Farmed teleost fish commonly suffer an increased rate of vertebral deformities. Vertebral deformities have an adverse impact on fish welfare, growth, and swimming behaviour. Under unfavourable conditions (inappropriate handling stress, increased mechanical load, increased water temperature) vertebral deformities are likely to progress and worsen over time1-2. However, an increased amount of recently published studies observe that deformities can recover and disappear or stabilise over time3-5. Here we present a detailed description of the process of recovery and containment of deformities in Atlantic salmon (Salmo salar).
Materials and Methods
Individual animals of Atlantic salmon were followed by x-ray imagining to analyse the development of vertebral deformities in two studies. The first study4 followed 200 PIT-tagged farmed animals which were x-rayed at three time points: prior to seawater transfer (100 g), six months after seawater transfer (1.1 kg), and 12 months post-seawater transfer (2.3 kg). The second study3 followed 135 PIT-tagged Atlantic salmon. These animals were x-rayed prior to seawater (50 g), seven months post-seawater transfer (720 g), and 16 months post-seawater transfer (4.5 kg). Further analytics included whole mount Alizarin red S staining, mineralised and non-demineralised histology, and mineral content analysis.
Results and Discussion
Studies detect two broad categories of vertebral deformity which determine further development. Category I. comprises deformities that affect the bone of the vertebral centra. These are characterised by either bent bone trabeculae and replacement of adipose tissue with ectopic cartilage in the bone marrow spaces (hyper-dense vertebrae), by vertical displacement of the vertebral centra, or by mild compression of the vertebrae. Importantly, the intervertebral space between the vertebral centra remain non-deformed and thus category I deformities were observed to recover.
Category II. comprises deformities with alterations in both, vertebral centra and intervertebral spaces. Two studies3,4 show that these deformities can stabilise as long as no more than three vertebrae are affected (stabilisation through vertebral fusion). Stabilised vertebral fusions do not further aggravate, do not affect animal welfare, and are also common in wild Atlantic salmon6. In cases with damage to more than three adjacent intervertebral spaces, recovery has not been observed. Instead, extended vertebral fusion centra develop.
These studies provide detailed insights into the development of vertebral deformities based on tracking individual animals from freshwater up to harvest size. Surprisingly, several types of vertebral deformities can recover and stabilise over time under favourable rearing conditions.
1. Fjelldal, P.G., Hansen, T.J., Lock, E.J., Wargelius, A., Fraser, T.W.K., Sambraus, F., El-Mowafi, A., Albrektsen, S., Waagbø, R., Ørnsrud, R., 2016. Increased dietary phosphorous prevents vertebral deformities in triploid Atlantic salmon (Salmo salar L.). Aquac. Nutr. 22, 72–90. https://doi.org/10.1111/anu.12238
2. Fraser, T.W.K., Witten, P.E., Albrektsen, S., Breck, O., Fontanillas, R., Nankervis, L., Thomsen, T.H., Koppe, W., Sambraus, F., Fjelldal, P.G., 2019. Phosphorus nutrition in farmed Atlantic salmon (Salmo salar): Life stage and temperature effects on bone pathologies. Aquaculture 511, 734246. https://doi.org/10.1016/j.aquaculture.2019.734246
3. 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/10.1016/j.aquaculture.2022.738430
4. Witten, P.E., Obach, A., Huysseune, A., Baeverfjord, G., 2006. Vertebrae fusion in Atlantic salmon (Salmo salar): Development, aggravation and pathways of containment. Aquaculture 258, 164–172. https://doi.org/10.1016/j.aquaculture.2006.05.005
5. Printzi, A., Mazurais, D., Witten, P.E., Madec, L., Gonzalez, A.A., Miahle, X., Zambonino-Infante, J.L., Koumoundouros, G., 2022. Juvenile zebrafish (Danio rerio) are able to recover from lordosis. Sci Rep 12, 21533. https://doi.org/10.1038/s41598-022-26112-2
6. Sambraus, B.F., Glover, K.A., Hansen, T., Fraser, T.W.K., Solberg, M.F., Fjelldal, P.G., 2014. Vertebra deformities in wild Atlantic salmon caught in the Figgjo River, southwest 30, 777–782. https://doi.org/10.1111/jai.12517