Evaluation of Marine By-Products in Fishmeal-Free Diets for Juvenile Largemouth Bass (Micropterus salmoides): Insights into Growth, Feed Utilization, Liver Health, and Intestinal Microbiota

The replacement of fishmeal (FM) in aquafeeds for carnivorous fish remains challenging due to reduced palatability and adverse effects on liver health and intestinal microbiota. Marine by-products-based additives containing fish protein hydrolysates and seaweed polysaccharides have shown potential to overcome these limitations. This study evaluated the effects of graded supplementation of Haiweisu (HWS), a multi-marine by-product formulated with squid viscera hydrolysate, small-molecule components from fish protein hydrolysate, seaweed polysaccharides, and seaweed residue as a carrier, in a FM-free diet for juvenile largemouth bass. Four isonitrogenous and isolipidic diets were prepared: a FM-free control diet (CON) and three diets supplemented with 10, 20, or 30 g/kg HWS (designated S10, S20, and S30, respectively). Each diet was fed to triplicate groups of fish (29.26 ± 2.61 g) for 56 days. Results showed that HWS supplementation linearly increased final body weight, weight gain rate, and feed intake, while significantly reducing the feed conversion ratio (p < 0.05). All HWS-supplemented groups exhibited markedly lower hepatic lipid accumulation and plasma total cholesterol levels compared with the CON group, accompanied by alleviated hepatocellular steatosis and inflammatory infiltration as revealed by Oil Red O and H&E staining. Moreover, HWS significantly enhanced intestinal microbiota alpha diversity (Ace, Chao, Sobs, and Shannon indices), decreased the relative abundance of the dominant genus Mesomycoplasma, and enriched potentially beneficial genera including Methylobacterium, Delftia, and Sphingomonas (p < 0.05). In conclusion, dietary HWS supplementation effectively improved growth performance, alleviated hepatic steatosis and inflammation, and beneficially reshaped the intestinal microbiota in juvenile largemouth bass fed a FM-free diet. These findings support HWS as a promising functional additive for sustainable FM-free aquafeeds in carnivorous fish species.