Abstract
Bone health is important for a viable and ethically sound Atlantic salmon aquaculture industry. Two important risk factors for vertebral deformities are dietary phosphorus and water temperature. Here, we explore the interplay between these two factors during a full production of Atlantic salmon. Salmon were fed one of three diets (low 4.4–5.0 g kg−1, medium 7.1–7.6 g kg−1, or high 9.0–9.7 g kg−1 soluble phosphorus) from 3 to 500 g body weight, followed by a common diet of 7.3 g kg−1 soluble phosphorus until harvest size at 4 kg. Additional groups were included to investigate the effects of water temperatures of 10 vs 16 °C (low and high diets only) and the switching of dietary phosphorus levels (from low to medium or high, from medium to low or high, from high to low or medium), starting at seawater transfer (~100 g body weight) and lasting for 4 months (~500 g body weight). During the experimental feeding period, the low phosphorus diet caused reduced bone mineralization and stiffness and a greater prevalence of vertebral deformities, compared to the medium and high phosphorus diets. However, the prevalence of severely deformed fish at harvest was reduced by switching from the low to either the medium or high phosphorus diets for 4 months after seawater transfer, followed by rearing on the standard commercial feed. Concurrently, switching from either the medium or high to a low phosphorus diet for the same period following seawater transfer had no effect on vertebral deformities at harvest. The higher water temperature for 4 months following seawater transfer increased the severity of deformities at harvest, irrespective of dietary phosphorus. Finally, low dietary phosphorus was associated with increased fillet damage, due to ectopic connective tissue around the spine, at harvest. In conclusion, dietary phosphorus levels of 5 g kg−1 for the initial 4 months in seawater are more of a risk factor for vertebral pathologies if preceded by low, but not medium or high, dietary phosphorus in freshwater. However, dietary phosphorus levels may not play a role in temperature induced radiologically detectable vertebral pathologies. Under the reported growing conditions and diet compositions, a combination of 7.5–7.6 g kg−1 soluble phosphorus during freshwater and 5.0 g kg−1 for the first 4 months in seawater, was sufficient for normal bone health and growth in Atlantic salmon.
Highlights
A normal developing skeleton is a prerequisite for sustainable production and animal welfare, but vertebral column deformities are a persistent concern for farmed Atlantic salmon (Salmo salar) (Fjelldal et al, 2012a)
There was no effect of dietary phosphorus or sex on body mass or length at any time point in freshwater or seawater, but Medium phosphorus (MP) fish had lower body condition than the Low phosphorus (LP) fish in freshwater and the High phosphorus (HP) fish had lower condition factors than the MP and LP groups throughout the seawater period
We found a combination of 7.5–7.6 g kg−1 (MP) soluble phosphorus during freshwater and 5.0 g kg−1 (LP) during the early seawater phase, was sufficient for bone health in Atlantic salmon under the current conditions
Summary
A normal developing skeleton is a prerequisite for sustainable production and animal welfare, but vertebral column deformities are a persistent concern for farmed Atlantic salmon (Salmo salar) (Fjelldal et al, 2012a). Dietary phosphorus supply has been highlighted as one of the key factors in the aetiology of vertebra deformities in farmed salmon (Bæverfjord et al, 2018), a phosphorus deficient diet alone in some cases may not induce skeletal deformities (Gil-Martens et al, 2012; Witten et al, 2016). Dietary phosphorus leads to insufficient bone mineralization and a reduced mechanical stiffness of the vertebrae (Baeverfjord et al, 1998), which can result in the development of compressed vertebrae (Fjelldal et al, 2009). Many papers have dealt with dietary phosphorus requirements during both freshwater (Fjelldal et al, 2012b; Helland et al, 2005; Ketola, 1975; Smedley et al, 2018; Vielma and Lall, 1998; Åsgård and Shearer, 1997) and seawater (Albrektsen et al, 2009; Baeverfjord et al, 1998; Fjelldal et al, 2009, 2012c) life stages separately, the combined effect of phosphorus nutrition in both environments has never been investigated
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