The presence of dark melanin spots in salmon fillets is a relevant quality problem for the salmon farming industry. It causes a decrease in the final product price, therefore, the dark melanin spots have become a very interesting topic of research both for industry and academia.

Founded by the Norwegian Seafood Research Fund (FHF), the Ph.D. work of Raúl Jiménez-Guerrero from the Norwegian University of Life Science (NMBU), together with Nofima’s researchers, showed that rib fractures can be a major cause of melanin spots.

“We have substantiated that there is a connection between rib defects and dark spots”, said Grete Bæverfjord, senior scientist at Nofima.

“We have investigated many possible causes, and now know that neither feed nor genetics is the main cause of dark spots in salmon fillets,” said Turid Mørkøre, senior scientist at Nofima and professor at NMBU.

Mørkøre and Bæverfjord investigated whether rib fractures could have anything to do with the problem. Using Nofima’s X-ray equipment at Sunndalsøra, researchers examined the salmon’s skeleton focusing on the ribs. Since very little research has been conducted on these bones, the work also involved classifying normal ribs and different types of injuries analyzing a group of salmon from the freshwater phase to slaughter and comparing them with a wild fish control.

The results reported several rib defects, with and without injuries, and showed that fractures and dark spots were mainly located in the same place in the fillet. In both freshwater smolt and wild salmon, there was an average of four rib defects per fish. After sea transfer, the number increased to ten defects per fish. The increase occurred during the final freshwater period and the first months at sea and remained stable until slaughter.

“We need to focus on what the fish are exposed to during this period to improve technology and routines so that the problem of dark spots can be prevented,” the scientists said.

However, researchers found both melanin spots without rib injuries and rib defects without dark spots, the results do not explain everything about the dark melanin spots. “We suspect, for example, that some melanin spots may be caused by muscle tears,” concluded Bæverfjord.

Reference:

Raúl Jiménez-Guerrero, Grete Baeverfjord, Øystein Evensen, Kristin Hamre, Thomas Larsson, Jens-Erik Dessen, Kjellrun-Hoås Gannestad, Turid Mørkøre. (2022) Rib abnormalities and their association with focal dark spots in Atlantic salmon fillets. Aquaculture, Volume 561, 738697 doi.org/10.1016/j.aquaculture.2022.738697