Investigating the development of spinal curvature in farmed New Zealand king salmon Oncorhynchus tshawytscha

Abstract

Spinal anomalies are a frequent cause of economic loss in intensive finfish aquaculture and raise ethical concerns. Despite an abundance of knowledge, there is a lack of general consensus about the relationships between different anomaly types, their pathologies and the factors which contribute to their development. Novel cases must therefore be investigated independently. The aim of this thesis was to investigate the bone and soft tissue changes associated with the development of spinal curvature in farmed New Zealand king salmon, a condition of unknown cause that is suspected to arise via a neuromuscular pathology. Using X-ray radiography and two novel non-invasive tools, dual-energy X-ray absorptiometry (DXA) and magnetic resonance imaging (MRI), it was discovered that spinal curvature constitutes an LSK (lordosis-scoliosis-kyphosis) curvature complex which initially appears on radiographs 7 months post-seawater transfer and induces secondary changes to the vertebral bodies, predominantly in the form of compressions. Although DXA detected no differences in bone mineral content, fat mass and lean mass between adult fish with and without curvature, MRI and subsequent histology revealed that curvature was strongly associated with progressive stages of inflammation and fibrosis of the peri-vertebral skeletal musculature and connective tissues, particularly the horizontal septum. Feeding different commercial diets throughout seawater production had no effect on curvature prevalence, but the results were confounded by other factors, namely a lack of differences in feed composition, feed supply delays and disruptions, rigor mortis, and pen movement. Collectively, the studies provide evidence that spinal curvature in king salmon is a late-onset neuromuscular condition that involves a chronic inflammatory process. Its development may be related to altered mechanical load caused by high intensity exercise, excessive musculature, fast growth, handling and transport stress, temperature, and/or insufficient bone mineralisation during the early seawater phase. Future research should prioritise evaluation of muscular forces applied to the spine during swimming, and the integrity of peri-vertebral connective tissues and vertebral bone. Sustained exercise during early life, minimisation of handling and other sources of stress, and diet manipulation may help to reduce curvature prevalence in farmed stocks while the contributing factors are still being elucidated.