STUDY ON THE ANNUAL DEVELOPMENT OF OVARY AND OUT-OF-SEASON SPAWNING IN LARGEMOUTH BASS MICROPTERUS SALMOIDES

In recent years, largemouth bass (Micropterus salmoides) aquaculture industry has developed rapidly and it has become an important aquaculture species in China. Due to the influence of climatic conditions and other factors, aquaculture development of the largemouth bass in central China is not as fast as other areas, accompanied by large price fluctuation, shortage of seedling and early sexual maturity, etc. These problems have seriously hindered the development of the largemouth bass industry in the central China region. To solve these problems to some extent, we studied annual changes in the ovarian development of largemouth bass. We analyzed the effects of water temperature and photoperiod change to ovarian development and explored the method of out-of-season spawning of largemouth bass. In the experiment, we used morphology and histology to compare the ovarian development characteristics of largemouth bass. We explored the effects of temperature and sex hormones on the development of gonads in largemouth bass through temperature-manipulation and maturation-promoting.　　In the present work, we found that the annual change of female GSI in the central China region was between 0.63% and 7.95%. During the period from mid-October to early December, when the water temperature dropped from 20.6℃ to 11.0℃, the ovaries began to develop to the stage Ⅲ and overwintered with stage Ⅲ. Largemouth bass initiated spawning in the middle of April and ended at the end of May. The absolute fecundity of about 80% of the female fish before spawning was between 45,000 and 65,000 eggs, but it was significantly affected by the increase of water temperature. About 15% of the mature eggs in the ovaries fail to spawn and degenerate and more than half eggs were not spawned with 4.6% GSI after the end of spawning. There was a significant negative correlation between GSI and MFI or HSI in the female largemouth bass from October to April of the following year, indicating that during this period, the energy stored by the body was transferred to the gonads. In the out-of-season spawning experiment of largemouth bass, we used well water to adjust the water temperature to promote the initiation and development of the gonads of largemouth bass. After 3 months of treatment, the ovaries of the temperature-manipulation group developed to the end of stage Ⅳ, with GSI of 4.06% for females and 0.89% for males, while the ovaries of the control group were in stage Ⅲ, with GSI of 2.52% for females 0.73% for males. Results showed that the gonadal development of largemouth bass can be regulated by temperature manipulation. Finally, we summarized the culture methods and treatment details in out-of-season spawning, to guide large-scale out-of-season spawning of largemouth bass.