Evaluation of processed soy ingredients for largemouth bass (Micropterus nigricans) fingerlings on growth performance, intestinal health, and disease susceptibility

Abstract

Largemouth bass is a critically important sportfish in the United States, and cultural practices have led to the potential of this species as a food fish. The aquaculture industry relies heavily on fishmeal (FM) as a source of protein in dietary formulations. Although an excellent protein source, FM is subject to supply shortages, increased demand, and variable pricing. Soy proteins are also considered an excellent protein source, but high inclusions may negatively influence growth performance and health in several aquaculture species. To evaluate the potential of soy proteins in largemouth bass diets, a feeding trial was conducted with largemouth bass fingerlings (initial weight: 6.15 ± 0.11 g) fed four formulated diets. Alongside a basal diet, the experimental diets included a solvent-extracted soybean meal (SBM), soy protein concentrate (SPC), or enzyme-treated soybean meal (ESBM), all replacing 50 % of FM. Endpoint growth results revealed no significant differences in growth parameters, but a difference in feed conversion ratio [(1.14–1.28); P < 0.001]. Qualitative histological differences were not significant for the thickness of the lamina propria, the thickness of connective tissue beneath folds, and vacuolization (P ≥ 0.344). Gene expression of targeted cytokines in the intestine were not significant for il1b, il8, il10, and tgfb (P ≥ 0.429). Additionally, no differences were detected in the alpha diversity of the gut microbiome, richness, evenness, or the Shannon index (P ≥ 0.082). However, significant differences in beta diversity were detected for Bray-Curtis (P = 0.002) and unweighted-UniFrac (P = 0.038). Significant differences in blood sera were found for lysozyme activity, albumin, alkaline phosphate, alanine transaminase, creatine, globulin, and total protein concentrations (P ≤ 0.044). Finally, the largemouth bass were subjected to an in vivo immersion-based challenge with Flavobacterium columnare (isolate ARS-LMB-23–5; 4.98 × 10⁶ colony-forming units (CFU) mL⁻¹ exposure). No significant differences in endpoint cumulative percent mortality (CPM) were detected among dietary treatment-fed largemouth bass [(81.7–100.0 %); P = 0.530], but a difference in Kaplan-Meier survival curves (P < 0.001) was discerned via Mantel-Cox testing. The comparable growth performance observed in this study demonstrates the potential of replacing FM with various soy protein types in largemouth bass fingerlings.