DOES DIFFERENT DIETARY FAT CONTENT AFFECT PHOSPHORUS REQUIREMENTS IN SEAWATER ATLANTIC SALMON?

Phosphorus is essential for growth and bone mineralisation. The requirements for phosphorus are higher during increased growth rate. Farmed Atlantic salmon (Salmo salar) showed a fast growth (0.98 mm.day^-1) during warmer water temperatures (7-14 °C) and longer daylight between April-July compared to slower growth (0.62 mm.day^-1) at 5-9 °C and shorter daylight between December-April (Drábiková et al., 2026). Salmon in this study required 3.7 g/kg available phosphorus between December-April and 4.6 g/kg available phosphorus between April-July to achieve regular growth and bone mineralisation (Drábiková et al., 2026). As increased dietary fat is also linked to accelerated growth the current study assesses the dietary phosphorus requirements in seawater stages of farmed Atlantic salmon fed different fat levels.

Materials and Methods

In August 2024, salmon post-smolts (4800 fish) were pit-tagged to follow individual fish growth and deformity development. Fish were measured (fork length was 30 cm and weight 290 g on average), 2909 fish were x-rayed, and all fish were divided among 24 sea-cages (5.2 x 5.2 x 6 m, width x length x depth) located at MOWI Feed Research Station (Aukan), Averøy, Norway. Animals were exposed to natural photoperiod and temperature and fed one of 12 formulated diets. At the start of the trail additional 50 animals were sampled for base-line data.

We tested two dietary fat levels ('low' with 30 % and 'high' with 36 % fat content) and six levels of dietary phosphorus (P1-P6) with increasing monoammonium phosphate supplementation (5.4-13.2 g.kg^-1 total phosphorus (tP) in a randomised 2 x 6 duplicated factorial design. 'Diet P1' contained 5.4-7.2 g.kg^-1 tP, 'Diet P2' had 6.3-8.2 g.kg^-1 tP, 'Diet P3' contained 7.6-9.3 g.kg^-1 tP, 'Diet P4' contained 9.2-12.0 g.kg^-1 tP, 'Diet P5' was formulated according to currently used levels in commercial diets for different salmon stages and contained 8.2-10.4 g.kg^-1 tP, and 'Diet P6' contained 10.3-13.2 g.kg^-1 tP.

Following the start of the trial, salmon were individually measured in December, May, and August with 20, 12, and 33 fish sampled per cage at the respective sampling points. For the purpose of this abstract we will discuss results from the final sampling done in August 2025. The analysis included vertebral deformity assessment using x-ray images, total mineral content (ash) of vertebrae as representatives of endoskeleton, total ash content of scales as representatives of dermal skeleton, and scale mineralisation using whole mount Alizarin red stain. Alizarin red bounds to calcium, i.e. mineralised part of the scale, while the non-mineralised part remains transparent but visible under brightfield light with partly closed aperture diaphragm. Assessment of scales represents a valuable non-invasive tool to assess severity of phosphorus deficiency in fish.

High energy increased salmon growth when combined with 'Diet P2'

Interestingly, at the final sampling in August, salmon fed 'Diet P2 high fat' were significantly longer (67.7 cm on average) and heavier (4200 g on average) compared to salmon fed 'Diet P2 low fat' with an average fork length of 64.9 cm and weight of 3868 g. As a result only 'Diet P2 low fat' fed salmon were significantly shorter and lighter than fish fed 'Diet P3-P4' at both fat levels with an average fork length and weight of 67.8 cm and 4200 g. Irrespective of dietary fat content, Atlantic salmon fed 'Diet P1' showed significantly reduced fork length and weight (58.7 cm, 3046 g for 'low' fat and 59.6 cm, 3020 g for 'high' fat diet groups) compared to salmon fed 'Diets P2-P6' at both fat levels.

Scale mineralisation was challenged into a greater extent than vertebrae mineralisation

At final sampling in August scale ash of salmon fed 'Diets P1-P2' at both fat levels as well as salmon fed 'Diet P3' at low fat level was significantly reduced compared to the remaining diet groups (Figure 1a). In contrast, vertebrae ash was significantly reduced only in salmon fed 'Diets P1-P2' at both fat levels relative to salmon fed 'Diets P4-P6'. Vertebrae ash of salmon fed 'Diet P3' was comparable to 'Diets P4-P6' and to those fed 'Diet P2' at high fat level (Figure 1b). Uniquely, scale mineralisation observed and quantified on whole mount Alizarin red stained scales was reduced not only in salmon fed 'Diet P1' but also in salmon fed 'Diet P2' with low fat content.

Figure 1. Scale (a) and vertebrae (b) ash (%) sampled in August 2025 from salmon fed different dietary phosphorus levels (P1-P6) and either 'low' or 'high' fat content. Different letters indicate statistically significant differences (p<0.05) among diet groups assessed by robust linear mixed effects model in R. About 40 scale were analysed in 6 fish per cage and 1 vertebra was assessed in 9 fish per cage.

What does this mean for skeletal health?

The observed reduction in growth and skeletal mineralisation will be elaborated in respect to the vertebral column health while potential to compensate phosphorus deficiency through the resorption of scales will be considered. We will discuss the possible advantages of either low or high fat levels with varying dietary phosphorus levels for phosphorus digestibility and reduced phosphorus excretion in seawater stages of Atlantic salmon and discuss the effect on dietary phosphorus requirements.

References

Drábiková, L., Kröckel, S., Witten, P.E., Riesen, G., Morris, P., Ostertag, A., Cohen-Solal, M., Fraser, T.W.K., Fjeldal, P.G. (2026) Phosphorus requirements in sea-cage farmed Atlantic salmon with an emphasis on bone health and digestibility. Aquaculture 610 (742915).