Eva Bonsak Thorstad

The recent invasion of pink salmon ( Oncorhynchus gorbuscha ) along the Norwegian coast has raised significant ecological concerns. Investigating regional differences in marine growth is an important step in evaluating the establishment of pink salmon in new environments. Pink salmon were captured in two northern Norwegian fjords during their return migration to in the summer of 2023; Varangerfjord in Finnmark and Oksfjord in Troms, representing the eastern and western extent of the core areas of the pink salmon invasion. Body size estimates (total length, weight, condition factor) were used to identify differences in sex-specific marine growth at the time of capture. Scales were analyzed to estimate marine somatic growth via the distance between circuli and the position of the last winter circuli (LWC). Estimates of cumulative circuli distance were divided into four marine growth periods that roughly correspond to 1) spring/summer, year one, 2) summer/fall, year one, 3) fall/winter, year one, and 4) spring, year two. Sex-specific and location effects were found in the body size of pink salmon, where males were heavier and had higher body condition than females and Varanger males had the highest estimates of body size. Scale readings revealed that growth rates were highest in the spring of both years (periods 1 and 4), with significantly decreased growth in the fall/winter (periods 2 and 3). Males from the established Varanger location exhibited the highest growth (17-21% total cumulative circuli distance) over all periods compared to the other groups, suggesting that these fish experience more favorable survivorship and marine growth conditions. These insights into pink salmon marine growth emphasize the importance of taking sex and location into consideration in further assessing the species’ expansive potential in Norway.

The invasive pink salmon ( Oncorhynchus gorbuscha Walbaum, 1792) is extending its distribution across the North Atlantic region, and important questions need to be answered in terms of spread, potential impacts, and threats to ecosystems and native species. To find evidence of pink salmon reproduction in Sweden, environmental DNA (eDNA) sampling and electrofishing were conducted in rivers along the Atlantic coastline. Pink salmon were detected in four out of five monitored rivers by use of eDNA from February to May 2024, albeit in low concentrations. No pink salmon were captured by electrofishing, and this method was largely unsuccessful due to flooding events and turbid waters. These findings provide the first indication of juvenile pink salmon presence in Sweden and represent the southernmost recorded eDNA-based detections of possible pink salmon reproduction in Europe and the North Atlantic. Further monitoring and research using eDNA are recommended to gain a comprehensive understanding of the recruitment and establishment of pink salmon in Sweden and other countries in the North Atlantic region.

Atlantic salmon have had a key role in European and North American oceans and human societies. Salmon are exploited and observed in rivers and along coasts, but there has always been great interest in knowing where salmon are going when they are at sea. Advances in electronic tagging and tracking, isotopic methods, and genetics have helped unravel a new understanding of salmon at sea. Ocean warming and changes to the Atlantic and Arctic food webs appear to be having an impact on the condition of salmon and their growth, survival, and age at maturity. Ultimately, salmon are threatened during their marine phase as the marine migration and major shifts in ocean currents, sea ice, and isotherms are apparently altering the distribution and abundance of their predators and prey at sea. However, there is still a paucity of knowledge about the relative importance of the different threats to Atlantic salmon at sea, particularly when comparing different regions and life stages (i.e. post‐smolts, sub‐adults, kelts). We argue that these knowledge gaps continue to limit effective management to curtail declining marine survival, and conclude with a series of key questions for the species in its marine phase.

Current knowledge on the continental shelf and transatlantic spawning migration of the European eel ( Anguilla anguilla ) is based on silver eels outmigrating from fresh water. However, an unknown proportion of eels grow in marine coastal waters during the continental part of their lifecycle and therefore start their spawning migration while they are already in marine waters. To study the onset of the European eel's spawning migration in marine waters, we tagged yellow eels in Sandnesfjord, Norway. One hundred yellow and pre‐migratory eels (stage FII or FIII) were tagged with acoustic transmitters in August, September or October in 2018–2019 and their subsequent movements were recorded within a hydrophone array until May 2023. Twenty‐two eels left the fjord in a period spanning from early October to early December, which partly coincides with the usual downstream migratory period among European eels. These eels were never detected again and had thus presumably transitioned to the silver eel stage. This transition occurred from a few months to up to 3 years after tagging. In all silver eels, an increase in vertical and horizontal movements within the fjord preceded their exit from the fjord. This sudden increase in activity occurred only for a few days before they left the fjord in most individuals, but this period was as long as 1 month in other individuals. Five of the silver eels undertook migrations up to several kilometres to a tidal stream before leaving the fjord. Eel migration was largely nocturnal, independent of moon or tidal phase. We conclude that the relatively short migration season and individual migration duration within the fjord, combined with the absence of significant environmental, physiological or anthropogenic stressors, may contribute to higher survival rates during the continental phase of the spawning migration compared to eels that start their spawning migration from fresh water, but this needs to be investigated in future studies.

Non-native pink salmon ( Oncorhynchus gorbuscha ) have increased rapidly in Norwegian rivers since 2017, yet their marine ecology remains poorly understood. To assess variation in marine growth, we analysed scales from 476 individuals returning to northern (Barents Sea region) and central (Atlantic region) Norway in 2019 and 2021. Clear differences in growth and body size emerged between regions and years, with fish returning in 2019 experiencing the poorest growth and reaching the smallest adult size. Differences in growth histories indicate that pink salmon captured in the two regions may have experienced distinct marine feeding resource conditions, possibly because they followed different migratory pathways, beginning in the first weeks at sea. Approximately one-third of individuals experienced distinct periods of depressed growth during their first spring and summer at sea, and these events were more common among fish returning to central Norway. Mid- to late-season growth during the first year at sea strongly influenced adult size, highlighting the importance of favourable conditions during this period. At the same time, it was shown that growth after the winter could more than compensate for poor growth up until then. The pronounced variability in growth performance suggests that environmental conditions during the marine phase strongly shape adult size and may influence the future invasion potential of pink salmon in the North Atlantic. As this invasive species continues to expand, continued monitoring of growth and improved understanding of their marine ecology and environmental drivers of growth will be essential for anticipating invasion dynamics and impacts on native ecosystems.

Introductions of species outside their native range, such as pink salmon ( Oncorhynchus gorbuscha ) in the Laurentian Great Lakes, can serve as unplanned experiments that provide new insights into ecological adaptation. We synthesize available information on the understudied Great Lakes pink salmon invasion and highlight how this case can inform research and management related to expansions and invasions of this species in the Pacific, Arctic and North Atlantic Oceans. Accidentally introduced to Lake Superior in 1956, pink salmon quickly spread to all five Great Lakes, displaying unexpected behaviours and life history plasticity. This invasion history demonstrates a remarkable ability of pink salmon to establish from a small founder population, colonize large areas, produce explosive year classes to rapidly increase in abundance, and complete a full life cycle entirely in freshwater. One of the most striking changes is a shift from their rigid 2‐year Pacific life cycle to a variable maturation age ranging from 1 to 4 years, likely influenced by prey availability as well as temperature and other environmental factors. We discuss implications for expansions elsewhere and outline five research themes necessary for understanding pink salmon dynamics in the Great Lakes with broader relevance for managing the species everywhere: (1) What drives rapid changes in abundance? (2) How do temperature extremes influence their ecology? (3) What causes departures from the 2‐year life cycle? (4) How important is it for the phenology of life history events to match new habitats? (5) What guides pelagic movements and straying in new habitats?

Pacific salmonids rely on precise sensory mechanisms, including the sagittal otolith, to support long-distance migrations. Vaterite replacement of otolith aragonite can impair sensory function, and compromise otolith-based reconstructions of environmental history. Otoliths from 1116 native and invasive Pink Salmon across the Pacific and Atlantic Oceans were examined for vaterite prevalence, coverage, and timing of formation. Visual assessment proved to be a reliable and efficient method for identifying otolith mineralogy, based on validation using X-ray diffraction ( R ² = 0.99, p < 0.001). We document exceptionally high vaterite prevalence in Pink Salmon from both Atlantic (97%) and Pacific (94%–96%) populations, with minimal geographic variation and no differences among native and invasive or wild and hatchery populations. Mean vaterite coverage within otolith pairs was 49 ± 27% ( n = 449). The prevalence of vaterite far exceeded that reported for other salmonids, including hatchery-reared populations. Our results indicate that vaterite formation can occur at any life stage, beginning as early as the juvenile freshwater stage and peaking early in the marine phase. Overall, our findings support an association between the unusually high prevalence of vaterite in Pink Salmon and a life-history strategy characterized by rapid growth.

Species redistributions, whether resulting from invasions or range expansions, pose similar challenges for conservation and management. Redistribution impacts on host ecosystems and species combine with those from climate change, which are already acute at northern latitudes. Using pink salmon Oncorhynchus gorbuscha, which are native to the Pacific Ocean, we employ knowledge exchange to inform decision-making in non-native marine areas: they are expanding their range to the Arctic Ocean and are invasive in the Atlantic Ocean. The predicted future marine distribution of pink salmon focuses effort on where and when pink salmon are present and informs on potential interactions with native species. Management actions taken in the Atlantic Ocean to reduce invasive pink salmon are resource-intensive, but removed salmon could be a food resource. Addressing identified gaps regarding the invasion potential of pink salmon, interactions among pink salmon and other species, and current mitigation efforts would support forward-thinking management decisions given predictions of continued environmental change. We also highlight steps that can be taken immediately to coordinate actions and better inform responses. Managed for production in the Pacific and as an invasion in the Atlantic, pink salmon provide a tangible approach to informed decision-making through collaboration for marine species on the move.

Freshwater ecosystems are highly biodiverse¹ and important for livelihoods and economic development², but are under substantial stress³. To date, comprehensive global assessments of extinction risk have not included any speciose groups primarily living in freshwaters. Consequently, data from predominantly terrestrial tetrapods^4,5 are used to guide environmental policy⁶ and conservation prioritization⁷, whereas recent proposals for target setting in freshwaters use abiotic factors^8–13. However, there is evidence^14–17 that such data are insufficient to represent the needs of freshwater species and achieve biodiversity goals^18,19. Here we present the results of a multi-taxon global freshwater fauna assessment for The IUCN Red List of Threatened Species covering 23,496 decapod crustaceans, fishes and odonates, finding that one-quarter are threatened with extinction. Prevalent threats include pollution, dams and water extraction, agriculture and invasive species, with overharvesting also driving extinctions. We also examined the degree of surrogacy of both threatened tetrapods and freshwater abiotic factors (water stress and nitrogen) for threatened freshwater species. Threatened tetrapods are good surrogates when prioritizing sites to maximize rarity-weighted richness, but poorer when prioritizing based on the most range-restricted species. However, they are much better surrogates than abiotic factors, which perform worse than random. Thus, although global priority regions identified for tetrapod conservation are broadly reflective of those for freshwater faunas, given differences in key threats and habitats, meeting the needs of tetrapods cannot be assumed sufficient to conserve freshwater species at local scales.