Wild salmon can defeat gyro

Recent research results shows that Norwegian wild salmon has the ability to develop genetic resistance against the deadly Gyrodactylus parasite.

Contact person
Hans Bernhard Bentsen

Senior Scientist
Phone: +47 64 97 04 46
hans.bentsen@nofima.no

The Norwegian management of the wild Atlantic salmon stock has during more than 20 years focused on eradicating the deadly Gyrodactylus parasite by treating infected rivers with rotenone poison and other chemicals.

However, full eradication has proven unsuccessful so far and the costs have now exceeded NOK 300 million.

Recent research results from a group including scientists from Nofima Marin shows that Norwegian wild salmon has the ability to develop genetic resistance against the parasite.

If proper action is taken, the mortality rate in infected rivers can be significantly reduced within a reasonable time with nature’s own methods.

The wild salmon is threatened

The wild Atlantic salmon stock is at record low levels, and is struggling even after several decades of international efforts to stop the decline. An excessively large proportion of the migrating salmon smolt does not survive in the sea.

In addition, the wild salmon stock of the Northeast Atlantic is threatened by the salmon parasite Gyrodactylus salaris during the first months of life in fresh water.

Every year, the parasite kills large amounts of salmon juveniles in Norwegian rivers, where nearly 50 percent of the Atlantic salmon stock spawns. Consequently, the parasite is contributing to a further reduction of the stocks.

There is concern that several local salmon stocks can become entirely extinct.

New approaches should be tested

Since 1986, the official management strategy against Gyrodactylus infections in Norwegian wild salmon stocks has been to eradicate the parasite from Norwegian rivers. This has so far demanded large resources and cost more than NOK 300 million.

The main measure has been to treat infected rivers with rotenone poison. The treatment has succeeded in smaller rivers, but has often failed in large, complex river systems despite repeated rotenone treatments.

In recent years, treatment with acid aluminium has been tested without the desired effect in complex rivers. Consequently, there is a fear that the eradication strategy is failing, and no alternative strategies have been developed. This threatens the future of the wild Atlantic salmon.

Recent research results now show that some individuals in a wild Atlantic salmon stock have the ability to survive when infected by the Gyrodactylus parasite, and that this ability is inherited.

This can be exploited in a strategy to enhance the genetic resistance in infected wild stocks of Atlantic salmon if future attempts to eradicate the parasite also fail.

Most infected river stocks are already cultivated by catching wild broodstock in the river and using hatcheries to produce offspring that are released into the river.

This makes it possible to implement measures that ensure that parent fish with high genetic resistance to the parasite are used.

The research results show how this can be achieved.

Important research results

The research results were presented in an article in the January edition of the recognised Canadian Journal of Fisheries and Aquatic Sciences.

The article was written by a group of Norwegian scientists from several major research institutions within salmon biology, genetics and parasitology – the Norwegian University of Life Sciences (UMB), Nofima, The Norwegian Institute for Nature Research (NINA) the University of Oslo (UiO) and the Centre for Integrative Genetics (CIGENE) – in collaboration with local stakeholders near the Drammen River.

The experiments were carried out with wild Atlantic salmon from the Drammen River, which has been infected by the parasite since 1987 and which is too large and complex to be treated with chemicals.

The offspring of captured, local wild Atlantic salmon were exposed to the Gyrodactylus parasite in a controlled co-habitant challenge test with naturally infected juvenile fish from the river. After two months, only about 10 percent of the experimental fish had survived, which agrees well with reports of mortality rates in infected rivers.

However, the least affected full-sib families had survival rates of up to nearly 50 percent.

This showed that the ability to survive Gyrodactylus infection is inherited.

The degree of heredity in the test implies that the survival rate will increase from 10 percent to 24 percent in one generation if only fish that survived the test are used as parents. Repeated selection is expected to increase the survival rate further with each new generation.

The results are indeed significant and clearly disagree with the prevailing understanding of many scientists as well as the wild salmon management authorities.

There has been an assumption that infected Norwegian salmon stocks are incapable of defeating the parasite and will, therefore, become extinct if the parasite is not eradicated.

What can be done?

The results show that wild Atlantic salmon has genetic potential to develop a steadily increasing resistance against Gyrodactylus through natural selection. So why has this not already happened?

Firstly, this will be a time-consuming process in the nature, where many years can pass from the selection among salmon juveniles occurs in the river until the survivors migrate to the ocean and return to the river as spawners.

Secondly, the combination of initially low survival rates and random incidents may result in spawning failure among the local salmon in some year classes.

Thirdly, large numbers of hatchery produced salmon juveniles are often released into infected rivers, juveniles that have been protected from the Gyrodactulus infection and consequently not been through natural selection.

Observations from the river Driva, where salmon juveniles have not been released in recent years, indicate that the survival rate is increasing even though the river has been infected since 1980.

In order to reduce the time it takes to develop genetic resistance against the parasite and safeguard against incidental setbacks under varying natural conditions, it is possible to exploit the use of hatcheries to test the resistance among offspring of wild broodstock. Fish that survive the controlled challenge test can then be reared in captivity until they reach sexual maturity and then be used as parents in the production of salmon juveniles for release.

This will both accelerate the development of genetic resistance and ensure that there are sufficient salmon juveniles in the river every year.

Since increased genetic resistance in the infected salmon stocks is unlikely to result in extinction of the parasite, the initial focus could be on river systems where the strategy to eradicate Gyrodactylus is in practice impossible, e.g. the Drammen River. Moreover, the measure can also be used as a backup insurance in rivers where eradication using chemicals has proven to give uncertain results.

In the long term, straying of spawners from river stocks with high resistance against the parasite may strengthen the survival ability in other stocks if the parasite spreads or becomes reintroduced from the nearby Baltic Sea rivers along the border to Finland and Sweden.

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