Melanin pigmentation, which arises in the aquaculture industry as dark spots on fish fillets, has long been an issue for salmon farmers, with consequences including lower final prices for their products.
The source of this discoloration has been the subject of many scientific investigations, and links have been made to various causes, such as rib fractures, fish genetic backgrounds, and adverse reactions to vaccines.
Researchers at the Norwegian University of the Environment and Life Sciences (NMBU) – a veterinary college in the Norwegian town of Ås where research into melanin spots has been ongoing for more than 20 years – have recently added to the subject’s body of research, saying they believe the development of these spots is linked to changes in fat tissue.
In a recently published peer-reviewed article, NMBU researchers describe the developmental stages of melanin spots as being in line with “fat necrosis,” a condition that can also occur in other animals and humans, where the body's immune system reacts against dying fat cells and their contents.
According to the research team, in humans, fat necrosis typically occurs in tissue that contains a lot of fat. Salmon, however, store significant amounts of fat in their muscles. Both bleeding and melanin spots occur mainly in the front areas of a fillet, which are also the fattiest.
“Previously, we mainly focused on muscle cells, but now, we have gone in depth on the fat cells. We started by mapping the fat's distribution in the muscles as a basis for understanding the changes in the spots,” NMBU Associate Professor Håvard Bjørgen said. “Dying fat cells release fat that can collect cysts or form so-called granulomas, which are chronic accumulations of inflammatory cells. The point is that the fish's immune system is unable to handle the enormous amounts of free fat, and thus, the fish gets chronic changes in the muscles.”
The researchers could not say for certain which characteristics of fish fat lead to the development of fat necrosis, though they gave suggestions as to what farmers could do to lessen the chances of melanin spots occurring.
“We consider it most likely that a high content of vegetable oils in feed gives a fat composition with too much polyunsaturated fatty acids and, correspondingly, too little saturated fat. This is a known cause of fat necrosis in other animals. In addition, an unfavorable relationship between the fatty acids omega-6 and omega-3 can act as fuel on the fire,” NMBU Professor Erling Olaf Koppang said.
Therefore, fillet dark staining is attributed to the oxidation of polyunsaturated fatty acids, he said.
“The pigment melanin is a powerful antioxidant. We see that the melanin is mainly around fat accumulations in the spots, and the melanin probably has a protective effect on the tissue that is exposed to oxidation,” Koppang said.
Sven Martin Jørgensen, the fish health lead at FHF – the Norwegian Seafood Research Fund, which has supported many of NMBU’s melanin pigmentation research, said he hopes the industry can use the most recent feedings to adjust their practices and ensure their operations limit these issues as much as possible.
“Now, it is up to the industry to put this knowledge to use and implement measures to avoid the problem of dark spots,” Jørgensen said. “It is natural to look at the content of the feed recipes, and here there are already breeding companies that, for example, have increased the omega-3 content in the feed with positive effects, but you should probably also look at the effects of reducing the fat content overall.”
The NMBU research was conducted in collaboration with the veterinary faculty at the University of Zurich, as well as with industry representatives from Mowi and Bremnes Seashore. The Marine Research Institute at Matre also contributed research material and expertise to the project.
In addition, human pathologists Britta Kleist at Sørlandet Hospital HF Kristiansand and Marius Lund-Iversen at Oslo University Hospital were central to understanding the development of melanin spots.
“We discussed our findings, and fat necrosis was quickly suggested as a key driving force behind the development of the spots. After an extensive literature search and review of a significant amount of research material, we were eventually able to put the pieces of the puzzle together,” Bjørgen said.
