Sexually mature male pink salmon have a hump-like protrusion on the front part of their backs, hence they are also named humpback salmon. Photo: Henrik H. Berntsen / NINA.

Pink salmon

NINA is monitoring the distribution of pink salmon in Norway.

Pink salmon, also called humpback salmon, are native to the northern parts of the Pacific Ocean. In the 1960s, pink salmon began to spread to northern Norwegian rivers after being released on the Kola Peninsula in Russia. Today, pink salmon can be caught throughout Norway, from the border with Russia in the north-east to the Swedish border in the south-east. However, pink salmon are most numerous in the northernmost counties of Troms and Finnmark. Pink salmon have been found to spawn in several Norwegian rivers.

Catches of pink salmon in Norway have increased sharply in recent years, and the species also appears to be expanding its core area to the south of the country. We do not know the exact reason for this increase, but analyses show that rising sea temperatures in the High North probably play an important role.

Two-year life cycle

Pink salmon have a two-year life cycle, and both males and females die after spawning. After hatching the following spring, the juveniles migrate relatively quickly to the sea. There, a pink salmon spends the next year grazing on crustaceans and fish, and during its second summer it returns to freshwater to spawn and complete its lifecycle. Fish that spawn in odd-numbered years thus have offspring that also spawn in odd-numbered years, and consequently do not spawn with fish that spawn in even-numbered years. Here in Norway, pink salmon spawn primarily in August.

In Norway, the odd-number-year population is most numerous, but there is also a smaller population of even-number-year fish.

A high-risk invasive species 

Pink salmon is a high-risk species, according to the Norwegian Biodiversity Information Centre’s list of alien species. If the pink salmon establishes itself in rivers over larger areas, that increases the chance there will be regular and numerous invasions of pink salmon in Norwegian rivers.

What problems do pink salmon cause?

A few pink salmon in a river is not a big problem, but if thousands come to spawn in a river, as is now seen in parts of Norway, there is a high risk that it can have negative effects on local salmon, sea trout and Arctic char populations. Pink salmon is a species whose population can grow rapidly, as has been seen in Finnmark County, where several thousand fish swim up some watercourses to spawn. This can affect other fish and the entire ecosystem. It can also affect the recreational fishing in the rivers.

• Pink salmon can carry disease organisms that infect salmon, trout and Arctic char. These may be new diseases and viruses that may be introduced to Norway with pink salmon, or pink salmon can contribute to the spread of disease between fish farms and wild fish in different rivers.
• Large schools of pink salmon can displace native salmon, and change and disrupt native salmon behaviour during the weeks and months they live in a river until spawning. This can also happen with sea trout and Arctic char. Pink salmon are known to sometimes behave aggressively towards other salmonids in rivers.
• Large schools of pink salmon, which displace and disrupt the behaviour of salmon, sea trout and Arctic char, can also be a problem for fishing.
• Pink salmon are mostly found in highest numbers near the lowest parts of a watercourse, but they have also been observed as far as several tens of kilometres up Norwegian rivers. They often spawn in the same areas as salmon and trout, but slightly earlier than salmon and trout, from early August to early September.
• If pink salmon become numerous and die in the rivers in large numbers, this can result in a large number of dead, rotting fish in the rivers throughout the autumn. Dead, rotting pink salmon can release nutrients to the water, which can affect the entire ecosystem in an unpredictable way. It can affect the growth of juvenile salmon, and the cumulative effect on salmon production in a river is difficult to predict — it can have a positive, but also negative, effect.
• If the supply of nutrients from dead pink salmon is too high, it can become a pollution problem. Large numbers of dead pink salmon in rivers can also affect terrestrial ecosystems. Dead and dying pink salmon can be a food source for different mammals and birds, making the combined effect of this new source of food unpredictable.
• Pink salmon are known to migrate to the sea during the same spring as they hatch, while salmon and trout stay in rivers for 2-4 years before migrating to the sea. This means the risk of competition with salmon and trout for food and space in the river is small. But where pink salmon feed for a short period in a river, as suggested by some observations in Russia and Norway, this can result in large numbers of pink salmon for a few weeks and increased competition with salmon and trout during this period.
• If pink salmon reside in estuaries for a period, they can affect other species there as well.
• If the effect of pink salmon becomes so great in some rivers that they displace juvenile salmon, this threatens river mussels (Margaritifera margaritifera) which rely on salmon and trout as hosts during their larval stage, but which cannot live in pink salmon.
• A number of studies from the Pacific Ocean show that in the years when there are large numbers of pink salmon in the sea, the competition for food reduces the growth and survival of other salmon species. In the same way, it has also been shown that birds can be negatively affected. There are much greater numbers of pink salmon in the Pacific than have been seen so far in the Atlantic and Barents Sea, but what has happened in the Pacific shows that the effects of pink salmon on other fish in the sea can occur if pink salmon become too numerous.
• When alien species establish themselves, it’s possible for negative effects to occur that no one has thought of before. If pink salmon are found in large numbers in the years to come, then there is also a possibility that they will change and better adapt to Norwegian river conditions, such as by altering their spawning period.
• The effects of pink salmon on salmon farming in Norway are primarily linked to pink salmon as a possible spread of fish diseases. The Norwegian Scientific Committee for Food and Environment has explored this issue in its risk assessment of pink salmon.

Until now, there has been little research on the effects of pink salmon in Norway, so there little specific information about these effects.

Pink salmon in other countries around the Atlantic

Catches of pink salmon have also increased since 2017 in Sweden, Denmark, Finland, the United Kingdom, Ireland, Iceland, the Faroe Islands and Greenland. In addition, some pink salmon have been recorded in German and French rivers, and on the eastern side of Canada.

Increased occurrences in Norwegian rivers will increase the risk of further spread to the other countries.

Also read:

Pink salmon life cycle. Illustration: Sigrid Skoglund.

Pink salmon - an alien species in Norway

Pink salmon (Oncorhynchus gorbuscha) is on the Norwegian Biodiversity Information Centre’s Black List of alien species, in the “high risk” category. There may be competition for spawning habitat between pink salmon and native salmonids (salmon, brown trout and Arctic char), although pink salmon normally spawn before these species. It is uncertain what effect dead and rotting pink salmon, which die after spawning, will have on a river’s ecosystem. It is also uncertain whether there is competition between fry of pink salmon and native Norwegian salmon fish species, because pink salmon fry migrate to the sea shortly after hatching. 

Catches of pink salmon in Norway over the last 45 years.

Click on the illustration to see the animation.