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Spinal anomalies are a recognised source of downgrading in finfish aquaculture, but identifying their cause(s) is difficult and often requires extensive knowledge of the underlying pathology. Late-onset spinal curvatures (lordosis, kyphosis, scoliosis) can affect up to 40% of farmed New Zealand Chinook (king) salmon (Oncorhynchus tshawytscha) at harvest, but little is known about their pathogenesis. Curvature development was radiographically documented in two related cohorts of commercially-farmed Chinook salmon throughout seawater production to determine (1) the timing of radiographic onset and relationships between (2) the curvature types, (3) the spinal regions in which they develop and (4) their associations with co-existing vertebral body anomalies (vertebral compression, fusion and vertical shift). Onset of curvature varied between individuals, but initially occurred eight months post-seawater transfer. There were strong associations between the three curvature types and the four recognised spinal regions: lordosis was predominantly observed in regions (R)1 and R3, kyphosis in R2 and R4, manifesting as a distinct pattern of alternating lordosis and kyphosis from head to tail. This was subsequently accompanied by scoliosis, which primarily manifested in spinal regions R2 and R3, where most of the anaerobic musculature is concentrated. Co-existing vertebral body anomalies, of which vertebral compression and vertical shift were most common, appeared to arise either independent of curvature development or as secondary effects. Our results suggest that spinal curvature in farmed New Zealand Chinook salmon constitutes a late-onset, rapidly-developing lordosis–kyphosis–scoliosis (LKS) curvature complex with a possible neuromuscular origin.

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Vertebral fusions are an established economic concern in farmed Atlantic salmon, but have not been studied in detail in farmed Chinook salmon. Two radiographic studies of vertebral fusions were performed in farmed Chinook salmon. Sixteen of 1,301 (1.2%) smolt and 201 of 2,636 (7.6%) harvest fish had fusions. There were no significant differences in the number of fused vertebrae/fusion in smolt compared with harvest fish. Secondly, tagged fish were repeatedly radiographed to determine the progression of the fusions. Nineteen (4.4%), 23 (5.3%) and 39 (9.0%) fish had fusions as smolt, after 129 days in sea water, and at harvest, respectively. There were no significant differences in the average number of vertebra/fusion between the three time points. Of the fusions that were observed in smolt, additional vertebra did not become fused in 81% of the lesions. Within the rare fusions that did progress due to the involvement of adjacent vertebra, an average of 1.6 vertebrae were added per year. Fish with fusions were significantly lighter than non‐affected fish at harvest. Fusions are common in farmed Chinook salmon; however, they are typically stable after development. As fish with fusions were lighter at harvest, reducing fusions may have an economic benefit.