Toward environmentally sustainable aquafeeds

Aquaculture is the fastest growing food production sector and currently supplies almost 50% of fish for human consumption worldwide. There are significant barriers to the continued growth of industrial aquaculture, including high production costs and harmful environmental impacts associated with the production of aquaculture feed. Most commercial aquaculture feeds are based on fish meal, fish oil, and terrestrial plant ingredients, which contain indigestible forms of phosphorus. Phosphorus loading from aquaculture effluent can lead to eutrophication in aquatic ecosystems. Formulating fish feeds using ingredients that contain highly bioavailable forms of phosphorus in nutritionally appropriate quantities will reduce phosphorus loading. Using both in vivo and in vitro experiments, we examined the digestibility of phosphorus in three experimental tilapia feeds supplemented with two freshwater microalgae (Spirulina sp., Chlorella sp.) and one marine microalga, Schizochytrium sp., relative to a reference diet containing fish meal and fish oil. We also calculated a phosphorus budget to quantify metabolic phosphorus waste outputs. The marine Schizochytrium-supplemented diet had the highest phosphorus digestibility and the lowest solid phosphorus discharge compared to the reference diet and the other experimental diets. The Schizochytrium ingredient also had the highest phosphorus digestibility among the three microalgae tested in vitro experiments. These results suggest that Schizochytrium sp. is a highly digestible source of phosphorus and findings on metabolic phosphorus waste outputs have implications for the formulation of sustainable diets for tilapia. Further research must examine the economic feasibility and environmental impacts of producing Schizochytrium sp. as an aquafeed ingredient.