Schizochytrium sp. can improve feed utilization, fillet DHA content, and non-specific immunity of juvenile Nile tilapia (Oreochromis niloticus) fed fish oil free diet

Abstract

An 8-week feeding trial was conducted to evaluate the impact of dietary Schizochytrium sp. on growth, fatty acid profile, non-specific immunity, and histopathology for Nile tilapia (Oreochromis niloticus) juveniles at initial individual weight of 0.88 ± 0.04 g. A control diet consisting of 40 % protein and 7 % lipid (D0, without Schizochytrium sp. supplementation) was used, along with five Schizochytrium sp. supplemented diets formulated at doses of 5 (D1), 10 (D2), 15 (D3), 20 (D4) and 25 g kg⁻¹ (D5), respectively. Each diet was randomly assigned to three replicate aquaria, each stocked with 50 uniform size and weight fish. At the end of the feeding trial it was found that the inclusion of dietary Schizochytrium sp. significantly enhanced the WGR (weight gain rate) and SGR (specific growth rate) of fish, particularly in the D2 group which exhibited the lowest FCR (feed conversion rate) and highest PER (protein efficiency rate). The whole-body crude lipid content decreased with increasing the levels of dietary Schizochytrium sp., while serum levels of TG (triglycerides) and TC (total cholesterol) also decreased concurrently. The DHA content of fillet was significantly up-regulated with the increase of dietary Schizochytrium sp. (P < 0.05). The activities of ALP (alkaline phosphatase), AST (aspartate transaminase) and ALT (alanine transaminase) in serum were found to be lower in Schizochytrium sp. supplemented treatment compared to the control group, with the lowest value observed in D2 group (P < 0.05). There was a positive correlation between serum lysozyme activity and dietary Schizochytrium sp. levels (P < 0.05). The malonaldehyde content initially declined followed by an increase in response to elevated dietary levels of Schizochytrium sp. with comparatively lower levels observed in the D2 group compared to other experimental groups (P < 0.05). Furthermore, the results of liver and intestinal morphology analysis revealed that 1 g kg^-1 dietary Schizochytrium sp. supplementation could maintain hepatocyte integrity and increase villus height in the intestine. Conversely, excessive intake of Schizochytrium sp. (≥2 g kg^-1) led to nuclear migration and vacuolation in hepatocytes. In conclusion, these findings indicate that dietary Schizochytrium sp. can enhance feed utilization, non-specific immunity, antioxidant capacity, as well as liver and intestinal morphology, ultimately improving the nutritional value of fillets, but excessive supplementation may result in liver tissue lesions.