Effects of partial replacement of fish meal by carob seed germ meal on growth performance and immune parameters of meagre (Argyrosomus regius, Asso, 1801)

Considering the Mediterranean market saturation with species like European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) as well as the high cost and low availability of fish meal (FM), efforts have been made to introduce new candidate species and more economic and sustainable diets in the Mediterranean aquaculture. In this way, meagre (Argyrosomus regius) presents a high potential to be produced at large scale due to its high growth rate and easily adaptation to captivity, in addition to its high nutritional value. Information regarding meagre dietary requirements is scarce. However, some studies suggested that meagre has a high protein requirement (about 50-54%) and a low lipid level (about 12-17%). Several plant protein sources are currently introduced in aquafeeds to reduce dependence on FM, being soybean meal (SBM) among the most used due to its high protein content and relatively balanced amino acids profile. Few studies have been made to introduce carob seed germ meal (CGM) in aquafeeds, a by-product of carob seed processing industry. CGM provides about 45 to 50% of protein, similar to SBM, presents a lower price and it is produced locally in Portugal. Information regarding the effects of CGM on fish performance and immune system are scarce and no studies are available regarding the utilization of this plant protein source on dietary formulations for meagre. Therefore, meagre were fed four experimental diets with graded levels of CGM, including 0% (diet control), 7.5% (diet CGM7.5), 15% (diet CGM15), and 22.5% (diet CGM22.5) to evaluate effects of this particular ingredient on growth performance, feed utilization efficiency, whole-body composition, haematology and cell-mediated and humoral immune parameters of meagre. The trial lasted 8 weeks and the immune parameters were evaluated at weeks 1, 2 and 8. All the experimental diets were well accepted by the animals and no mortality was recorded during the course of the study. The values of feed efficiency, protein efficiency ratio and N retention decreased in fish fed diets with 15 and 22.5% of CGM incorporation compared to fish fed the control diet. However, growth performance was unaffected by dietary CGM inclusion. Haematocrit and mean corpuscular volume decreased from week 1 to week 2, thereafter increasing at week 8. Values of red blood cells counts augmented during feeding trial while total white blood cells showed the opposite pattern, with lower values at the end of the trial compared to week 1. Differential cell counting showed that lymphocytes were the most abundant leucocytes in meagre plasma, followed by thrombocytes, monocytes and neutrophils. The relative proportion of peripheral thrombocytes increased significantly from week 1 to week 2, decreasing at week 8, while lymphocytes followed a different pattern, decreasing from week 1 to week 2, and increasing at the end of the trial. Lymphocytes were also affected by dietary composition, with a significant decrease in fish fed diet with 7.5% of CGM incorporation compared to fish fed other dietary treatments. No significant effects were verified in monocytes and neutrophils proportion. While alternative complement pathway activity in plasma increased significantly in fish fed diets with CGM, plasma total immunoglobulins (Ig), nitric oxide, peroxidase and intestinal bactericidal activities decreased significantly over time. Anti-protease activity augmented from week 1 to week 2 and intestinal total Ig decreased from week 1 to week 2, increasing at the end of the trial. No significant effects were verified in plasma bactericidal activity. Results suggest that GCM incorporation did not induce an inflammatory response. The observed variations among sampling times could be related to the fast growth and development of meagre. Based on these results, it is possible to partially replace FM by CGM up to 7.5%, without compromising growth performance and health status of fish. However, higher incorporations may bring adverse effects on meagre growth and feed utilization efficiency. Due to the limited effects of CGM on meagre immune parameters, results suggest that this vegetable ingredient seems to be a promising alternative protein source to FM. Nevertheless, time-course studies with higher levels of incorporation as well as higher FM replacement are necessary to evaluate the effects of this plant feedstuff in a long-term growth trial. A challenge with pathogenic bacteria may bring further insights to evaluate the resistance of fish fed this alternative protein source during infection by opportunistic pathogens.