Effects of replacing fish meal with plant-based protein on growth, physiological and biological indices, and intestinal histology in tongue sole, Cynoglossus semilaevis Güntuer

This study was conducted to replace fish meal protein (FM) with plant-based protein in the tongue sole (Cynoglossus semilaevis Günther) diet without affecting growth performance, physiological and biological indices, or intestinal histology. Six plant ingredients (wheat gluten, soy protein concentrate, soybean meal, cottonseed meal, peanut meal, and corn gluten meal) were selected, and eight plant-based diets were formulated to replace 20% (PP20), 30% (PP30), 40% (PP40I, PP40II, PP40III, and PP40IV), 60% (PP60), and 80% (PP80) of FM. Each diet was assigned randomly to triplicate groups of 15 fish [initial weight, (255.21 ± 0.79) g] per 500-L aquarium. The fish were maintained in flow-through aquaria and fed twice daily to apparent satiation for 9 weeks. The results showed that weight gain, specific growth rate, feed efficiency ratio, protein efficiency ratio, protein retention, whole-body crude protein, and plasma triglycerides were not affected by the different plant-based protein diets compared with those of the FM-based diet (P > 0.05). However, whole-body crude lipid and plasma cholesterol levels were negatively correlated with increasing plant-based protein level (P < 0.05). The hepatosomatic index in fish in the PP30 treatment was significantly lower than that in fish in the FM treatment (P < 0.05). In addition, the viscerosomatic index of the P40III diet group was significantly higher than that of the three other 40% FM-substituted diets. The histological results showed that intestinal villus of the hindgut were seriously damaged in the PP80 treatment, whereas they remained intact in the other treatments. According to these results, 20%, 30%, 40%, 60%, and 80% FM can be replaced by plant-based protein without affecting survival, growth performance, or feed efficiency of C. semilaevis. However, replacing 80% of the FM with plant-based protein affected the physiological and histological indices, which would likely reduce growth performance and feed intake in long-term trials. Thus, our results indicate that up to 60% of FM can be replaced by plant-based protein in the tongue sole diet without affecting growth, physiological and biological indices, or intestinal histology.