WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2025 29 tissues. If we cut through the vertebral column we can see the mineralised and non-mineralised parts of the vertebral centra and the non-mineralised intervertebral spaces sitting in between the centra (Figure 7). This gives us answers about the functional stability of the vertebral column on a cellular and tissue level and helps us understand the distribution of the minerals within the bone. Vertebrae of animals on ‘45%P’ that appeared smaller on x-rays showed a regular size when observed through the microscope. We noticed large areas of non-mineralised bone tissue formed during the study, sitting on top of the regularly mineralised bone which developed before our experiment (Figure 7). The edges of vertebral structures in ‘45%P’ fed animals showed signs of bending due to their soft and rubbery nature associated with the lack of minerals. The bending of the vertebrae manifested through shorter vertebrae in length and longer in height (Figure 8). This indicates a potential for future deformity development if the animals were kept on a phosphorus-deficient diet. All structures of the vertebral column were otherwise welldeveloped even if the minerals were missing. Bone cells were actively forming a protein-based bony network prepared to capture minerals once available. How Much Phosphorus is Necessary to Ensure Strong Vertebrae? The mineralised — hard — part of the bone of the vertebrae forms and strengthens in response to mechanical load (Laerm 1976). We therefore assessed the strength of vertebrae by compression with a tool called a texture analyser (Figure 9). Based on the results we could see that the vertebrae of animals fed ‘45%P’ diet developed soft, rubbery vertebrae that compress under mechanical load 30% more easily than vertebrae from the other groups (Figure 9 IV. a). By the time we sampled the animals in July salmon on ‘75%P’ diet showed 18% softer vertebrae than animals fed more phosphorus (Figure 9 IV. b). The rest of the fish had similarly strong vertebrae which shows (CONTINUED ON PAGE 30) FIGURE 5. Three x-ray images and illustrations of the same Atlantic salmon at the freshwater to seawater transfer at 50g, in seawater at 700g, and at harvest at 4.5kg. We can observe progressive development of vertebral fusion already at the seawater transfer, 6 months before it becomes a severe deformity in the seawater phase (modified after Drábiková et al. 2022). FIGURE 6. X-ray images of salmon vertebral column in which we can see that vertebrae (dotted rectangle) of animals on ‘45%P’ diet appeared smaller and the spaces between the vertebrae larger compared with the salmon on 75%- 107%P diets (modified after Drábiková et al. 2026). FIGURE 7. (A) X-ray image of Atlantic salmon. (B) A more detailed image to show locations of histological sections (a-c). Orange rectangle surrounds the vertebral centra and yellow rectangle specifies area with intervertebral space (asterisks in a) and vertebral endplates (triangle in b). (b) Salmon fed phosphorus deficient diet – 45%P – developed characteristic extended areas of non-mineralised bone (black arrows) as opposed to the rest of the groups (yellow arrow in c) (modified after Drábiková et al. 2026). FIGURE 8. Vertebrae of animals fed 75%P and more had regular square vertebrae with a ratio between length and height close to 1 while vertebrae of animals fed 45%P had rather compressed vertebrae shorter in length and longer in height indicating a likelihood for future deformity development if the animals were kept on a phosphorus-deficient diet.
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