Effects of Attractants on the Growth Performance, Antioxidant Capacity, Immunity, and Histology of Largemouth Bass Larvae ( <i>Micropterus salmoides</i> )

This study evaluated the effect of four attractants on the growth performance, antioxidant capacity, immunity, and histology of largemouth bass larvae (<i>Micropterus salmoides</i>). A total of 75,000 larvae (~1.25 mg) were randomly divided into 15 tanks (five groups with three replicates), with 5000 larvae per tank. The experimental diets were isonitrogenous (54.45% crude protein) and isolipidic (13.20% crude lipid), including a control group (CON, no attractants), taurine (TAU) group (8 g/kg, trimethylamine oxide (TMAO) group (0.25 g/kg), dimethyl-β-propiothetin (DMPT) group (5 g/kg), and nucleotides (NTs) group (0.8 g/kg). The feeding trial was conducted in tanks (40 cm × 60 cm × 80 cm) for 21 days. The larvae were fed with <i>Artemia</i> three times daily for 7 days. The 21-day feeding trial started on the 8th day, alternated with feed six times daily from the 8th to 15th day, then fully transitioned to experimental diets with adjusted feed sizes. The results showed that TAU, DMPT, and NT groups significantly increased the final body weight (FBW), weight gain (WG), and specific growth rate (SGR) (<i>p</i> < 0.05), while TMAO, DMPT, and NT groups significantly increased the survival rate (SR) (<i>p</i> < 0.05), compared with the CON group. During the air stress challenge, compared with the CON group, all treatments significantly prolonged survival time (<i>p</i> < 0.05). Compared with the CON group, the TAU and NT groups significantly increased the activities of superoxide dismutase (SOD), catalase (CAT), and glutathione (GSH) content, and decreased the malondialdehyde (MDA) content (<i>p</i> < 0.05), and the total antioxidant capacity (T-AOC) in the NT group was significantly increased (<i>p</i> < 0.05). All treatments significantly increased the activities of acid phosphatase (ACP) and alkaline phosphatase (AKP; <i>p</i> < 0.05). Compared with the CON group, the histological intestine indicated that all treatments significantly increased the intestinal fold height and decreased the muscular layer thickness, crypt depth, and fold width (<i>p</i> < 0.05); the histological liver indicated that the TMAO and NT groups could improve liver cells. In conclusion, all attractants exerted positive effects on largemouth bass larvae, with TAU, DMPT, and NT demonstrating significant growth enhancement associated with improved antioxidant capacity, immune response, and tissue histology. Further research is needed to clarify mechanisms and optimal doses.