The inhibitory effect of low temperature on the gonad development and growth of out-of-season largemouth bass broodstock

Abstract:

To explore the effects of low temperature on the gonadal development and growth performance of largemouth bass (Micropterus salmoides), 600 one-year-old largemouth bass with an average weight of 365.2 ± 28.8 g and gonadal development at stage III were randomly divided into 4 groups. Three experimental groups were reared at water temperatures of 12 ± 0.5 °C, 15 ± 0.5 °C and 18 ± 0.5 °C (named T12, T15 and T18), while the control group was reared under natural conditions with a water temperature of 22.5 ± 1.5 °C. Each group had 3 replicates, each containing 50 fish of both sexes. The experimental period was 30 days. During this period, the weight (Wt), hepatosomatic index (HSI), viscerasomatic index (VSI), gonadsomatic index (GSI), and serum sex steroid hormone concentrations of largemouth bass in each group were randomly sampled and measured. Gonadal tissues were collected for histological observation and the expression levels of genes related to gonadal development were detected by RT-PCR. The results showed that the Wt of largemouth bass in T12 and T15 groups was slightly different but significantly lower than that in T18 and the control group (P < 0.01). The HSI and VSI of largemouth bass in the T12 group were significantly lower than those in the other three groups, but there was no significant difference between T15 and T18 groups (P > 0.05). The GSI of largemouth bass in T12 and T15 groups was significantly lower than that in T18 and the control group (P < 0.05). Histological observation of gonadal tissues showed that the testes and ovaries of largemouth bass in the T12 group only developed to stage IV, while about half of the cells in the testes and ovaries of the T15 group developed to stage V. The testes and ovaries of the T18 and control groups basically developed to stage V. The concentrations of testosterone (T) and estradiol (E₂) in male largemouth bass in the T12 and T15 groups were significantly lower than those in the other two groups. The expression levels of male-specific genes dmrt1 and gsdf in the testes and female-specific genes foxl2 and cyp19a1a in the ovaries of the T12 and T15 groups were slightly different but significantly lower than those in the other two groups (P < 0.05). In conclusion, water temperatures below 15 °C can effectively inhibit the gonadal development of largemouth bass. The research results provide basic data for improving the breeding technology of largemouth bass.