Melanized Focal Changes in the Fillets of Farmed Atlantic Salmon: A Clinical Report From a Deep-Sea Production Site

Melanized focal changes (MFCs) are a persistent challenge in Atlantic salmon (Salmo salar) aquaculture, affecting product quality and fish welfare. These lesions are generally considered to arise from red focal changes (RFCs), which are often attributed to handling-related trauma, but underlying mechanisms remain poorly understood. In this case report, we investigated the occurrence and histopathological features of RFCs and MFCs in farmed Atlantic salmon reared in a submerged deep-sea system, where handling disturbances were minimized. Environmental conditions, including dissolved oxygen levels, were monitored throughout production. Macroscopic lesions exhibiting red, black or mixed discoloration were sampled from the ventral fillet at slaughter (average weight 3.3 kg) and examined histologically. RFCs showed haemorrhage, adipocyte necrosis, extracellular lipid release and early pseudocyst formation within adipose tissue. MFCs displayed diffuse granulomatous inflammation dominated by foamy and pigmented macrophages, with lipid-filled pseudocystic structures and persistent adipose involvement. Lesions exhibited a polyphasic pattern consistent with fat necrosis-associated inflammation. The presence and progression of RFCs into MFCs under submerged, minimally handled conditions demonstrate that handling-related trauma is not necessary for lesion initiation. The strong association with adipose tissue suggests endogenous processes-related to lipid metabolism, tissue vascularization, local hypoxia or nutrition-play a central role. Declining environmental oxygen preceding slaughter may exacerbate hypoxia within affected tissues, although its precise influence remains unclear. These findings refine current concepts of MFC pathogenesis and emphasize intrinsic biological drivers over mechanical injury in the development of these lesions in Atlantic salmon.