The role of phosphorus in salmon aquaculture
Phosphorus is an essential nutrient for Atlantic salmon (Salmo salar, L.) due to its role in cellular metabolism and bone mineralisation. As the level of phosphorus in water is low and its form in readily available ingredients is often poorly digested, commercial fish feeds are supplemented with highly digestible inorganic phosphorus, a limited and expensive resource. It is therefore essential that salmon are fed only the necessary amounts to not only reduce costs, but to prevent the release of excess phosphorus into the environment. The required dietary phosphorus levels during the grow-out phase of farmed salmon, which is when feeding intensity is highest, remain to be specified. The range of total phosphorus in commercial diets can be relatively high (9.4–3.0 g/kg total P) (Chatvijitkul et al., 2017) compared to the recommendations (8.0–10.0 g/kg total P) (National Research Council, 2011). The Norwegian annual production of Atlantic salmon was 1.5 Mt in 2021 while the average phosphorus emissions from salmon farming reached 16.3 kt (Pandit et al., 2023). With the salmon industry expected to reach 5 Mt annually by 2050, it is essential to ensure proper phosphorus management.
Experimental design
In the current study, sea-cage reared salmon were located at the MOWI Feed Research Station (63°N, 7°E) from December 2022 (1.8 kg), through April (2.8 kg), until July 2023 (4.2 kg). The temperature was 5–9 °C (December-April) and 7–14 °C (April–July). Salmon were fed one of six diets (A–F) formulated to contain a total phosphorus level of either below (Diets A–D, 6.1, 8.0, 8.7, and 9.5 g/kg, respectively), at (Diet E, 10.4 g/kg), or above (Diet F, 11.2 g/kg) the current recommendations. The study involved 24 sea-cages, with 4 replicates per diet and 90 fish per sea-cage. Diets with 57.4% plant protein and 7.5% fish meal were supplemented with mono-ammonium phosphate (MAP) (Diets B–F). The MAP inclusion rate in Diet A was negligible and raw materials were the main source of dietary phosphorus. Response parameters were growth, phosphorus digestibility, phosphorus excretion and several skeletal health indicators, namely bone mineralisation and bone deformities since inadequate or excess dietary phosphorus content can play a role in deformity development. In addition, bone mechanical strength measured as resistance of the vertebra to deformation under stress, was analysed as it is a sensitive biomarker for detecting sub-optimal phosphorus levels.
Growth and bone health
In all diet groups, growth was slower from December–April compared to later in April–July. From December–April, all diet groups grew at comparable rates of about 0.62 mm/day and 0.35% body mass/day. The general growth rate in animals fed Diets B–F increased between April–July to 0.98 mm/day and 0.55% body mass/day. However, during this latter period, Diet A was not able to sustain the same growth rate with the fish growing significantly slower at 0.77 mm/day and 0.47% body mass/day. According to radiological examination there were no dietary differences in the prevalence of deformities except for the occurrence of increased radiolucent spaces due to large areas of non-mineralised bone in those fish fed Diet A. In April, animals fed Diet A already showed a significant reduction in vertebral ash content and mechanical strength by 20% and 39%, respectively, compared to those fed Diets B–F. In July, vertebrae of animals fed Diet B also exhibited a 7% and 18% reduction in ash and mechanical strength, respectively, compared to animals fed Diets C–F.
Phosphorus digestibility and excretion
Phosphorus digestibility was lower in December-April compared to April–July where there was a most notable increase in animals fed Diets B–D. The increased phosphorus digestibility led to an increase in the level of available dietary phosphorus from 2.3, 3.7, 4.1, 4.6, 5.6, and 5.8 g/kg in Diets A–F, respectively, in the period December–April, to 2.5, 4.2, 4.6, 5.1, 5.8, and 6.1 g/kg in April–July.
Conclusions
The levels of available phosphorus required for regular bone mineralisation and mechanical strength were 3.7 g/kg (Diet B) in the period December–April and 4.6 g/kg (Diet C) in April–July. Therefore, there is potential to reduce the total dietary phosphorus content in current commercial feeds, here represented by Diet E (10.4 g/kg), by 16–24%. This will not only reduce feed costs, but may also significantly lower the total phosphorus loss into the environment by estimated 13% reduction in solid phosphorus excretion and 37% reduction in dissolved phosphorus excretion (Fig. 1).
References
Chatvijitkul, S., Boyd, C.E., Davis, D.A., 2017. Nitrogen, phosphorus, and carbon concentrations in some common aquaculture feeds. J World Aqua Soc 49, 477-483
National Research Council, 2011. Nutrient requirements of fish and shrimp. The National Academic Press, Washinghton D.C.
Pandit, A.V., Dittrich, N., Strand, A.V., Lozach, L., Hernández, M.L.H., Reitan K.I., Müller, D.B., 2023. Circular economy for aquatic food systems: insights from a multiscale phosphorus flow analysis in Norway, Front. Sustain. Food Syst. 7:1248984