this rapid expansion is intensifying environmental impacts. Poorly digested feed ingredients increase nitrogen and phosphorus emissions in aquaculture effluents. The heavy reliance on ocean-derived forage fish such as anchovies and sardines for fishmeal (FM) and fish oil (FO)—particularly in salmonid aquafeeds, the largest consumers—raises major sustainability concerns. Moreover, feed inputs, including fishmeal, fish oil, and crop-based meals, are among the dominant contributors to environmental impacts across categories. These challenges have driven the search for sustainable alternative aquafeed ingredients. Microalgae-derived proteins and lipids have emerged as promising replacements for FM and FO. We aim to valorize large volumes of microalgal co-products remaining after nutraceutical oil extraction into low-emission, cost-viable feed ingredients for rainbow trout aquaculture. We report a study developing and evaluating low-emission, low-phosphorus (P) aquafeed formulations that replace FM and FO using protein-rich, defatted biomass from Nannochloropsis sp. combined with whole cells of Schizochytrium sp.
We conducted an 82-day feeding trial to evaluate low-phosphorus diets replacing fishmeal and fish oil (FMFO) for effects on growth performance, phosphorus loading, and environmental impacts. We formulated four experimental diets: (1) a conventional fishmeal–fish oil (FMFO) diet with standard P levels; (2) a fish-free diet with standard P levels (lab-developed reference); (3) a fully fish-free diet with reduced phosphorus (low-P fish-free feed); and (4) a fishmeal-free diet with reduced phosphorus (low-P FM-free feed). We then applied the Cruz Aquafeed Sustainability Tool to quantify environmental impacts of the diets, including biotic resource use (BRU), global warming potential (GWP), water use (WU), land use (LU), marine eutrophication potential (MEP), and freshwater eutrophication potential (FEP). Growth performance of rainbow trout fed microalgae-based low-phosphorus diets was comparable to that of fish fed the reference diet, with no significant differences in feed conversion ratio, weight gain, protein efficiency ratio, specific growth rate, or survival. Both the fish-free and fishmeal-free low-phosphorus diets significantly reduced solid phosphorus loading compared to the conventional and fish-free reference diets. Environmental impact assessment indicated that BRU was significantly lower for microalgal-based feeds compared with the reference diet, whereas GWP, WU, LU, FEP, and MEP did not differ significantly among dietary treatments.
Results demonstrate that incorporating N. oculata co-product with Schizochytrium sp. whole cells can fully replace FMFO with a low-P microalgal diet for trout aquaculture, while maintaining growth performance, reducing phosphorus discharge, and achieving GWP comparable to conventional reference feeds.