The Invasive Humpback Salmon is Back in Sweden

The invasive humpback salmon has once again been observed in Swedish waterways, most recently in the Ätran River in Halland. The species has large spawning migrations and its impact on ecosystems is not yet known. It is therefore considered a potential threat to the already severely depleted Swedish Atlantic salmon. Researchers and authorities want to stop its spread before it is too late.

At the end of June, fish counters recorded two humpback salmon in the Ätran River in Halland, according to Fiskdata.se. The species has been seen in Sweden before, but each time it causes new concern.

In Norway, the development has been rapid. Tens of thousands of humpback salmon have made their way up rivers, and in some fjords, more humpback salmon have been caught than Atlantic salmon.

The species has also been seen in Denmark this year – according to the magazine , an angler caught two humpback salmon on the north coast of Själland. This is very unusual in Danish waters.

The species does not belong in Sweden

The humpback salmon originally comes from the northern Pacific Ocean and was introduced into northern Russian rivers by the Soviet Union in the 1950s. From there, it has spread via the Barents Sea and along the coast of Norway – and now to Sweden. Here, it has only been found in small numbers, but experts are monitoring developments closely.

The humpback salmon (Oncorhynchus gorbuscha) has a two-year life cycle. It is divided into so-called odd-year and even-year populations, depending on which year they spawn. In Europe, it is mainly the odd-year spawning salmon – i.e. those that spawn in 2023, 2025, and so on – that have managed to establish themselves.

New species – already classified as high risk

According to Ida Ahlbeck Bergendahl, Environmental Assessment Specialist at SLU’s Department of Aquatic Resources, the humpback salmon is “a new species in our waters” – but how it affects our ecosystems is still unclear.

“There are major gaps in our knowledge,” she says.

Despite the uncertainty, the Swedish Species Information Center has recently classified humpback salmon as a species with a very high risk – the most serious category used in Sweden. According to Michael Diemer, investigator at the Swedish Agency for Marine and Water Management’s Unit for Action Coordination, this is precisely why humpback salmon is now being proposed for inclusion on Sweden’s list of invasive species.

“The fact that we are proposing it for the national list is a clear sign that we are concerned about future developments,” says Michael Diemer.

“It is classified as very high risk. That says a lot about how we view the future,” he adds.

The development is also being monitored closely at the regional level. Erika Axelsson at the Halland County Administrative Board does not currently see any impact on the ecosystem in Sweden, but raises concerns about what may come.

“I’m not worried now, but I am concerned about the development. It seems to have an ability to adapt, and we’ve seen what happened in Norway,” she says.

Better to prevent than cure

If humpback salmon were to begin reproducing on a larger scale in Sweden, it could have significant ecological consequences. The species spawns earlier than the native Atlantic salmon, is aggressive, and can compete for both spawning grounds and food.

“There is a fear that it will outcompete our salmon in the waterways, change the behavior and spawning grounds of our salmon,” says Ida Ahlbeck Bergendahl.

After spawning, humpback salmon die. If they become common, large quantities of dead fish could affect both water quality and the nutrient balance.

“There could be a lot of dead fish, bacteria, and fungi. This would be a nutrient input that our systems are not used to,” she continues.

Michael Diemer emphasizes that now is the time to act.

“It’s always much easier to prevent an undesirable development than to try to deal with it later,” he says.

Monitoring is ongoing – but more research is needed

SLU is monitoring the spread of the species by taking water samples and analyzing environmental DNA. So far, no spawning has been confirmed.

“We haven’t found anything yet, but the samples haven’t been fully analyzed,” says Ida Ahlbeck Bergendahl.

Only a few fish have been found in Sweden, and the risk is currently considered low. However, SLU has not received research funding to investigate how humpback salmon affect Swedish salmon. Norway also lacks the funds to study the effects, despite the species being widespread there.

The Halland County Administrative Board supports SLU’s work, both financially and practically. Among other things, they have helped set up cameras and are assisting in the collection of environmental DNA.

“It’s a good tool, but there are uncertainties. It’s better when we get a specimen or a picture,” says Erika Axelsson, county fisheries consultant at the Halland County Administrative Board.

According to Axelsson, humpback salmon have previously been found in both the Fyleån and Lagan rivers. She believes there are signs that spawning may have occurred, but this has not yet been confirmed.

Report – and kill

The authorities are urging anglers to kill any humpback salmon they catch, not to release them back into the water, and to report their findings via the app Rappen. County administrative boards along the west coast are also collecting scale samples from caught specimens.

^{About Deep Sea Reporter: Our ambition is to examine and report on issues related to the sea and the life that exists beneath its surface. We operate in the public interest and are independent of political, commercial, and other interests in society.}