What is Black Spot on Salmon Fillets and Why Does it Matter?
The black spot on salmon refers to unattractive dark pigmentation found in the fillets of Atlantic salmon, often affecting over 20 percent of reared fish. These spots are typically more than 1 cm in size and can significantly affect the fillet’s market value. In commercial seafood farming, these blemishes present both economic and aesthetic challenges, prompting extensive research into their origins.
The dark pigmentation, known scientifically as melanized focal changes, is often accompanied by similar but visually distinct “red spots.” These two types of discolorations were long thought to be biochemically related. However, recent studies have refuted this theory, revealing separate cellular mechanisms for each.
The Biochemistry Behind Black Spot on Salmon
To investigate the biochemical basis of these discolorations, Professor Turid Mørkøre of the Norwegian University of Life Sciences collaborated with Professors Kazumasa Wakamatsu and Shosuke Ito of Fujita Health University, Japan—renowned experts in melanin biochemistry.
Their study, published in the International Journal of Molecular Sciences, used a comprehensive panel of biochemical assays to analyze the pigmentation in salmon fillets.
Key Findings:
- Black spots contain eumelanin, a type of melanin also found in human skin and hair.
- Red spots do not contain detectable melanin, but exhibit DOPA-derived products indicative of hemorrhage.
- The biochemical discontinuity supports the theory that red and black spots originate from different cellular sources.
This study is the first to conclusively determine that the pigment in black spots is eumelanin, produced by specialized immune cells called melanomacrophages—a type of cell unique to ectothermic vertebrates such as fish and amphibians.
Red Spots vs. Black Spot on Salmon: Distinct Causes and Characteristics
Although both types of lesions affect salmon fillets, they do not share a common origin and exhibit distinct biochemical and cellular signatures:
- Red spots are likely caused by localized hemorrhaging. This happens when red blood cells leak into the muscle tissue, often due to physical trauma, handling stress, or spontaneous rupture of blood vessels. These areas show evidence of oxidative stress, such as DOPA-derived products, but lack detectable melanin.
- Black spots, in contrast, result from chronic inflammation. These are characterized by an accumulation of melanomacrophages—specialized immune cells in fish that produce the dark pigment eumelanin. This pigment gives the black spots their characteristic appearance and signals a long-term immune response rather than an acute injury.
Intermediate discolorations that appear reddish-brown have sometimes been interpreted as transitional forms, leading to early assumptions that red spots evolved into black spots. However, the biochemical discontinuity demonstrated in the study confirms these are separate phenomena with distinct biological pathways.
“The study by Wakamatsu and coworkers is an important component in the characterization of pigmented lesions in the Atlantic salmon,” says Professor Erling Koppang, a leading specialist in salmon pathology. “It reinforces earlier genetic and histological evidence that black pigmentation results from tyrosinase gene expression and eumelanin deposition.”
Aquaculture Implications of Black Spot on Salmon
The new findings have major implications for both aquaculture operations and scientific research:
- Financial impact: Fillets with visible spots are often downgraded in quality, resulting in substantial revenue losses for producers. In large-scale operations, even a modest increase in incidence can translate into millions in lost profit.
- Differentiated treatment strategies: Recognizing the separate causes of red and black spots allows aquaculture managers to tailor prevention measures. For example, reducing handling stress might help prevent red spots, while improving biosecurity and anti-inflammatory protocols could address black spot formation.
- Improved diagnostics: By using targeted assays to detect biomarkers like DOPA derivatives or melanin, fish health professionals can better diagnose the underlying cause of discoloration, informing more effective interventions.
These insights point to a multifactorial approach for improving fish health and product quality. Moving forward, integrating biochemical monitoring into regular aquaculture practices may help mitigate the prevalence of both red and black spots, thereby improving yield and consumer satisfaction.
How Melanin and Melanomacrophages Cause Black Spot on Salmon
Melanin is a large, complex polymer derived from the amino acid tyrosine through enzymatic oxidation. Among its types, eumelanin is the dark pigment confirmed in the black spots on salmon. Until now, assumptions about melanin’s role were based on limited histological staining methods, such as Fontana Masson.
Wakamatsu and Ito’s assays confirmed that the black pigment in salmon fillets is indeed eumelanin, marking a significant step forward in the biochemical understanding of these lesions.
Conclusion: Reducing Black Spot on Salmon Through Biochemical Insights
The discovery of eumelanin in black spot lesions, and the biochemical distinction from red spots, is a milestone in aquaculture science. By identifying that these blemishes stem from different biological processes—hemorrhage vs. immune response—researchers can now pursue targeted strategies to reduce their occurrence. This not only helps restore the market value of Atlantic salmon but also supports healthier, more sustainable aquaculture practices.
Frequently Asked Questions (FAQs) About Black Spots on Salmon
What causes black spots on salmon fillets?
They are caused by immune cells called melanomacrophages that produce eumelanin during chronic inflammation.
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Are red and black spots related?
No. Red spots are associated with hemorrhage, while black spots are linked to melanomacrophage activity. They are biochemically and cellularly distinct.
Is the presence of black spots harmful to eaters?
While unappealing, black spots are not harmful to human health. The concern is primarily cosmetic and economic.
Can fish farming practices reduce spot occurrence?
Possibly. Reducing stress, injury, and infections in reared fish may lower the risk of both hemorrhages and chronic immune responses.
