Soybean protein concentrate as a fishmeal replacer in weaning diets for common sole (Solea solea) post-larvae: Effects on the growth, biochemical and oxidative stress biomarkers, and histopathological investigations

An experiment was conducted to evaluate the use of soybean protein concentrate (SPC) meal as a partial replacement for fishmeal (FM) protein at levels of 0%, 25%, 50%, 75%, and 100% (SPC0, SPC25, SPC50, SPC75, and SPC100 groups, respectively) in weaning diets (similar to 55% crude protein) for post-larvae of common sole (Solea solea). During the weaning period, larvae were fed on the experimental diets simultaneously with the life food for 15 days. After completing the weaning period, post-larvae (5.5 mg) were fed with the experimental diets for further 30 days in triplicates. No significant (P > 0.05) variations were recorded in the growth performance and survival rate up to the 50% SPC level, after which the larvae growth and survival rates were significantly (P < 0.05) declined. A gradual decrease in carcass protein simultaneously with significant increases in carcass lipid, triglyceride, and low-density lipoprotein cholesterol levels was observed as SPC levels in diets increased. SPC induced significant (P < 0.05) increases in aspartate and alanine aminotransferases activities, and their highest values were observed in the SPC75 and SPC100 groups. The oxidative stress biomarkers (superoxide dismutase and catalase as well as reduced glutathione and malondialdehyde levels) showed their highest values in postlarvae fed the SPC100 diet. Histological investigations showed esophagus inflammation and stomach hyperplasia in SPC75 and SPC100 groups with no pathological intestinal signs. Hepatocyte infiltration and massive lipid accumulations with signs of necrosis were recorded in fish fed with 75% and 100% SPC diets. The second-order polynomial regression between the inclusion levels of SPC and growth indices recommends replacing FM protein with 35% SPC in weaning diets for common sole post-larvae.