Discolouration of the dorsal white musculature of farmed Atlantic salmon (Salmo salar L.) in Norway, characterised by histological and transcriptional methods

Focal melanisation in the white muscle of farmed Atlantic salmon (Salmo salar L.) has become a common finding in Norway. Depending on the magnitude of such changes, muscle tissue (fillets) are graded to lower quality, leading to immense economic losses. In 2010, the cost to Norwegian aquaculture of melanised changes in fish fillets was estimated to approximately NOK 500 million (Hjeltnes et al., 2016). In particular, major costs are associated with removing melanised changes in fillets. The purpose of the present work was to investigate the nature of focal melanised changes located in the dorsal muscle of seawater-farmed Atlantic salmon, and to compare these with similar changes in the muscle of the abdominal wall. The material was sampled from two fish farms, one in Western and one in Mid Norway. All the fish investigated had been vaccinated intraperitoneally with oil-adjuvanted vaccines. Gross pathological inspection, histological and immunohistochemical methods were used to characterize the changes. In addition, infectious agents were searched for by molecular approaches. Focal and diffuse melanised changes, which were observed in dorsal musculature, were compared with focal melanised abdominal changes. The results showed non-distinguishable pathological processes in both muscle regions. Presence of piscine orthoreovirus was identified in the different changes. No bacteria or fungi were detected in the investigated material. These results suggest that melanised changes in dorsal and abdominal areas are of the same nature as concerns origin and manifestation.M-H