Urinary phosphorus excretion in fish: environmental and aquaculture implications

Global aquaculture production has expanded rapidly in recent decades, resulting in a significant increase in effluent discharge rich in phosphorus and nitrogen. In freshwater and brackish water ecosystems, phosphorus is often the primary limiting nutrient. Environmental sustainability of aquaculture, therefore, hinges on reducing phosphorus in its effluents, particularly urinary phosphorus due to its soluble nature, which can directly contribute to eutrophication and harmful algal blooms such as red tides. In contrast, fecal phosphorus, which is primarily insoluble, poses a lower environmental risk. However, much of the past research has focused on improving phosphorus digestibility to reduce fecal phosphorus excretion, while urinary phosphorus has been largely overlooked. Urinary phosphorus represents the excess phosphorus absorbed from the diet. Therefore, dietary strategies should aim to reduce the available phosphorus content in fish feed to just below their nutritional requirements, ensuring that fish growth and health are not compromised. This approach could significantly reduce or even eliminate urinary phosphorus excretion, greatly enhancing the sustainability of aquaculture. This review highlights key aspects of urinary phosphorus and examines various strategies for its reduction, with a focus on practical in situ techniques.