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Spinal curvatures and onset of vertebral deformities in farmed Chinook salmon, Oncorhynchus tshawytscha (Walbaum, 1792) in New Zealand

Perrott, M. R., Symonds, J. E., Walker, S. P., Hely, F. S., Wybourne, B., Preece, M. A., Davie, P. S.
Journal of applied ichthyology 2018 v.34 no.2 pp. 501-511
Oncorhynchus tshawytscha, X-radiation, fish, harvesting, phenotype, radiography, scoliosis, seawater, New Zealand
The prevalence and onset of radiological skeletal deformities in tagged Chinook salmon (Oncorhynchus tshawytscha) were investigated after 129 days in seawater at grading for removal of fish that failed to thrive (mean weight ± SEM, 432 ± 6.5 g) and again at harvest after 395 days (mean weight ± SEM, 3,721 ± 43 g). A scoring system identifying four categories of the radiographic deformity phenotype was established based on literature and evaluation of X‐rays of harvest‐size Chinook salmon. Deformity categories were: spinal curvature or Lordosis, Kyphosis, Scoliosis (LKS); Fusion; Compression and/or reduced inter‐vertebral (IV) space; Vertical shift. Of the 432 fish surviving to harvest and for which there were weights and diagnostic radiographs, 38.4% were affected by at least one deformity. Late onset LKS, detected at harvest, was the most prevalent deformity in 29.4% of all harvested fish. LKS was present alone with no other potentially confounding deformities in 10% of harvested fish. Cranial lordosis, a common LKS variant, in the postcranial region comprised half of LKS‐deformed fish. LKS commonly co‐existed with compression, a combination prevalent in harvest fish at 13.4%. Compression and/or reduced IV space detected in 22.0% of harvested fish was commonly associated with other deformity phenotypes (84/95 of fish with compression). Fusion and vertical shift were present in 7.6% and 4.6% of harvested fish, respectively. More than 77% of fish with any type of deformity developed the deformity within 9 months of harvest. Fusions that were visible in radiographs at grading persisted in fish that survived until harvest. In contrast, LKS, the most visible harvest deformity, was difficult to detect in radiographs from the earlier time point, suggesting that this economically important deformity develops at a relatively late stage of seawater growth. Deformed fish at harvest were smaller (mean ± SEM 3,479 ± 76 g) than normal (mean ± SEM 3,875 ± 51 g). Fish with no deformity grew at a significantly (p < .05) faster rate than fish that developed a deformity during this period or were already deformed at grading. The scoring system performed with a sensitivity of 92.4% and a specificity of 97.6% for Chinook salmon >500 g, and thus has a potential utility for other farmed salmonids.