28 DECEMBER • WORLD AQUACULTURE • WWW.WAS.ORG How Much Phosphorus do Salmon Need to Maintain Fast Growth? One of the benefits of farming Atlantic salmon is their fast growth. Today’s farming takes about 2 years to supply the market with a 5 kg fish. On a large scale this is due to selective breeding, nutrition, and farming advances. On a regional scale there are other parameters that increase speed of growth: optimal temperature and oxygen, longer day length, etc. Salmon growth in length is strongly associated with the extension of the vertebral column. Therefore, we focus on the growth increase in mm per day. Fish grew slower from December to April followed by a phase of a faster growth until July. Slow growth coincided with colder temperatures and shorter daylengths and faster growth was present during longer days with warmer temperatures (Figure 3A). Seeing that fish from all groups had a comparable growth between December-April suggests that even animals on ‘45%P’ diet received enough phosphorus (Figure 3B). Only later in July did we observe that animals fed ‘45%P’ — the lowest amount of phosphorus, started to grow slower than the rest. Diets with ‘75%P’ and higher phosphorus content supported regular development of farmed salmon during the phase of faster growth. Can we however base our dietary phosphorus recommendations purely on growth? Unlikely so. Organisms use phosphorus for growth and maintenance at the expense of bone mineralisation. The health of the vertebral column is critical for active swimming. So, the next step in our study was to determine which diets provide salmon with a robust vertebral column. Have we observed any bone abnormalities? Overall, we noted a low occurrence of deformities on x-ray images consistent across the distinct phosphorus groups. We looked at the number of animals with progressive vertebral fusion and severe compressions, commercially relevant deformities that can lower fillet quality by worsening over time and creating white scar tissue in the muscles (Figure 4). The frequency of these deformities varied between 6-12% with the least number found in animals on ‘75%P’ diet. Admittedly, the process of a severe fusion development is lengthy and can take over 10 months (Witten et al. 2006, Drábiková et al. 2022), while our trial lasted 7 months. So, would it in fact be possible to detect such severe deformities? The answer is yes! Indications that a progressive vertebral fusion is about to develop can be observed on x-ray images up to 6 months before it creates a serious problem (Figure 5, Drábiková et al. 2022). We therefore looked at the inner structures of the vertebral column to see if we could find any changes that predicted future development of deformities. Especially, because all salmon fed the lowest ‘45%P’ diet showed smaller vertebrae on x-rays compared to the rest of the groups (Figure 6). Why Do Vertebrae of Salmon on Phosphorus Deficient Diets Become Smaller, and Does it Impair Health of the Vertebral Column? X-ray images show us the mineralised — radio dense — tissues while the non-mineralised soft tissues remain obscured. Histology reveals both, the tissues without minerals as well hard mineralised TABLE 1. The phosphorus (P) content in the diets containing 57.4% plant protein, 7.5% fish meal, and monoammonium phosphate as a phosphorus source. Available P refers to the portion of the mineral in the diet that is effectively used by the animal and represents one of the results of the study (Drábiková et al. 2026). Availability of P was lower in April (blue values) compared with July (red values). Diet 45%P 75%P 80%P 90%P 100%P 107%P Available P (g/kg) 2.3 – 2.5 3.7 – 4.2 4.1 – 4.6 4.6 – 5.1 5.6 – 5.7 5.8 – 6.1 Comparison with the 45%* 75% 80% 90% 100% 107% currently used diets * Numbers in this row compare the content of phosphorus in our diets to the content of commercially available salmon diets. FIGURE 3. (A) Seawater temperature and day light were steadily increasing from December to July. (B) This was reflected in a slower growth during the first period of the study and a faster one in the second period with ‘45%P’ diet not able to sustain it (modified after Drábiková et al. 2026). FIGURE 4. X-ray image of a salmon with progressive vertebral fusion, severe compressions (red rectangle) that can result in the development of white scar tissue in muscles (black arrow).
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