Swine blood hydrolysates as promising ingredients for European seabass (D. labrax) diets

In this work, we hypothesised that peptide-rich swine blood hydrolysates (BH) could be beneficial ingredients for European seabass diets, due to their potential antioxidant, mineralbinding, immunomodulatory or antimicrobial activities. Furthermore, this inclusion could address T. maritimum infections that occur in aquaculture farms, with high impact on fish wellbeing and aquaculture production, while promoting circular economy. Three fractions of swine BH were studied: one obtained by autohydrolysis (AH); two obtained enzymatically and submitted either to a micro- (MF) or nanofiltration (NF). Dried hydrolysates were included in isolipidic and isoproteic diets for European seabass: a fishmeal (FM) based diet (positive control, PC), a commercial-based diet where 50% of FM was replaced by vegetable proteins (negative control, NC) and three diets where 3% of each BH was added to the NC. Diets were assigned to triplicate groups of 71 juveniles (initial weight 12.3±1.4 g), fed three times daily ad libitum in a recirculating saltwater system. Growth, nutrient utilisation and whole-body composition were evaluated after 12 weeks. At the end of the trial, ten fish per tank were infected with Tenacibaculum maritimum (3.5×105 cfu/L), in a two-hour water bath, and mortality was assessed for 8 days. Fish fed PC had the highest final weight, followed by NC and NF. NC had a significantly higher weight than the MF and AH groups. Specific growth rate (SGR) and feed conversion ratio (FCR) of fish fed NF showed no significant differences from PC and NC. Fish fed AH also displayed similar SGR to NF and NC but significantly higher than MF. Despite no differences regarding final body composition, lipid and energy retention and gain were lowest in fish fed MF, which also had the lowest condition factor. Protein retention was unaffected by dietary treatments. Regarding the infectious challenge, NF had the lowest mortality rate, differing significantly from the NC and MF groups. Mortality for AH and PC groups did not differ from NF. MF showed the highest mortality. Overall, results indicate that dietary supplementation with NF improves European seabass resistance to T. maritimum without affecting growth rate and can hence become a valuable ingredient for this species.