Reducing Fish Meal Dependency in Juvenile Yellowtail Diets Using Composite By-Product Protein Mixtures

A 6-week feeding trial evaluated composite protein mixtures as partial replacements for fish meal (FM) in diets for juvenile yellowtail (Seriola quinqueradiata). Five diets were prepared: a control diet (C) with FM as the main protein source, and four test diets in which 25% and 35% of FM protein were replaced by either a composite mixture of SSM (44% shark by-product meal + 56% surimi by-product mixture; SS25 and SS35) or SMM (58% shark by-product meal + 42% other by-product mixture; SM25 and SM35). Fish (initial mean weight 0.85 g) were stocked at 30 fish per 500-L tank, with three replicate tanks per treatment. Growth performance indicators, including final mean weight, percent weight gain, specific growth rate, daily feeding rate, and survival, did not differ significantly among treatments (p > 0.05). Feed efficiency was significantly higher in SS25 and SS35 than in the control (p < 0.05), whereas no significant differences were observed among the by-product-based diets. Whole-body proximate composition was unaffected, except for crude ash. The levels of key n-3 fatty acids, eicosapentaenoic acid and docosahexaenoic acid, were significantly lower in SM25 and SM35 than in the control (p < 0.05). These results indicate that up to 35% of FM protein can be replaced with shark by-product-based mixtures without compromising growth in juvenile yellowtail.