From Catchment to Coast—Ecological impact of juvenile Pacific pink salmon

Get ready for the 2025 pink salmon run when locals and fishing associations will trap as many pink salmon as possible before they enter northern Norway’s rivers! The media will be out in force too, describing how pink salmon threaten northern coastal ecosystems. But how much do we really know?

Large numbers of pink salmon are relatively new to Norway’s coasts and rivers, and so is our knowledge about their potential impacts on these ecosystems. The Fram Centre’s Catchment to Coast programme has focused on understanding some of the interactions of pink salmon with coastal ecosystems—a priority for those managing our northernmost rivers. Local management and interest groups in Finnmark have expressed their concerns about lack of knowledge on the degree to which pink salmon are spreading into even smaller tributaries, disturbing spawning of native salmonids, impacting river water quality with their rotting carcasses, and competing for food with native salmonids. These concerns were repeatedly expressed to Bror Bonde from the Arctic Sustainability Lab at UiT – The Arctic University of Norway, during interviews in 2024. Bror set out to determine what main ecological knowledge is lacking and hindering the effective management of pink salmon. 

One concern described as a major risk to native fish was that large numbers of pink salmon juveniles could feed on freshwater invertebrates and thus compete with native salmonids. Pink salmon eggs are laid in “redds”, nests that adult females create and fiercely protect while their bodies slowly decay and they turn into the many “zombie fish” haunting rivers in August and September. Long after the adult pink salmon have become carcasses, supplying energy and nutrients to the river and coastal food webs, pink salmon juveniles hatch and begin their journey down the river to enter coastal and offshore waters rich in marine food. In their native Pacific areas, pink salmon juveniles are thought to travel directly to the sea without feeding, unless the sea is very far away, or migration starts late. Understanding when juvenile pink salmon hatch and begin feeding in the river is crucial to assess their competition with native salmon for food and space.

To better understand this interaction, over 400 juvenile pink salmon were captured in 10 rivers in northern Norway between April and July 2022, through targeted sampling aided by local fishing associations. Hallvard Jensen from the Norwegian Institute of Bioeconomy Research and Rune Muladal from Naturtjenster i Nord searched through the ice breaking up in Skallelva to find juveniles. Jenny Jensen from Akvaplan-niva repeatedly sampled juveniles in Eibyelva to study the progression of emergence from redds throughout the spring and summer. Karl Øystein Gjelland and colleagues from the Norwegian Institute for Nature Research collected pink salmon smolts during routine sampling in Skjoma. All fish were processed by Sigurd Slåtang in the freshwater ecology laboratory at UiT to look for stomach contents and to prepare samples to be sent to Antti Eloranta at the University of Jyväskylä in Finland. There, Antti and his team disentangled the contributions of freshwater and marine food sources in the assimilated diets of juvenile pink salmon and native salmonids. 

We found that pink salmon juveniles still used their yolk sacs for sustenance during their freshwater phase in April and May. But by mid-May, some pink salmon smolt had started feeding in the rivers Kvalsundelva and Rungelva. In early July, free-swimming juvenile pink salmon in Kongsfjordelva near Varangerfjorden began feeding on freshwater invertebrates, no longer relying on their yolk sac. On 2 July, smolt caught in Skallelva were transitioning to feeding: some had stomach contents, while others still relied on their yolk sac. 

Overall, our findings show that pink salmon smolts start to feed on freshwater prey upon reaching a body length of 30–35 mm. The invertebrates that they feed on, such as non-biting midges (Chironomidae larvae and pupae), blackfly larvae (Simuliidae), and mayfly larvae (Ephemeroptera), are also a primary food source for native Atlantic salmon and brown trout. Therefore, there is potential competition for the same food source between pink salmon and native salmonids later in the summer. 

Is the observed late migration to the sea a widespread strategy for young pink salmon in northern Norway, or is it only found in specific rivers like Kongsfjordelva? Predictions of emergence timing do not support an evolution toward late spawning and emergence in pink salmon in northern Norway. The presence of large numbers of fish suggests that, despite late spawning not being favoured, a significant number may still use this strategy. Attaining a larger body size in the river through external feeding also has advantages to survive cold coastal waters around Troms and Finnmark. However, we still lack basic knowledge on the pink salmon smolt run, such as when it starts, peaks, and ends. 

Although pink salmon smolts are potential competitors to young native salmonids, they may also serve as food for them. Young Atlantic salmon and trout can feed on salmon eggs, and some larger individuals can also feed on pink salmon smolts. Stable isotope analyses showed that as Atlantic salmon and brown trout increase in size, they increase the amount of marine-derived nutrients in their body from feeding on pink salmon smolts and eggs even though they are still in the rivers. We know from local contacts that pink salmon smolts have been found in the stomach of brown trout showing that pink salmon young can be a valuable food source to native fish.  

Rune Muladal surveyed 16 rivers in Troms and Finnmark in spring 2024 to assess pink salmon spawning and hatching success after trapping efforts in 2023. He concluded that the juveniles were at the same developmental stage but more numerous than in similar registrations in 2022, indicating an even larger smolt run from these rivers in 2024. Young pink salmon juveniles are continuing their journey from Catchment to Coast in northern Norway. Those leaving late have the added bonus of loading up on some freshwater invertebrates to sustain them onwards to the Norwegian coast and finally the North Atlantic.