Introduction
Pacific white shrimp is the most economically valuable shrimp species in the world. Fish oil serves as a primary source of essential fatty acids such as docosahexaenoic acid (DHA) and eicosapentaenoic acid which play critical roles in supporting growth, immune function, and disease resistance in shrimp. However, the decrease in fish stocks and the rise in global demand of fish oil are driving up feed prices and threatening marine ecosystems, underscoring the need for sustainable alternative resources. Schizochytrium sp. (SHC), a marine microalga rich in DHA, has emerged as a promising substitute for fish oil. SCH demonstrates environmental sustainability and ensures stable large-scale production and supply, with the distinct advantage of not competing with food sources intended for human consumption. This study was conducted to evaluate the effects of dietary fish oil replacement with SCH oil on the growth performance, innate immunity, antioxidant capacity, and disease resistance of P. vannamei.
Material and methods
A control diet (Con) was formulated to contain 10% fish meal (anchovy and tuna byproduct) and 4% cod liver oil. The experimental diets were prepared by replacing the fish oil in Con diet with SCH oil and soybean oil at graded levels of 12.5%, 25%, 37.5%, 50%, 62.5%, 75%, 87.5% and 100% (designated as S12.5, S25, S37.5, S50, S62.5, S75, S87.5 and S100, respectively). Total 945 shrimp (0.09 ± 0.01 g) were randomly stocked into 27 tanks (240 L) with 35 shrimp per tank. Triplicate groups of shrimp were fed one of the diets four times daily for 56 days. After the feeding trial, each dietary treatment were randomly stocked into 27 tanks (120 L) with 18 shrimp per tank in triplicate groups for a challenge test. Vibrio parahaemolyticus suspension was added to each tank to achieve a final bacterial concentration of 3.5×105 CFU/mL. The challenge test was carried out over a period of 10 days (240 h) monitoring the mortality hourly.
Results
Growth performance (final body weight, weight gain and specific growth rate) and feed conversion ratio were significantly improved in all SCH oil groups than in Con group. Lipase activity was significantly higher in shrimp fed S50, S62.5, S75, S87.5 and S100 diets than in those fed Con diet. Fatty acid oxidation activity was significantly higher in shrimp fed S87.5 and S100 diets than in shrimp fed Con and S12.5 diets. Superoxide dismutase activity was significantly higher in shrimp fed S25 to S100 diets than in shrimp fed Con and S12.5 diets. Catalase activity was significantly higher in all SCH-supplemented groups compared to that of Con group. Malondialdehyde levels were significantly lower in shrimp fed S12.5 to S87.5 diets than in those fed Con diet. Total antioxidant capacity was significantly higher in shrimp fed S50, S62.5 and S87.5 diets than in shrimp fed Con diet. Triglyceride and aspartate aminotransferase levels were significantly lower in all SCH-supplemented groups compared to those in Con group. The proportion of Vibrio spp. on TCBS agar was significantly lower in shrimp fed S50 to S100 diets than in shrimp fed Con diet. Total hemocyte count was significantly higher in shrimp fed S25 and S37.5 diets compared to those in Con group. The Hyalinocyte count was significantly higher in shrimp fed S25, S37.5, S50 and S75 diets compared to that in Con group. During the V. parahaemolyticus challenge test, Con group showed significantly lower survival compared to S12.5 to S87.5 groups.
Conclusion
Dietary fish oil replacement with SCH oil and soybean oil had positive effects on growth performance, feed efficiency, digestive enzyme activity, antioxidant capacity, immune responses, gut microbiota balance and disease resistance in P. vannamei. Based on quadratic regression analysis of weight gain, the optimum replacement level of fish oil by SCH oil in shrimp diet was estimated to be approximately 70%.