Potential use of meat and bone meal in diets for gilthead seabream (Sparus aurata) juveniles

Fishmeal (FM) has been the preferred protein source for aquafeeds, in particular for carnivorous species. However, current FM inclusion levels threaten the expansion of the intensive production of those species. In this context, research has been focusing on evaluating more cost-effective and sustainable alternative ingredients to FM. Meat and bone meal (MBM) is a desirable product for carnivorous fish diets as it generally possesses a high protein content, relatively balanced amino acid profile, high digestibility and palatability and lacks anti-nutritional factors. Also, it is produced worldwide with a steady availability. Recent unban of the use of these ingredients inside the European Union emphasizes the necessity to evaluate it as new potential ingredient for FM replacement. Gilthead seabream (Sparus aurata) is an important economic species in Mediterranean aquaculture but overproduction, associated with increasing price of feeds, led to a decrease in profitability of the intensive production of this species. Therefore, present study aimed to evaluate FM replacement with MBM on growth, digestibility, feed efficiency utilization and gut microbiota of gilthead seabream juveniles. Three experimental diets were formulated (45% CP; 20% CL): a control diet (FM100), with FM as the main protein source, and MBM50 and MBM75 where FM was replaced at 50% and 75%, respectively. Triplicate groups of juvenile gilthead seabream (25 ± 0.72 g) were fed for 83 days with the experimental diets. A 50% substitution did not significantly affect growth (DGI of 2.48 and 2.51 for FM100 and MBM50 diets, respectively), feed utilization efficiency (FCR of 1.51 and 1.53; PER of 1.51 and 1.50, for FM100 and MBM50 diets, respectively). However, a 75% substitution led to a significant decrease on growth rate (DGI of 2.25) and feed utilization (FCR of 1.72; PER of 1.29), although feed intake (g kg ABW-1 day-1) was significantly higher (26.1 compared to 24.2 for diet MBM50). Whole-body composition was mostly unaffected by the experimental diets with the exception of lipid and energy content, which were significantly lower in fish fed the diet MBM75. Protein and essential amino acid retention were unaffected by the experimental diets while lipid and energy retention were significantly reduced with the increase of FM substitution. Crude protein digestibility was high (>89%) and unaffected by the experimental diets while energy digestibility was significantly higher for diet MBM50 (95.2 %), compared to the control (82 %). ADCs of essential amino acids were high (>92%) for all experimental diets and statistically similar or higher for diet MBM50, compared to the control diet, but lower for MBM75 when compared to MBM50. FCUP Potential use of meat and bone meal in diets for gilthead seabream (Sparus aurata) juveniles 2 MBM significantly modulated gastrointestinal microbiota with a decrease in operational taxonomic units (OTUs) and species richness but an increase in replicate similarity with increasing MBM inclusion rate. MBM appeared to promote the development of Vibrio, Bacillus and Mycobacterium genera while colonization by Staphylococcus and Corynebacterium genera appeared to decrease. Overall, results indicate that half of FM could be replaced by MBM, in diets for gilthead seabream juveniles, without compromising growth performance and feed utilization with good results in nutrient and EAA digestibility and retention. Further studies are required to study fatty acid profile, digestibility and retention, and the effect of dietary MBM inclusion on general intestine health fish, fish wellbeing and immune status as well as on flesh quality traits of gilthead seabream.