Introduction
The main minerals of bone are calcium and phosphorus. While teleost can obtain calcium in sufficient amounts from water they depend on dietary P supply. Dietary P deficiency is considered a risk factor for the development of vertebral deformities in farmed Atlantic salmon (Salmo salar). In commercial diet formulations dietary P requirements are met by addition of inorganic P compounds such as mono-ammonium phosphate (MAP) which are highly digestible (up to 90%) (Morales et al., 2018). Recent studies on early seawater stages of Atlantic salmon show that low-P diets result in the development of non-mineralised bone matrix that retains the capacity to mineralise without causing vertebral deformities (Witten et al., 2016; 2019). High P diets, on the other hand, may increase P excretion associated with waste of P resources and environmental pollution. We here present results from two studies that show the effect of low and high dietary P on freshwater stages of Atlantic salmon and the long-term effects of dietary P history at harvest. Study 1 analysed the effects of dietary P as a single factor for skeletal health. Atlantic salmon parr were fed low and high P diets and growth performance, plasma P content, and vertebral deformities were analysed. Study 2 followed individually tagged animals with low and high dietary P history from study 1 up to harvest. Progression and regression of deformities related to the animals’ dietary freshwater history were analysed.
Materials and Methods
Study 1: Freshwater parr stages of Atlantic salmon (start weight 13.5 g) were fed MAP supplemented diets with a low (0.35%), regular (0.56%), or high (0.93%) estimated available P for 11 weeks (week 0 - 12). Animals were subjected to different analytical methods such as radiography, whole mount Alizarin red S staining, histology, plasma analysis, and bone mineral content analysis.
Study 2: The remaining animals from the study 1 (45 animals per diet group) were tagged and fed a commercial diet with regular P content for 66 weeks in seawater up to harvest (4.5 kg). Deformity development was monitored in individual animals by repeated radiology (at week 15, 47, 84) up to harvest. Additional analysis was done by Alizarin red S whole mount staining, histology, and bone mineral analysis.
Results and Discussion
Study 1 (freshwater): The prevalence of deformities in Atlantic salmon (week 12) was low and there was no effect of low or high P diet on the development of deformities. Animals fed low-P diet showed low-mineralised vertebrae characterised by extended areas of non-mineralised bone. Plasma P levels showed a 3-fold reduction in low-P fed animals. While bone formation continued, bone mineralisation was discontinued. In contrast, regular-P and high-P diet animals had fully mineralised bone. The high-P diet had no further effect on bone mineralisation. Interestingly, the growth of animals fed different levels of dietary P was comparable, no growth retardation was observed in low-P diet animals (Drábiková et al., 2021).
Study 2: Vertebral body deformities either affected bone trabeculae of the vertebral centra (rectangle in Fig. A) or the intervertebral spaces (arrow in Fig. 1A). Deformities of the vertebral centra with intact intervertebral spaces (low-mineralised, hyper-dense, compressed, and vertically shifted vertebrae) (Fig. 1A), had the capacity to recover in seawater (week 47). The prevalence of all deformities was low, except low-mineralised vertebrae and hyper-dense vertebrae that were frequent in low-P diet animals. In seawater low-mineralised vertebrae were completely mineralised and hyper-dense vertebrae recovered. Deformities which affected both the vertebral centra and the intervertebral joints, equally prevalent in all diet groups, were either contained (stabilised vertebral fusion) (Fig. 1B) or progressed over time (fused vertebral centra) (Fig. 1C). Deformities could also develop late in the seawater (week 47-84) (late-onset vertical shift and compression) (Fig. 1D). At harvest (week 84), the number of progressive deformities was low and independent of dietary P history.
In current studies the dietary P in freshwater has an immediate effect on bone mineralisation but no effect on the development of bone deformities and growth. There was also no long-term effect on the incidence of deformities at harvest. This study shows that under favourable conditions specific deformities can recover in seawater. Moreover, a high-P diet freshwater history had no effect on growth or development of deformities.
References
Drábiková, L., Fjelldal, P.G., De Clercq, A., Yousaf, M.N., Morken, T., McGurk, C., Witten, P.E., 2021. Vertebral column adaptations in juvenile Atlantic salmon Salmo salar, L. as a response to dietary phosphorus. Aquaculture 736776. doi:10.1016/j.aquaculture.2021.736776
Morales, G.A., Azcuy, R.L., Casaretto, M.E., Márquez, L., Hernández, A.J., Gómez, F., Koppe, W., Mereu, A., 2018. Effect of different inorganic phosphorus sources on growth performance, digestibility, retention efficiency and discharge of nutrients in rainbow trout (Oncorhynchus mykiss). Aquaculture 495, 568–574. https://doi.org/10.1016/j.aquaculture.2018.06.036
Witten, P.E., Owen, M.A.G., Fontanillas, R., Soenens, M., Mcgurk, C., Obach, A., 2016. A primary phosphorus-deficient skeletal phenotype in juvenile Atlantic salmon Salmo salar: The uncoupling of bone formation and mineralization. J. Fish Biol. 88, 690–708. https://doi.org/10.1111/jfb.12870
Witten, P.E., Fjelldal, P.G., Huysseune, A., McGurk, C., Obach, A., Owen, M.A.G., 2019. Bone without minerals and its secondary mineralization in Atlantic salmon (Salmo salar): the recovery from phosphorus deficiency. J. Exp. Biol. jeb.188763. https://doi.org/10.1242/jeb.188763