Comparison of Physiological Responses to Plant-Protein- and Carbohydrate-Based Fishmeal Substitutes in Largemouth Bass (Micropterus salmoides)

Developing cost-effective fishmeal alternatives is crucial due to the continuously increasing costs. Carbohydrates are the cheapest alternative, with soybean meal (SM) being the most widely adopted substitute. Nevertheless, few systematic comparisons have addressed their metabolic differences in low-fishmeal feeds. A 60-day rearing experiment examined the physiological responses of Micropterus salmoides to fishmeal replaced by SM versus α-starch (ST). Results showed that dietary SM and ST did not alter the intestine/body length ratio or condition factors. SM-fed fish had a significantly higher final body weight, specific growth rate, and carcass index than those fed ST at 5–15% supplementation. High (≥15%) ST increased hepatopancreas and adipose indices, hepatic and muscular lipids, serum triglyceride, cholesterol, tissue glycogen, and hepatic aspartate aminotransferase and alanine aminotransferase activities. The SM groups possessed higher intestinal digestive enzyme activities, while trypsin increased in the ST groups at 15–20% substitution. Histologically, massive hepatic lipid vacuolation occurred in high-level (≥15%) ST groups. Moderate SM thickened the intestinal muscular layers and widened the villi, while ≥15% SM induced severe intestinal morphological damage and upregulated intestinal inflammation-related gene expression. Moreover, 5–15% SM stimulated the mTOR pathway and amino acid absorption gene expression, while suppressing protein degradation-related gene transcription. Higher (≥15%) ATP/AMP ratios were found in high-level SM groups than in their ST counterparts. Importantly, ST exerted a higher protein efficiency ratio at 15% inclusion. This study systematically characterized the physiological disparities between plant-protein and carbohydrate fishmeal alternatives for M. salmoides, guiding the reduction of physiological costs from different fishmeal replacement strategies.