Evaluation of low-fish meal diets containing animal byproducts and plant protein sources for juvenile olive flounder Paralichthys olivaceus

This study aimed to assess the effectiveness of low-fish meal diets (LFMD), incorporating animal (tankage meal, tuna byproduct meal and poultry byproduct meal) and plant (soy protein concentrate and wheat gluten meal) protein sources as a fish meal (FM) replacement in juvenile olive flounder Paralichthys olivaceus. A control diet (FM70) was prepared to contain 70% of sardine and anchovy FM (1:1, v:v). Six experimental diets were formulated reducing FM levels in FM70 diet down to 35, 30, 25, 20, 15 and 10% (designated as FM35, FM30, FM25, FM20, FM15 and FM10, respectively) with increasing proportions of animal and plant protein sources. Fish (initial mean body weight: 22.3 +/- 0.01 g) were fed each experimental diet for 12 weeks. At the end of the feeding trial, growth performance, survival and biological indices were not affected by dietary FM levels. Feed utilization was significantly decreased in FM15 and FM10 groups than in FM70 group. The gene expression of tumor necrosis factor-alpha (TNF-alpha) was significantly upregulated in FM10 group compared to FM70 group. In muscle fatty acid profiles, a reduction in palmitoleic, eicosapentaenoic and docosahexaenoic acids was observed, while oleic and linoleic acid levels increased with the decrease in dietary FM levels. These results indicate that the combined use of animal and plant protein sources would be a good option as a FM substitute for juvenile olive flounder. Considered with the decrease in feed utilization and upregulation of TNF-alpha gene expression, the dietary FM levels could be reduced only down to 20%. These results could be helpful to research the alternative sources of FM substitutes in feed formulations for carnivorous fishes.