Abstract:The rapid expansion of the global aquaculture industry continues to drive up the demand for high-quality protein sources in aquatic feeds. As a traditional high-quality ingredient, fishmeal faces supply constraints, with its annual production growth lagging behind the rapid growth of the industry, thereby keeping aquaculture feed costs persistently high. This situation has prompted the search for sustainable and low-cost alternative protein sources. Currently, although various plant-based and animal-based proteins have been identified as potential substitutes for fishmeal, single protein sources often suffer from limitations such as imbalanced amino acid profiles, the presence of antinutritional factors, poor palatability, or susceptibility to oxidation, making it difficult to achieve high-level replacement of fishmeal. In contrast, terrestrial composite proteins, formulated through the scientific blending of multiple protein sources, can effectively compensate for these shortcomings by enabling complementary amino acid profiles and reducing the impact of undesirable factors, thus representing a more promising alternative strategy. Against this background, as one of the major species in freshwater aquaculture in China, the largemouth bass (Micropterus salmoides) has become an ideal model for studying novel protein replacement strategies due to its significant economic and scientific research value. To investigate the feasibility and optimal replacement ratio of a low-cost terrestrial compound protein, composed of chicken meal, meat and bone meal, and black soldier fly protein (ratio 30:65:5), as a substitute for fishmeal in the diet of juvenile largemouth bass (Micropterus salmoides), a control diet (D1) containing 48% fishmeal was formulated. The compound protein was used to replace 16.67% (D2), 33.33% (D3), 50% (D4), and 66.67% (D5) of the fishmeal in the diet, respectively, formulating five groups of isonitrogenous and isolipidic experimental diets. Juvenile largemouth bass with an initial body weight of (3.26 ± 0.05) g were selected for a 9-week indoor feeding trial. The results showed that compared with the D1 group, there were no significant differences in the weight gain rate and specific growth rate among all replacement groups (D2-D5). The D2 group exhibited the highest values, which were significantly higher than those of the D5 group. The viscerosomatic index of the D2 and D4 groups decreased significantly. In terms of digestion, the pyloric caeca protein digestibility and stomach amylase and lipase activities in the D2 group were significantly higher than those in the D1 group. Stomach protein digestibility in the D5 group was significantly improved. The foregut lipase activities in groups D3-D5 also increased significantly. Regarding antioxidant indices, most serum (T-AOC, CAT, GSH) and liver (CAT, SOD, GSH-Px) indices in the D2 group showed no significant difference from the D1 group, but serum SOD activity and liver MDA level decreased. Serum GST and CAT activities and MDA levels increased in groups D3-D5, while liver GST and MDA levels decreased. In terms of muscle quality, the texture properties and conventional nutritional composition of the D2 group showed no significant difference from the D1 group. However, muscle moisture, hardness, and other texture parameters in the D5 group improved significantly, although the protein content decreased. The above results indicate that this terrestrial compound protein can replace up to 50% of the fishmeal (reducing fishmeal inclusion to 24%). Among the tested levels, the 16.67% replacement ratio (40% fishmeal inclusion) demonstrated optimal performance in terms of growth, digestive enzyme activity, and antioxidant homeostasis. This study provides data support for the development of efficient low-fishmeal compound feeds for largemouth bass.
