Locally-sourced animal protein hydrolysates in high-plant-protein diets can promote European seabass growth and nutrient utilization, reducing reliance on fishmeal

Upcycling meat and fish by-products into bioactive protein hydrolysates promotes zero-waste practices within the circular bioeconomy and provides locally-sourced alternatives to replace fishmeal (FM) in aquafeeds. In this study, novel hydrolysates were developed from blue shark skin, fish by-products, and swine processed animal protein, aiming to reduce the high-quality FM inclusion in European seabass (Dicentrarchus labrax) diets. Four isoproteic (53%) and isolipidic (18%) plant-based diets were formulated: a control (CTRL) diet containing 12.5% FM was compared to three experimental diets, including 3% of each hydrolysate at the expense of FM-protein: SHARK, FISH, and SWINE. Seabass juveniles (13 g) stocked in 160 L tanks (3.9 kg/m3) were fed the diets in triplicate, three times daily until satiety, for 89 days. All diets were equally well accepted, promoting similar feed intake, daily growth index (1.6), and efficient feed conversion ratio (1.0–1.1). All fish at least quadrupled their initial size, but those fed the SWINE diet showed the highest body weight (55 g) and significantly higher condition factor (1.22), hepatosomatic index (HSI) (1.6), and viscerosomatic index (VSI) (8.4). Intestinal integrity remained similar across treatments, and apparent digestibility coefficients (ADCs) of protein and fat were above 96%. Iron (Fe) ADC was at least doubled with hydrolysates inclusion. Whole-body and muscle composition were similar across groups. However, the muscle metabolome of the SHARK-fed fish exhibited higher levels of glycolytic intermediates and lower levels of glucogenic amino acids (AAs) and fumarate, suggesting an increased catabolic activity. However, these changes were not reflected in fish growth performance or muscle flesh quality, which remained similar among treatments.