Redesigning Aquafeeds: Insect, Algae, and By-Product Blends Sustain Growth and Nutritional Value in European Sea Bass Under Feeding Constraints

Background: Adopting novel feed ingredients and aligning feeding strategies with these formulations are key to improving aquaculture sustainability. This study assessed the combined effects of alternative protein and lipid sources and feeding regime on growth, nutrient utilization, and body composition of European sea bass (Dicentrarchus labrax) juveniles. Methods: Two isoenergetic and identical digestible protein diets (39%) were formulated: a control (conventional fishmeal/fish oil (FM/FO) and plant proteins, containing 20% FM and 6% FO) and an alternative diet replacing 50% of FM and 25% of vegetable proteins with a blend of poultry by-products, insect meal, and single-cell protein (Corynebacterium glutamicum) and totally replacing fish oil with alternative lipid sources (microalgae and by-product oils). Fish (28 g of initial body weight) were fed for 210 days either to apparent satiety (AS) or under moderate restriction (85% and 65% of AS). The number of fish used was 65 fish per 500 L tank (triplicate for each experimental group). Growth performance, feed conversion, nutrient efficiency ratios, protein retention, and proximate and fatty acid composition were measured. Results: The alternative diet significantly improved growth, feed and nutrient efficiency, and protein retention compared with the control. Whole-body fatty acid profiles of fish fed the alternative diet showed higher contents of nutritionally important fatty acids, including DHA. Restricted feeding at 65% of AS enhanced nutrient efficiency ratios and protein retention relative to 85% and AS, but reduced growth. Feeding to AS produced the highest feed intake and growth but poorer feed conversion and nutrient efficiency. No significant interaction between diet and feeding strategy was observed. Conclusions: Incorporating novel protein and lipid sources can improve sea bass performance and product nutritional value while supporting sustainability. Feeding at ~85% of AS may offer a practical compromise between growth and efficient nutrient utilization.