Assessment of the feasibility of including high levels of rapeseed meal and peanut meal in diets of juvenile crucian carp (Carassius auratus gibelio♀ × Cyprinus carpio♂): Growth, immunity, intestinal morphology, and microflora

• High inclusion of RSM and PNM retarded the growth of crucian carp.
• High inclusion of RSM and PNM induced a mild change in intestinal morphology.
• High inclusion of RSM and PNM disturbed the immunity of crucian carp.
• High inclusion of RSM and PNM enhanced the intestinal secretion of mucus.
• Mucus formed a peritrophic membrane-like structure.

Two experiments were conducted to evaluate the feasibility of including high levels of rapeseed meal (RSM) and peanut meal (PNM) in juvenile crucian carp (Carassius auratus gibelio♀ × Cyprinus carpio♂) diets by investigating the growth performance, immunity, intestinal morphology, and microflora of these animals. Fish meal (FM)-based diets were used as controls in both experiments. In experiment 1, RSM and PNM replaced 250 (RSM25 and PNM25) and 500 (RSM50 and PNM50) g kg− 1 of FM in test diets. In experiment 2, RSM and PNM substituted 500 g kg− 1 of FM in test diets (RSM50′ and PNM50′, respectively). The diets in experiment 2 were isonitrogenous and isocaloric, and supplemental crystalline amino acids were added to compensate for the reduction in essential amino acids. More than 100 g kg− 1 of FM was maintained in the test diets in both experiments to ensure palatability, and fish were fed these diets for 8 weeks. No significant changes in feed intake were noticed among groups in both experiments. In experiment 1, the growth rate (GR) of fish fed test diets was significantly lower than in fish fed control diets; the feed conversion ratios (FCR) were higher in fish fed test diets than in fish fed control diets with the exception of fish fed the PNM25 diet. The lower GR observed in fish fed test diets was partly due to reduced dietary energy and protein levels. No significant differences were observed in the GR of fish in experiment 2; however, the FCR of fish fed the RSM50′ diet increased, whereas the protein efficiency ratio decreased in fish of both test groups, suggesting that growth retardation would have occurred over a longer period. No significant changes were observed in serum superoxide dismutase activity in both experiments. Compared with control groups, the lymphocyte proliferation index (LPI) decreased in fish fed the RSM50 diet but not in those fed the RSM50′ diet; the LPI increased in fish fed the PNM50 and PNM50′ diets. The secretion of mucus, which formed a peritrophic membrane-like structure, was enhanced when diets included high levels of RSM and PNM. Mild intestinal histological changes, but no significant inflammatory responses or intestinal microflora changes, were observed. From these results, we came to the general conclusion that the inclusion of high levels of RSM and PNM in the diet not only retarded growth but also produced health risks to the crucian carp.