Yellowtail kingfish farming is growing in Europe with high emphasis on recirculating aquaculture systems (RAS), while in Australia, Japan and Latin America the industry continues to expand predominantly through sea-cage farming. It is a carnivorous species that requires high dietary protein, and to date, fishmeal (FM) -rich feeds are common to support its rapid growth. However, this brings the challenge of high phosphorus (P) content on these diets which could be poorly retained given a more complex matrix bone-bound P, resulting in significant P excretion and discharge. The latter continues to be of environmental concern and stricter regulations governing P release to the environment are arising across the globe regardless of culture systems. For instance, Australia is introducing discharge regulations to sea-cage systems to protect the Great Barrier Reef. Similarly, for European RAS yellowtail producers, addressing phosphorus management is now central for achieving both compliance with strict nutrient limits and broader sustainability goals. With this in mind, this study aimed to aid on the management of P in Seriola rearing facilities by investigating the effects of fishmeal inclusion and dietary P levels on performance, phosphorus retention, and effluent discharge.
A seven-week feeding trial was conducted with Yellowtail kingfish (initial weight 65 ± 1.5 g, final weight 340 ± 15.5 g) maintained at a constant temperature of 23 ± 0.2 °C at the BioMar Aquaculture Technology Centre, Hirtshals, Denmark. Four experimental diets were formulated and manufactured to provide two fishmeal (30 and 47%) and two total P levels (16 and 19 g/kg) on a factorial design basis. Results showed that lowering total dietary P significantly improved its retention, while decreasing excretion of ortho-phosphate and total phosphorus rejection by 23 and 19%, respectively. Importantly, fish performance was not affected with lower P in the diet. Similarly, reducing fishmeal also had an effect in the excretion of ortho-phosphate decreasing its discharge by 5.2%; however, no effects were found in total phosphate rejection. Moreover, no interaction was found between the two tested factors. This indicates that to help to mitigate the environmental impact while maintaining fish performance, it is fundamental not only the optimization of total dietary P, but also its bioavailability through the selection of proper sources.