Riddle of dark spots in salmon solved

Photo: NMBU-Norwegian University of Life Science

Photo: NMBU-Norwegian University of Life Science

Dark spots on the muscle filets of farmed salmon have posed a significant problem for many years. New research has revealed that a virus associated with heart and skeletal muscle inflammation is the cause of the spots.

The dark spots are so-called melanin spots that are black, brown or red colour pigments. The spots are cut off the filet manually. Consequently, they create additional work and significant financial losses in the production lines of the fish farming industry.

Up to 20 % of the salmon at some fish farms exhibit these spots, and the problem exists in salmon along the entire Norwegian coast. It has been estimated that the melanin spots cost the Norwegian fish farming industry around one hundred million Euros per year.

The black spots are comprised of inflammation effects and scar formation, a sign of both acute and old tissue damage. Salmon have a special type of immune cells that produce melanin. It is these cells that give rise to the discolouration. The discolourations can be quite small, however they can also be as large as several centimetres in diameter or even still larger. They are found everywhere in the filet.

Since we also know that other salmon-producing countries such at the UK, Ireland, Chile, Canada and the US have the same problem, it becomes an issue involving large sums of money in added costs for the industry.

Virus that also attacks the heart and skeleton

Photo: NMBU-Norwegian University of Life Science

Photo: NMBU-Norwegian University of Life Science

In co-operation with the Institute of Marine Research and the Lerøy Seafood Group, Norwegian University of Life Sciences Professors Espen Rimstad and Erling Olaf Koppang have found the cause of these changes. Groups involved with research in virology and anatomy at the Norwegian University of Life Sciences (NMBU) have found large quantities of a virus in the black spots. The veterinary environment has worked with the virus, which is named Piscine orthoreovirus (PRV), for a number of years, including making antibodies to the virus and tools for diagnosing it.

An antibody that has been developed to identify possible occurrences of the virus has shown that there are large quantities of the virus in the black spots. The virus has been shown earlier to be associated with heart and skeletal muscle inflammation (HSMI), the viral illness most frequently encountered in farmed Norwegian salmon. The findings were thus extremely surprising. Further examinations of fish from experiments at the Matre Research Station, and different Lerøy Seafood Group sites clearly show that infection with PRV in these dark spots is the cause.

It is also clear that environmental factors can influence the frequency of the dark spots, possibly indirectly by providing conditions favourable for PRV infections. The specifics of how the virus initially establishes itself in the musculature are still unknown.

Vaccination was believed to have been the cause 
There has been much speculation about the cause of these changes. Impact injuries, nutrition and in particular vaccination have all been advanced as possible causes. These theories have in part proved to be untenable or have lacked a dimension because they have not been able to explain why a long-lasting inflammation develops. For example, the theory involving adverse effects of vaccinations was ruled out after an experiment at the Institute of Marine Research’s Research Station at Matre in which unvaccinated fish developed the condition on an equal footing with vaccinated fish. Furthermore, studies have shown that the condition arises more frequently at higher temperatures.

Easier to solve once the cause has been found
There are many more or less arbitrary factors behind the results that researchers currently possess. These findings constitute an excellent example of how research results usually cannot be planned. The large quantities of a virus in the dark spots were extremely surprising, and that they are of great significance to the filet being discoloured is beyond dispute. The next goal is also clear: now that we know what the problem is, how can we solve it? As always, an answer will generate several additional questions, and much still remains to be understood about how the condition is actually triggered.

By Mette Risbråthe

The research has been published in Veterinary Research

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