30 DECEMBER • WORLD AQUACULTURE • WWW.WAS.ORG that feeding salmon more than 80% of the currently used phosphorus level does not improve bone strength. These findings combined with the histological and mineralisation assessment show that the diet reduced in phosphorus by 25% (75%P) worked well during the slow growth period. Later, salmon needed marginally more phosphorus due to faster growth and metabolism. The ‘80%P’ diet met the requirements for proper bone health during this period. A Better Use of Phosphorus Can Reduce Waste Finding the right concentration of phosphorus to fulfil salmon needs while maximising the amount that the fish retain and use is essential to optimise feed formulations. We already discussed that the grow-out phase of salmon requires 20% less phosphorus than currently given. Do the salmon become better in using phosphorus the less we give them? Whether a fish uses a nutrient better or worse depends on a multitude of factors. It can vary with the age, species, or metabolism of the fish, and with water temperature. The source of the nutrient, its concentration, and the rest of the ingredients can also play a part in effective nutrient use. In our study, salmon digested phosphorus better at a larger size, at an increased growth rate, and at higher temperature. In other words, the use of phosphorus in April-July was superior to the use in December-April. The improvement in phosphorus use also depended on its levels in the diet. The less phosphorus in the diet the higher the improvement. The most notable increase in phosphorus use was in animals on ‘75%P’ diet. The increased phosphorus use in diets with a reduced phosphorus content in our study is consistent with others. Sugiura et al. (2000), Koko et al. (2010) and Sambraus et al. (2020) also showed that salmon and rainbow trout (Oncorhynchus mykiss) on low phosphorus diets have an increased capacity to retain phosphorus. We could calculate that phosphorus excretion rose with its increasing dietary content (Figure 10). If salmon farms applied the updated dietary phosphorus recommendations they could decrease their solid faecal and dissolved urinary wastes by 13% and 37%, respectively. This is especially noteworthy since the latter represents a more potent environmental pollutant. These findings highlight the potential of precision nutrition strategies to increase fish’s phosphorus use and reduce waste from sea-cages. Application of Optimised Phosphorus Nutrition in Atlantic Salmon Overall, we looked at how much phosphorus farmed salmon (1.8 – 4.2 kg) kept under natural light in sea-cages need to build healthy skeletons while minimising a wasteful oversupply. At temperatures between 5 – 9 °C salmon required 3.7 g/kg phosphorus (‘75%P’ diet). The requirement increased to 4.6 g/kg phosphorus (‘80%P’ diet) during the warmer period with 7 – 14 °C and faster growth (Drábiková et al. 2026). It is yet to be determined if these phosphorus levels would also suffice the phosphorus demand during the fast salmon growth period between July and October. Modern salmon diets completely rely on phosphorus mineral supply, but this study provides convincing evidence that the currently used phosphorus level of 5.8 g/kg exceeds the requirements during the grow-out phase of salmon. If we target the actual salmon needs we could lower the amount of phosphorus use by 16 – 24%. As observed in this study, such adjustments can translate in a 37% reduction in phosphorus waste over a six month period. Future Opportunities The reduction in phosphorus loading could however begin even earlier in production. Research on trout by Koko et al. (2010) showed promising results with alternating feeding regimes. Trout fed diets with low (2 g/kg) and optimal (3 g/kg) phosphorus in alternating cycles of one to four weeks were compared against a group fed the FIGURE 10. The calculated estimated rate of phosphorus (P) excretion in animals fed different levels of available P. The cross-out areas specify the potential reduction in dissolved and solid P excretion between the currently used P levels in commercial diets (A, B) and the updated requirements based on this study (A’, B’). The statistical significance between diet groups is specified with different lowercase letters. A finding that the newly set requirements of 4.6 g/ kg available P can reduce the dissolved phosphorus excretion, a more potent environmental pollutant, by 37% is especially encouraging. FIGURE 9. The texture analyser (I.) is used to compress vertebral centra (white arrow) with a slowly downward moving piston (green arrow) (I.-III.). III. Shows a complete compression of a vertebra which was fractured and permanently damaged due to excessive mechanical stress. The measurements of the texture analyser are simultaneously recorded, and specific parameters are then acquired from the load-deformation and stress-strain curves (panel IV.) (modified after Drábiková et al. 2026).
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