Investigating Alternative Organic Ingredients for Sustainable Meagre (Argyrosomus regius) Aquaculture: Effects on Growth, Gut Microbiota and Plasma Biochemistry

The increasing global demand for sustainable seafood calls for innovative solutions that reduce reliance on marine resources. Organic aquaculture, with its focus on environmental sustainability and animal welfare, represents a promising avenue. However, one of the major constraints to its expansion is the limited availability of certified organic feed ingredients. In this study, we evaluated the feasibility of replacing fishmeal (FM) in the diets of meagre (Argyrosomus regius), a promising aquaculture species, by increasing the inclusion of organic pea protein meal (0%, 7%, 18% and 27.5%), while supplementing all pea-protein enriched diets with fixed levels of functional organic ingredients (5% yeast and 2% brown seaweed). Four isoenergetic diets—C (control), PEA 7%, PEA 18% and PEA 27%—were administered to triplicate groups of 40 juvenile meagre over 85 days. We assessed growth performance, feed utilisation, body composition, plasma biochemistry, and gut microbiota composition. All pea protein inclusion levels (7%–27.5%) showed comparable growth to the control diet, with no significant differences in final body weight (PEA 7%: 173 ± 22.5 g; PEA 18%: 189 ± 11.7 g; PEA 27%: 178 ± 23.7 g vs. control: 171 ± 10.0 g) or feed conversion ratio (FCR), demonstrating successful FM replacement up to 27.5%. The PEA 18% group showed superior lipid utilisation with 109% ± 7.21% gross lipid efficiency (GLE; vs. 75.4% ± 10.8% in PEA 7% and 85.3% ± 5.47% in control) and optimal protein metabolism (albumin [ALB]: 0.71 ± 0.07 g/dL vs. control: 0.62 ± 0.05), indicating enhanced nutrient utilisation at intermediate inclusion. High pea inclusion (27.5%) significantly increased beneficial Lactobacillus (8.74% ± 3.30% vs. control: 4.92% ± 3.96%) while maintaining microbial diversity, suggesting pre-biotic effects without dysbiosis. Overall, the results demonstrated that organic pea protein as well as organic seaweed and yeast can be successfully incorporated into meagre diets without compromising growth, feed efficiency, or fish health. These findings highlight the potential of organic pea meal as a sustainable protein alternative to FM in organic aquafeeds for meagre.