Early Physiological and Gut Microbiota Responses to Dietary Fishmeal Reduction in Mugil cephalus Under Biofloc Technology

The early intestinal response of Mugil cephalus to fishmeal replacement with alternative protein sources under biofloc technology (BFT) was assessed. Juvenile mullet (82.68 ± 0.05 g) were distributed among three dietary treatments with decreasing fishmeal inclusion levels of 15%, 5%, and 0%, corresponding to FM15, FM5, and FM0, respectively. After 27 days of feeding, intestinal samples were analyzed for microbiota composition, transcriptomic responses and enzymatic activity. The FM0 group exhibited reduced intestinal enzyme activity, whereas no significant differences were observed in terms of gut microbiota among the diets. Furthermore, transcriptomic analysis revealed differentially expressed genes associated with enriched GO terms and KEGG pathways related to lipid metabolism. This response was consistent with the lower mesenteric fat indices and whole-body lipid content observed in fish fed the FM0 diet, suggesting an early metabolic adjustment associated with complete fishmeal replacement and the higher dietary protein content. All diets exhibited high protein efficiency ratios (9.98 ± 2.42) and low feed conversion ratios (0.40 ± 0.17), with no significant differences observed among them. However, complete fishmeal replacement negatively affected intestinal physiology and lipid storage, indicating that these alterations were not fully compensated for by the biofloc system and may not be physiologically sustainable over longer periods. Further studies over an extended period could be carried out to evaluate the long-term effects of these diets.