Evaluation of extruded full-fat soybean as the substitution for fish meal in diets for juvenile Scophthalmus maximus based on growth performance, intestinal microbiota, and aquaculture water quality

Abstract

Extrusion technology is applied to improve the utilization of soy products, providing an alternative to fish meal (FM) as a component of feed. In this study, a 60-day feeding of turbot (Scophthalmus maximus) was conducted to evaluate the effects of the partial substitution of FM with extruded full-fat soybean (EFS) on growth performance, digestion capacities, intestinal microbiota, and aquaculture water quality. Four isonitrogenous (427.95 ± 0.38 g/kg crude protein) and isolipidic (9.80 ± 1.13 g/kg crude lipid) diets were formulated to contain FM with EFS substitution of 0% (EFS 0%), 10% (EFS 10%), 20% (EFS 20%), and 30% (EFS 30%). Each diet was fed twice a day in triplicate to juvenile turbot (initial body weight, 5.35 ± 0.07 g). The results showed that the turbot fed with EFS 30% had significantly lower body weight, specific growth rate, and protein efficiency ratio than turbot with other diets. In addition, the EFS 30% significantly decreased total serum cholesterol, low-density lipoprotein contents, intestinal folds height, and apparent digestibility coefficients of dry matter and crude protein. It also increased pathogenic Cyanobacteria at the phylum level, decreased probiotics, Rhodobacterales, and Pseudomonadales at the order level, and decreased leuconostoc and Psychrobacter at the genus level. Compared with other treatments, the EFS 30% displayed the lowest proportion of nitrogen for growth and the highest fecal nitrogen loss. Furthermore, the water quality indicators such as ammonia nitrogen discharge, total suspended solids, and fine solid accumulation (< 15 μm) reached the peak in the EFS 30% group. In conclusion, EFS is a feasible turbot feed that can substitute up to 20% of FM with no adverse effects on growth performance, digestive function, intestinal microbiota, and aquaculture water quality.