The Analysis of Transitional or Caudal Vertebrae Is Equally Suitable to Determine the Optimal Dietary Phosphorus Intake to Ensure Skeletal Health and Prevent Phosphorus Waste in Salmonid Aquaculture

A prolonged dietary phosphorus (P) deficiency can result in reduced growth and vertebral deformities in farmed Atlantic salmon (Salmo salar). Severe deformities can impair swimming and lead to chronic stress associated with muscular fibrotic scarring. Conversely, excess dietary P contributes to farm effluents and environmental pollution. Vertebral centra ash content and mechanical strength both respond rapidly to suboptimal dietary P supply, but measuring all of salmon’s 59 vertebrae is time consuming. As such, this study assessed whether vertebrae from two commonly assessed regions (transitional and caudal) vary in their response to different dietary P levels. Atlantic salmon with an initial average weight of 1.8 kg (December 2022) were fed one of four experimental diets containing an increasing level of inorganic P (6.1–10.7 g/kg total P, 2.3–5.8 g/kg available P). Animals were distributed across 16 sea cages in a quadruplicated design. The regional differences in vertebral centra were assessed at two sampling points: in April 2023 following a slow growth period, and in July 2023 following a fast growth period. The growth of the caudal vertebrae in length surpassed the extension of the transitional vertebrae during the fast growth period. The bone mineralisation measured through vertebral centra ash and mechanical strength was however comparable between the regions, indicating that the rate of mineralisation was adjusted to the growth of the vertebrae. Only two parameters, yield point, which specifies the amount of energy that vertebra can absorb before it is permanently compressed, and toughness, a measure of stress per unit volume required to cause a fracture, showed regional differences. Considering transitional vertebrae, the estimated requirements were 4.1 g/kg available P in April and 4.4 g/kg in July, while the requirements based on caudal vertebrae were 3.7 g/kg in April and 4.6 g/kg in July. As such, both the transitional and caudal regions are equally suitable for a prompt recognition of suboptimal dietary P levels.