Panu Orell

Pink salmon originate from the North Pacific area but were introduced into northwest Russia from the late 1950s onwards. Since 2017, the alien species has increased dramatically in abundance and rapidly invaded adjacent areas of the North Atlantic region. In the large Teno River in northernmost Norway and Finland, running to the Barents Sea, various monitoring methods originally designed for assessment of Atlantic salmon populations have been used to observe the development in abundance and distribution of pink salmon in the main stem and in a number of tributaries. In addition, environmental DNA (eDNA) sampling has enabled monitoring of these trends across an even wider set of tributaries. The first observations of pink salmon were made in the 1960s, and variable but mostly low catches were recorded in the following decades. In recent years, the total number of pink salmon entering the Teno system increased rapidly from c. 5000 in 2017 to c. 180,000 in 2023. Initially, the invading pink salmon were occupying the main stem, large tributaries and headwater rivers of the catchment, even up to a distance of 250–350 km from the sea. However, in recent years, a greater number of smaller tributaries have been occupied as demonstrated by eDNA detections and other observations. The largest spawning aggregations of pink salmon have been observed in the main stem of the Teno River. Future development in the abundance and dispersal of pink salmon in the Teno system depends strongly on the extent and success of the mitigation efforts in intercepting and removing pink salmon by a weir and trap close to the estuary.

The Näätämö River, in the far north of Europe, hosts a large Atlantic salmon (Salmo salar) stock with high socio-economic value. The catchment has near-pristine environmental conditions, although there are recent signs of stock declines. It nevertheless offers a good opportunity to monitor the status of a near pristine wild salmon system. This study aimed to characterize the fine-scale genetic structure of Atlantic salmon in the Näätämö River and evaluated the possibility to assign individuals to their population of origin. We genotyped juveniles sampled from eight locations using a 60 K single nucleotide polymorphism (SNP) panel to characterize within-river genetic diversity and structure. We also tested the performance of a previously designed 180 SNP panel to assign individuals to their population of origin. The genetic structure of the Näätämö River salmon population appears weak (F_ST ranging from 0.001–0.035), possibly due to natural straying. The performance of the predesigned panel in assigning individuals to their population of origin was moderate to high (82–85%), but customization could further enhance its accuracy. Additionally, we described the life-history diversity of salmon within the river using a long-term scale archive of c. 22,000 individuals collected over 48 years, identifying 59 unique life-history strategies. We suggest that to protect the self-sustaining salmon population in the Näätämö River, it would be important to manage the salmon fishery and conserve the population by incorporating up-to-date knowledge of stock status, considering their genetic structure, substantial gene flow and diverse life history.

The recent, rapid spreading of non‐native pink salmon Oncorhynchus gorbuscha in the North Atlantic area has raised concerns about their possible negative impacts on native salmonid species. Potential interactions include competition for food resources during the short freshwater phase of juvenile O. gorbuscha, but little is known about their feeding behavior in the newly occupied North Atlantic rivers. Using stable isotope and stomach content analyses, patterns of freshwater feeding of non‐native O. gorbuscha fry were studied in a large Fennoscandian river, the Teno, that discharges to the Barents Sea. Changes in stable isotope values (δ¹³C, δ¹⁵N, δ³⁴S) and stomach contents from the period of emergence (April to mid‐May) to estuarine entry (late May/June) were examined and provided both temporally integrated and short‐term indicators of freshwater feeding dependency. In addition, the occurrence of juvenile O. gorbuscha and changes in their length and weight during their emergence/migration period were investigated. Juvenile O. gorbuscha were at the spawning grounds from April through to mid‐May with abundance peaking in mid‐May. Fish moved to the estuary by late May and their abundance decreased toward June, and their body size increased concurrently. Stomach analyses indicated no feeding activity in April–early May in the spawning areas, but the stomach fullness indices increased markedly in fish sampled in the estuary in May and June. The most important prey items in stomachs were Chironomidae and Ephemeroptera larvae. Significant changes in all analysed stable isotopes were detected among sample periods, with a peak in mid‐May and June showing significantly lower values than other sample periods. A change from the higher values reflective of parental marine feeding to the lower values reflective of freshwater feeding indicated active in‐river feeding by juveniles during the study period. The documented active freshwater feeding of non‐native juvenile O. gorbuscha suggests potential resource competition with native fluvial fishes, particularly salmonids.

The spawning and egg development of invasive pink salmon Oncorhynchus gorbuscha were investigated in the large River Teno in the Barents Sea area where they spawned for the first time on a large scale in 2021. The spawning period started in early August and egg development was rapid. All eggs were eyed by mid‐September and the first juveniles hatched in late September. In early October most eggs had hatched. Degree‐days in water temperature suggested that egg deposition had mostly taken place in early August. Early egg development is discussed in relation to possible consequences for survival.

Understanding the drivers of evolution is a fundamental aim in biology. However, identifying the evolutionary impacts of human activities is challenging because of a lack of temporal data and limited knowledge of the genetic basis of most traits. Here, we identify the drivers of evolution toward maturity at an earlier age in Atlantic salmon through two types of fisheries-induced evolution acting in opposing directions: an indirect effect linked with harvest of a salmon prey species (capelin) at sea (selection against late maturation) and a direct effect due to net fishing in rivers (selection against early maturation). Because capelin are harvested as an aquaculture feed protein source, we hereby determine an indirect path by which salmon aquaculture may influence wild salmon populations.