ALBERT

Description: The short tail phenotype represents one of the main causes for downgrading of farmed Atlantic salmon (Salmo salar) at slaughterhouses. Prevalence of short tail is variable and the aetiology is suspected to be multi-factorial. Risk factors have been identified but descriptions of the aetiology and the pathology of the condition are still rare. In the current study, a radiological and histological analysis of short tails has been performed, examining six normal and six downgraded individuals from a slaughterhouse in southern Norway. In the short tail phenotype, vertebral bodies were shifted and bent at the contact zone of adjacent vertebral bodies. Changes either affected the entire spine or were located at the medial caudal-spine. While the internal bone structure of the vertebrae was similar in deformed and non-deformed animals, a lack of intervertebral space apparently caused a shortening of the vertebral column and corresponded to an elevated condition factor in deformed individuals. Histological analysis revealed different degrees of proliferation of cartilaginous tissues, which replaced the intervertebral notochord tissue. The displacement of adjacent vertebral bodies and the development of cartilage in between vertebral bodies suggest mechanical forces as a possible cause for the observed deformations, since mechanically-induced overload and a subsequent direct contact of bones are factors that can stimulate heterotopic cartilage development and pseudoarthrosis