To improve the breeding efficient of fish, it is crucial to provided essential nutrients based on the nutritional demands. Isoleucine is an essential amino acid, and it plays a significant role in growth and metabolism of largemouth bass. Therefore, this study aimed to investigate the effects of dietary isoleucine on the growth and muscle development of largemouth bass (Micropterus salmoides) by conducting an 8-week feeding experiment. Fish (initial weight 19.97 ± 0.04 g) were randomly distributed into six treatments with three replicate groups of 20 fish and fish were fed diets with varying isoleucine levels (1.47%, 1.68%, 1.93%, 2.30%, 2.46%, and 2.72% of dry diet). Results revealed that isoleucine supplementation increased the specific growth rate (SGR) and significantly decreased the feed conversion ratio (FCR). Moreover, histological analysis of the muscle tissue revealed a significant correlation between the size and density of muscle fibers and isoleucine levels. This alteration may be attributed to the downregulation of myostatin and muscle growth-related genes, MyoD and MyoG, respectively. Additionally, fish fed an isoleucine-supplemented diet showed downregulated expression of apoptosis-related (Caspase-3, Caspase-8, and Caspase-9) and ubiquitin-proteasome pathway-related (Ub, Murf1, Mafbx, and Psmc1) genes (1.93%−2.30%). The muscle and plasma total protein contents showed similar patterns of variation to the gene described above. Furthermore, isoleucine supplementation significantly increased body protein levels and decreased body lipid levels. Quadratic regression analysis revealed that the optimal isoleucine levels for juvenile largemouth bass were 1.997% and 1.837% in the dry diet (4.21% and 3.87% of dietary protein) for SGR and FCR, respectively.
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