New discoveries point to internal processes as main cause of melanosis
Fish raised in submersible cages also exhibited melanosis. Scientists highlight the importance of biological factors such as lipid metabolism, hypoxia, and tissue vascularization in the pathophysiology.
Published
Melanosis, one of the main alterations affecting the quality of salmon fillets, could originate from internal biological processes rather than damage associated with handling, according to a recent study by Norwegian scientists.
Among the factors that could influence this change in the salmon muscle, the following have been described: diet, infectious agents, vaccination, traumatic injuries, mechanical stress during handling, environmental factors, genetics, among others.
This new research analysed the appearance of red focal changes (RFC) and their progression to melanised focal changes (MFC) in fish farmed in submersible cages, which involve minimal human intervention and handling, which allowed the authors to largely rule out mechanical trauma as a cause of melanosis.
Experts monitored environmental conditions, including dissolved oxygen levels, throughout production. They also took samples of macroscopic lesions exhibiting red, black, or mixed discoloration from the ventral fillet at harvest (average weight 3.3 kg) for histological analysis.
Intrinsic biological factors
The results showed that RFCs are characterised by haemorrhages, adipose tissue necrosis, and lipid release, while MFCs exhibit persistent granulomatous inflammation, with an abundance of pigmented macrophages and pseudocystic structures. This pattern suggests a progressive process associated with fat necrosis.
However, one of the most relevant findings of the study is that these lesions developed even in the absence of intensive manipulation, indicating that endogenous factors would play a key role in their origin.
“The presence and progression of RFCs to MFCs under immersion and minimal manipulation conditions demonstrate that manipulation-related trauma is not necessary for the onset of the injury. The strong association with adipose tissue suggests that endogenous processes - related to lipid metabolism, tissue vascularisation, local hypoxia, or nutrition - play a fundamental role,” the experts explained.
The researchers also suggested that low levels of dissolved oxygen prior to harvest could exacerbate hypoxic conditions in affected tissues, although its specific impact still requires further investigation.
“These findings refine current concepts on the pathogenesis of MFC and emphasise the importance of intrinsic biological factors over mechanical injury in the development of these lesions in Atlantic salmon,” the scientists concluded.
Review the abstract of the study titled “Melanized Focal Changes in the Fillets of Farmed Atlantic Salmon: A Clinical Report From a Deep-Sea Production Site”, here.
