December,and there was no significant difference in relative fecundity and fertilization rate among the treatment groups.The results of natural and out-of-season spawning effects showed that the fertilization rate of each treatment group in out-of-season spawning was higher than that of natural spawning.The results indicated that out-of-season spawning could be successfully carried out under the condition of 18NP at a low cost.（2）The annual sexual size dimorphism of largemouth bass was analyzed under indoor small water culture conditions.It was found that at the age of 240 days post-hatching（21.93±1.81 cm,154.33±49.94 g）,the weight of male fish was significantly larger than that of female fish,and the weight of the males（384.10±13.53 g）was significantly higher than that of the females（328.54±15.48 g）by 16.91%at 360 DPH.At the early stage when the growth of female fish was significantly slower than that of male fish（150 days post-hatching and 180 days post-hatching）,it was observed that the volume of oocytes in cortical alveoli stage and vitellogenesis stage increased significantly in the ovaries.The ratio of 11-KT/17β-E₂ of male fish was highest at 150 days post-hatching,and the ratio of 17β-E₂/11-KT of female fish was highest at 180 days post-hatching,which indicates that the sexual size dimorphism was closely related to the gonad development of male and female fish.We speculated that the large energy consumption for ovarian development was the main reason for the sexual size dimorphism.（3）The 10 different artificial induced-spawning schemes indicated that two-injection protocol（2 ug/kg LHRH-A2+1000 IU/kg HCG,25 ug/kg LHRH-A2,5 mg/kg DOM）can effectively reduce the effect time and realize in vitro fertilization of broodstocks at 2years old,and the oosperms obtained from artificial insemination had the characteristics of normal development.In this study,the artificial controllable out-of-season spawning of largemouth bass was successfully realized.This study provides important technical guidance for factory out-of-season spawning of largemouth bass,provides theoretical basis and solution for the relationship between sexual size dimorphism and gonad development and the accompanying problems,and lays a practical foundation for hybrid breeding and polyploid breeding.