γ-Aminobutyric Acid Effectively Modulate Growth Performance, Physiological Response of Largemouth Bass (Micropterus Salmoides) Under Combined Stress of Flow Velocity and Density

The circular aquaculture model of largemouth bass pond engineering has the characteristics of high yield and efficiency, but it is prone to stress caused by flow velocity and density, which affects the yield of largemouth bass (Micropterus salmoides). γ-Aminobutyric acid (GABA) is believed to have the effect of improving growth and stress tolerance. We divided the largemouth bass into three groups: a control group, a flow rate and density combined stress group, and a combined stress feed supplemented with GABA (0.9%) group, and conducted a 60-day aquaculture experiment. The results showed that the final weight, weight gain rate (WGR), specific growth rate (SGR), and feed efficiency (FE) of largemouth bass significantly decreased in the combined stress group (P < 0.05). The serum aspartate aminotransferase (AST), alanine transaminase (ALT) activity, and glucose (GLU), malondialdehyde (MDA) level of largemouth bass significantly higher than the control group, and the serum lysozyme (LZM) activity and total antioxidant capacity (T-AOC) were significantly lower than the control group (P < 0.05). After adding GABA, the final weight, WGR, SGR, and FE decreased, and the serum GLU levels, AST, ALT activity, and MDA levels were downregulated, and the serum LZM activity and T-AOC of largemouth bass were upregulated. But most of the above are still at the level of the control group. Under combined stress, the messenger ribonucleic acid (mRNA) expression levels of growth hormone (GH), insulin-like growth factor-Ⅰ (IGF-I), B-cell lymphoma-2 (Blc2), nuclear transcription factor E2-related factor 2 (Nrf2), catalase (CAT), and superoxide dismutase (SOD) genes were significantly reduced (P < 0.05), while the mRNA expression levels of heat stress protein 70 (HSP70), heat stress protein 90 (HSP90), nuclear factor kappa-B (NF-κB), interleukin-1β (IL-1β), Bax and keap1 genes were significantly increased (P < 0.05). After the exogenous addition of GABA, all the above genes have a certain degree of callback, but GH, HSP70, HSP90, IL-1β, Bax, Nrf2, CAT, and SOD have not yet reached the level of the control group. These results indicate that adding GABA to feed can alleviate the adverse effects of combined stress of flow rate and density to a certain extent and provide insights for solving the problems in the circular aquaculture model of largemouth bass.