WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2025 27 other sources including plants. Salmon diets can now be up to 60% plant-based and contain as little as 7% fishmeal. Unfortunately, all plant phosphorus is unavailable to salmon. This means the raw materials used in their diets end up adding to the phosphorus waste rather than fulfilling their requirements. To counter the low phosphorus availability in raw materials, feed formulators add a highly available form of phosphorus mineral (Morales et al. 2018). Although phosphorus can be recovered from animal waste, uneaten feed, or sludge, it is still technologically challenging, expensive, and often out of touch with current regulations. Therefore, feed producers rely on a mined phosphorus mineral with limited sources, with the main ones located in China, Morrocco, USA, and Russia. This compounds the need for a careful management of diet formulations. Yet despite its limited nature, fish farmers often request diets oversupplied with phosphorus by up to 30%, convinced that it supports better growth and bone mineralisation, while preventing the development of bone deformities (Albrektsen et al. 2018, Chatvijitkul et al. 2017, Aas et al. 2022). Indeed, salmon have an increased phosphorus demand during exponential body and bone growth at the initial freshwater stages. A lack of phosphorus in the first feeding fry can manifest through deformed jaw and vertebral column structures later in life (Fjelldal et al. 2016, Fraser et al. 2019). Later, in seawater stages of salmon their growth levels off and so does the need for phosphorus. These changes should be reflected in their feed formulations. One of the negative aspects of phosphorus oversupply is that it binds to calcium and zinc in the intestine making these minerals unavailable for the animal (Porn-Ngam et al. 1993, Vorland et al. 2017). So its oversupply may also inhibit salmon from utilising other important minerals from their diet. Can We Reduce Phosphorus Emissions from SeaCages Without Compromising Salmon Welfare? The current dietary phosphorus recommendations for salmon in sea-cages are based on research from the 1970’s and 1990’s (National Research Council, 2011). Salmon farming has developed a lot since then, with higher growth rates and plant-rich diets. Therefore, we recently went about reassessing the requirements to see how little phosphorus could be included in salmon feed without compromising welfare of salmon grown in sea-cages and simultaneously minimising phosphorus waste from fish farms. This study builds on a decade-long history of research into dietary phosphorus nutrition and skeletal health (Witten et al. 2016, Drábiková 2021, 2023). Over the years, this collaboration has brought together a multidisciplinary team: the Evolutionary Developmental Group at Ghent University with established expertise on analysis of bone on a cellular level, the Reproduction and Developmental Biology Group at the Institute of Marine Research, specializing in bone mechanics and deformity assessment, and the MOWI Feed team, providing knowledge on feed formulation and fish nutrition. In addition, the collaboration with the BiIOSCAR Inserm team at the University of Paris Cité has helped in analysing salmon bone using standard medical imaging techniques such as CT scanning. This study can be found as an open access article in a peer-reviewed Aquaculture journal (Drábiková et al. 2026). A Practical Approach Using Sea-Cages With the aim to replicate practical farming conditions we stocked 2160 salmon in 24 sea-cages (Figure 2). This study took place between December 2022 and July 2023. MOWI Feed’s nutrition experts, Saskia Kröckel and Guido Riesen, formulated 6 diets with varying phosphorus levels (Table 1). Diets were produced by the Norwegian Institute of Food, Fisheries and Aquaculture Research NOFIMA. We refer to the diets as the percentages of phosphorus they contained in respect to the currently used level. As an example ‘45%P’ contained 45% of the phosphorus level found in commercial diets ‘100%P’. (CONTINUED ON PAGE 28) FIGURE 2. Salmon used in this study were reared at the MOWI Feed Research Station, Norway (63°N, 7°E). There were 4 cages per group, each with 90 fish. FIGURE 1. The signs of phosphorus deficiency include curled spines (yellow arrows), vertebral centra (red arrow) with bent inner structures (white arrows), and large areas of non-mineralised bone tissue (orange asterisks) which are normally mineralised as shown in vertebrae of Atlantic salmon fed sufficient level of phosphorus (black asterisks) (modified after Drábiková et al. 2021).
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