Bone health is an essential aspect of overall health and well—being of fish. Bones provide support to the body, protect vital organs, and store minerals such as calcium and phosphorus. All vertebrates need sufficient minerals for their Ca—P based skeletons. Fish mainly obtain calcium from the water, but not phosphorus as ocean is empty o f phosphorus. Dietary phosphorus deficiency is considered a nutritional risk factor for the development of vertebral deformities in farmed Atlantic salmon (Salmo salar, L). Bone health is important not only for fish welfare, but also for farmers, as bone defects (vertebral deformities) may lead to bad fillet quality and eventually downgrading of the fish at slaughterhouses.
Our recent studies have shown no correlation between dietary phosphorus levels and growth in freshwater Atlantic salmon (Drábiková et al., 2021, 2022) . The growth of post-smolt Atlantic salmon deficient in phosphorus (with a 50% lower dietary P content) was restricted compared to those reared in seawater with a sufficient phosphorus content, despite no significant differences in the condition factor or feed conversion ratio between the experimental groups (Witten et al., 2016, 2019). Fish fed a low phosphorus diet develop extended areas of non-mineralised bones without having significant effects on the prevalence of vertebral deformities in Atlantic salmon (Drábiková et al., 2021, 2022; Witten et al., 2016, 2019). Excessive dietary phosphorus intake (50% above the dietary P requirement) has no additional benefits in terms of bone mineralization, but ultimately results in water waste . Furthermore, elevated CO2 levels in water can prevent dietary induced osteomalacia (bone softening) in Atlantic salmon (Drábiková et al., 2023) .
In conclusion, fish need an optimum phosphorus diet and environment to keep their bones healthy. This is critical for closed aquaculture systems where excess nutrients (phosphorus) build up in the water.
Drábiková, L., Fjelldal, P.G., Yousaf, M.N., De Clercq, A., Morken, T., McGurk, C., Witten, P.E. (2023) . Elevated water CO2 can prevent dietary-induced osteomalacia in post-smolt Atlantic salmon (Salmo salar, L.). Biomolecules 2023, 13, 663.
Drábiková, L., Fjelldal, P.G., De Clercq, A., Yousaf, M.N., Morken, T., McGurk, C., Witten, P.E. (2022) What will happen to my smolt at harvest? Individually tagged Atlantic salmon help to understand possible progression and regression of vertebral deformities. Aquaculture, 559, 738430. https://doi.org/https://doi.org/10.1016/j.aquaculture.2022.738430
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. https://doi.org/10.1016/j.aquaculture.2021.736776
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
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