Phosphorus requirements in sea-cage farmed Atlantic salmon with an emphasis on bone health and digestibility

Commercial fish feeds are supplemented with highly digestible inorganic phosphorus (P), a limited and expensive resource. As fish excrete excess dietary P, it is necessary to ensure they are fed the correct amounts to reduce costs and the release of phosphorus into the environment. The specific P requirements during the grow-out phase of farmed Atlantic salmon (Salmo salar, L.), which is when feeding intensity is highest, are unknown. In the current study, sea-cage reared salmon under natural light were fed one of six diets with increasing inorganic P levels (6.1-11.2 g/kg total P), from December 2022 (1.8 kg, sampling point I.), through April (2.8 kg, sampling II.), until July 2023 (4.2 kg, sampling III.). Response parameters were P digestibility, P retention, growth, vertebral deformities, vertebral mechanical strength and mineral content, bone microstructure, and microscopic location of bone minerals. Growth of the animals was lower between December-April (0.62 mm/day) with temperatures ranging from 5 to 9 °C compared with the period between April-July (0.98 mm/day, 7-14 °C). Phosphorus digestibility followed a similar trajectory with higher values in the second part of the study. Vertebral deformities were not affected by different dietary P levels. A regular somatic growth, bone mineralisation, and bone mechanical strength were achieved in animals fed 3.7 g/kg available P between December-April and in animals fed 4.6 g/kg available P between April-July. This shows the potential to reduce total dietary P content by 16 –24 % compared to current commercial feeds, without compromising bone mineralisation or skeletal health.