Toward Sustainable Aquaculture: Microalgae Fully Replace Fishmeal and Fish Oil in Rainbow Trout Diets While Maintaining Growth, Nutritional Quality, and Cost Viability

As global demand for affordable, high-quality protein continues to rise, commercial aquaculture has emerged as a critical solution. However, the industry's reliance on ocean-derived fishmeal (FM) and fish oil (FO) for aquafeed poses sustainability concerns. Marine microalgae offer a promising alternative due to their comparable nutrient profiles and potential for large-scale, sustainable production. In this study, we conducted a nutritional feeding experiment with juvenile rainbow trout to evaluate the efficacy of fish-free, microalgae-based diets formulated with protein-rich Nannochloropsis oculata (defatted biomass) and DHA- and antioxidant-rich Schizochytrium sp. (either whole-cell or oil), combined with canola oil as replacements for FM and FO. Diets included a reference diet and three experimental diets with partial or full FMFO replacements: 75% inclusion of N. oculata and Schizochytrium whole-cell (NSW75), 100% inclusion of both (NSW100), and 100% inclusion of N. oculata with Schizochytrium oil (NSO100). The fully fish-free diet, NSW100, supported growth, feed conversion, and survival rates comparable to the FMFO control. Whole-body fatty acid profiles reflected dietary inclusion levels, with similar total n-3 long-chain polyunsaturated fatty acids (LC-PUFA), including DHA, for fish fed the microalgal and reference diets but reduced EPA in fish fed the microalgal diets. No significant differences were observed across treatments in amino acid profiles, macronutrients, or mineral deposition. Notably, cost analysis revealed that fish-free diets had the lowest values, though not significantly, for formulated feed costs and economic conversion ratios (ECR), highlighting their commercial potential. These findings demonstrate that microalgae-based aquafeeds combining N. oculata and Schizochytrium sp. can viably replace FMFO without compromising fish performance, nutritional quality, or production economics—marking a key advance towards more sustainable aquaculture.