Macroalgal Inclusion in Aquafeeds: A Strategy to Enhance Growth Performance, Feed Efficiency, and Immune Resilience in Mediterranean Teleost Fish

The rapid intensification of global aquaculture has led to an urgent need for sustainable alternatives to conventional protein sources like fishmeal, which is increasingly scarce and expensive. Macroalgae have emerged as a premier candidate for blue sustainable growth due to their rapid growth rates, lack of requirement for arable land, and rich profile of bioactive compounds. This study investigates the potential of a macroalgal blend, incorporating Ulva lactuca and Gracilaria gracilis, as a functional ingredient in the diet of juvenile Mediterranean species, such as Sparus aurata. A 90-day feeding trial was conducted to evaluate three experimental diets with varying inclusion levels of macroalgal meal: 0% (Control), 5%, and 10%. Growth performance was monitored bi-weekly, and at the end of the trial, blood and tissue samples were collected for physiological analysis. Our results demonstrated that the 5% inclusion level significantly optimized growth, showing a 14.5% increase in Specific Growth Rate (SGR) compared to the control group. Furthermore, the Feed Conversion Ratio (FCR) improved by 9%, dropping from 1.42 to 1.29 (p < 0.05), which indicates superior nutrient utilization. Beyond growth, the macroalgal diet triggered a robust systemic immune response. Serum lysozyme activity, a key marker of innate immunity, increased by 22%, while phagocytic activity in head-kidney leucocytes was significantly upregulated. Additionally, liver antioxidant capacity was enhanced, as evidenced by a reduction in lipid peroxidation markers. These findings suggest that low-level macroalgal inclusion serves not only as a nutritional substitute but also as a powerful nutraceutical. Such innovations are crucial for reducing the ecological footprint of aquaculture while ensuring the health and welfare of farmed aquatic species in a changing environment