Abstract. Marine heatwaves (MHWs) are increasing globally, yet their long-term variability and trends remain poorly characterized across the high-latitude northeastern North Atlantic. We analysed daily sea surface temperature from the TOPAZ reanalysis during 1991–2025 to quantify the spatial characteristics, temporal trends and dominant modes of surface MHW variability across the North Sea, Norwegian Sea and Barents Sea and assessed associations with large-scale climate variability. We also examined MHW exposure within Norwegian particularly valuable and vulnerable areas (SVOs). MHWs occurred at an average frequency of 1.7 events yr-1 and lasted 22.6 ± 6.6 days, with southern and coastal regions experiencing more frequent and intense events, whereas northern and sea ice-influenced regions experienced fewer but generally longer events. MHW frequency, cumulative intensity and annual MHW days increased over the study period, while trends in mean and maximum intensity were weaker and more spatially heterogeneous. The fraction of open water affected by MHWs increased by 21.7 % per decade, reaching 99 % in 2025. The two leading modes of detrended annual MHW day variability explained 53.5 % of the total variance and represented a coherent basin-scale pattern and a north–south dipole. Statistical associations with the East Atlantic pattern, previous summer North Atlantic Oscillation and previous year Atlantic thermal conditions suggest contributions from both atmospheric circulation and oceanic preconditioning. These lagged atmospheric and oceanic relationships suggest potential for improving MHW early-warning frameworks. MHW exposure was substantial within ecologically important SVOs. This increase in MHW frequency, persistence and extent across the region, has the potential to reduce thermal refugia and increase risks to fisheries, aquaculture and high-latitude marine ecosystems. These findings provide a regional baseline for ecological impact assessment and long-term monitoring, supporting ecosystem-based management and adaptation strategies in a rapidly changing subarctic region.

Most of the previously large cod stocks in the North Atlantic are depleted to very low levels. A notable exception has been the Northeast Arctic cod inhabiting the Barents Sea. This cod stock reached a record high level around 2013, but since then has declined sharply, with older fish being fished out and few new recruits entering the stock. More specifically, since 2006 the average recruitment at age 3 per unit spawning stock biomass has dropped to a very low level. Here we review recent literature and data to evaluate potential causes of poor recruitment. We find that in most years the large spawning stock was successful in producing offspring up to the 0-group stage, but the survival thereafter was reduced-leading to few recruits into the fishable stock. Evidence points to consequences of northwards shifts of spawning locations, reduced inflow of Calanus and density dependent factors influencing pre-recruit survival at times when the spawning stock was at a record high level. However, large uncertainties in the causes of recruitment failure remain at a time when the Barents Sea ecosystem is experiencing unprecedented warming. A precautionary measure to avoid further decline and potential stock collapse in the current situation is to lower fishing pressure and improve management by setting reference points based on the recent period with low productivity. Including demographic characteristics like age diversity as explicit management targets to ensure spatial and temporal spread in spawning should be a goal for future method development.

Marine heatwaves (MHWs) are prolonged periods of extreme ocean warming that significantly impact marine ecosystems, fisheries, and aquaculture. In August 2024, northern Norway experienced one of the most intense MHWs recorded since 2012, which coincided with a sharp rise in salmon lice infestations at aquaculture sites in the area. This study investigates the atmospheric and oceanic drivers of this event, focusing on the interaction of local meteorological and oceanic conditions with large-scale climate variability. Using a combination of ocean model hindcast data (Norkyst), atmospheric reanalysis data (ERA5), and in situ observations, we characterized the MHW and identified key contributing factors. Our analysis revealed that the MHW was driven by a combination of weakened local wind patterns, high air temperatures, and strong stratification, alongside external heat supply from northward advection of warm and fresher water, facilitated by a positive phase of the summer North Atlantic Oscillation (NAO). The positive NAO phase enhanced southwesterly winds, which transported warm and humid air masses into the region, increasing the total heat flux from the atmosphere and further intensifying local warming. The ecological impacts of this MHW included increased salmon lice abundance, posing significant challenges to wild and farmed salmon populations in a region that hosts the world’s largest salmon aquaculture industry. Understanding the drivers of MHWs in northern Norway is essential for assessing their predictability and informing management strategies to mitigate their effects. This study highlights the importance of advancing regional MHW forecasting to enhance resilience in fisheries and aquaculture sectors under a warming climate.

Arctic amplification of climate change is causing sea ice to retreat at unprecedented rates, potentially opening up large vulnerable Arctic areas for oil and gas exploration and new shipping routes. This rapid warming marginalizes sympagic species habitats making them more sensitive to other anthropogenic pressures. Here, we assess potential impacts of hypothetic oil spills from the northernmost licensed oil field Wisting and additional neighbouring spill sites in areas currently not open to oil exploitation on the key ice-associated Arctic fish species polar cod (Boreogadus saida). We do this by developing and running combined data-driven models for the ocean, oil spill dispersal and fate, and the early life stages of polar cod. Sea ice and the Polar Front act as natural barriers limiting the exchange of polar cod eggs and larvae and oil spill between Atlantic and Polar Water. However, both barriers vary seasonally so that the sea ice retreats and the Polar Front weakens towards summer causing significant increases in oil exposure to early life stages of polar cod under varying oil spill scenarios investigated here. Previous literature emphasizes that fall feeding conditions must be sufficient for juvenile polar cod to allocate lipids and survive their first winter. Here, we show that less than half the exposed individuals experience these suitable feeding conditions in the fall. The seasonal exposure intensity suggests a need for petroleum regulations with temporal and spatial limitations varying through the year. However, even with these seasonal dynamic regulations in place, climate change induced by the use of fossil fuel will likely reduce these natural barriers through continued sea ice retreat and a weakening of the Polar Front thereby reducing their barrier effects. Risk assessments of anthropogenic impacts on key Arctic ecosystem components in the vicinity of the ice edge zone and the Polar Front will therefore have to be updated to account for these major changes.

Marine heatwaves are prolonged periods of extreme ocean warming that can strongly impact marine ecosystems, fisheries, and aquaculture. In August 2024, northern Norway experienced one of its most intense marine heatwaves on record, with sea surface temperatures exceeding 18 °C—more than 4 °C above the long-term average. This event contributed to an unprecedented salmon lice outbreak at aquaculture sites, causing increased fish mortality and economic losses. Here we use numerical ocean model hindcast data, atmospheric reanalysis data, and in situ observations to identify the factors behind this event. Local weather conditions, including high solar radiation, unusually warm air temperatures, and weak winds, caused increased heat transfer to the sea surface and reduced ocean mixing. Additionally, large-scale atmospheric pressure patterns intensified warm air advection and freshwater transport along the coast, reinforcing local stratification and warming. These findings highlight the importance of advancing regional marine heatwave forecasting to support fisheries and aquaculture resilience under a warming climate.

Risk assessments of oil spills in the marine environment typically only address dissolved oil. However, oil droplets may adhere to some organisms and result in prolonged exposure not disrupted by diverging dispersal trajectories. Here, we present a framework for quantifying exposure of dissolved and droplet oil on adhesive fish eggs and implement the framework in a well-established numerical model for ocean, oil and fish eggs. Utilizing the model for a case study with an oil spill in the habitat of the world's largest haddock stock, Northeast Arctic haddock, shows that droplets for a given concentration cover a much larger domain than dissolved oil. Impact assessments not taking oil droplets into account may therefore severely underestimate the impact of oil spills and result in decisions with inappropriate weighting of societal benefits to ecosystem impacts. Finally, our results call for more experimental work on the impact of oil droplets on marine species.

The oil industry's expansion and increased operational activity at older installations, along with their demolition, contribute to rising cumulative pollution and a heightened risk of accidental oil spills. The lesser sandeel (Ammodytes marinus) is a keystone prey species in the North Sea and coastal systems. Their eggs adhere to the seabed substrate making them particularly vulnerable to oil exposure during embryonic development. We evaluated the sensitivity of sandeel embryos to crude oil in a laboratory by exposing them to dispersed oil at concentrations of 0, 15, 50, and 150 µg/L oil between 2 and 16 days post-fertilization. We assessed water and tissue concentrations of THC and tPAH, cyp1a expression, lipid distribution in the eyes, head and trunk, and morphological and functional deformities. Oil droplets accumulated on the eggshell in all oil treatment groups, to which the embryo responded by a dose-dependent rise in cyp1a expression. The oil exposure led to only minor sublethal deformities in the upper jaw and otic vesicle. The findings suggest that lesser sandeel embryos are resilient to crude oil exposure. The lowest observed effect level documented in this study was 36 µg THC/L and 3 µg tPAH/L. The inclusion of these species-specific data in risk assessment models will enhance the precision of risk evaluations for the North Atlantic ecosystems.

The Northeast Arctic stock of haddock (Melanogrammus aeglefinus) is the most northerly stock of the species. It is one of the largest haddock stocks but not the most studied. Similar to the more extensively studied sympatric cod (Gadus morhua) and herring (Clupea harengus) stocks, it relies on Atlantic currents for egg and larvae transportation from spawning grounds to nursery areas in the Barents Sea. The exact locations of the spawning grounds have not been known and the maps that have been published differ substantially. Here, we present results from the first survey dedicated to exploring the spawning habitat of Northeast Arctic (NEA) haddock, and a particle drift and larvae growth model with physical forcing from a high‐resolution ocean model. Gadoid eggs, later identified as haddock by DNA analyses, as well as spawning individuals were sampled at the survey. Spawning haddock was found in Atlantic water in temperatures 3.6–6.2°C and depths of 240–560 m. Sampled eggs were mostly in early development stages, suggesting that they were largely spawned in the surveyed area. Using the drift model, we found that most of the modelled particles released as eggs at our surveyed area ended up in the nursery area in the Barents Sea. A crude estimate of the spawning stock biomass based on the survey suggests that the most updated spawning ground map of NEA haddock might be too restricted in its extent. Particle drift and growth with physical forcing for the years 2012–2022 indicated small interannual variation in aberrant drift, and that growth driven by ambient temperature could be important in explaining interannual variation in haddock recruitment.

Climate change and harvesting result in temporal and spatial changes and variability in spawning, and thus in offspring ambient drift conditions. As a result, variable survival of offspring and thereby in recruitment are expected. This is especially true for species with long reproduction migration as is the case for some Atlantic cod stocks. We utilize biophysical model simulations to analyze survival from spawning until age 1 resulting from different scenarios of spatial and temporal changes in spawning. We find that survival is 1.5–2 times higher when spawning is shifted southwards as compared to northerly shifts. In general, survival is more sensitive to shifts in spawning location than in spawning time. Early spawning is only favourable if spawning is concurrently shifted farther north. A future spawning scenario with a northward shift in spawning grounds beyond what has been observed historically suggests reduced offspring survival and increased sensitivity to the timing of spawning.

The reproductive success of marine ectotherms is especially vulnerable in warming oceans due to alterations in adult physiology, as well as embryonic and larval survival prospects. These vital responses may, however, differ considerably across the species' geographical distribution. Here we investigated the life history, focusing on reproductive ecology, of three spatially distant populations (stocks) of Atlantic cod (Gadus morhua, Gadidae) (50-80 degrees N), in the Irish/Celtic Seas-English Channel Complex, North and Barents Seas, under past and projected climate. First, experimental tracking of spawning behaviour evidenced that the ovulation cycle is highly distressed at >= 9.6 (+/- 0.25)degrees C (T-up). This knife-edge threshold resulted in erratic spawning frequencies, whereas vitellogenin sequestration remained unaffected, indicating endocrine rather than aerobic scope constraints. Cod in the Celtic Sea-English Channel are, therefore, expected to show critical stock depensation over the next decades as spawning grounds warm above T-up, with Irish Sea cod subsequently at risk. Second, in the relatively cooler North Sea, the northward retraction of Calanus finmarchicus (Calanidae) and Para-Pseudocalanus spp. (Clausocalanidae) (1958-2017) limit cod larvae feeding opportunities, particularly in the southernmost subarea. However, the contrasting increase in Calanus helgolandicus (Calanidae) does not counteract this negative effect, likely because cod larvae hatch ahead of its abundance peaks. Overfishing again comes as a twin effect. Third, in the still relatively cold Barents Sea, the sustainably harvested cod benefit from improved food conditions in the recent ice-free polar region but at the energetic cost of lengthier and faster spawning migrations. Consequently, under climate change local stocks are stressed by different mechanistic factors of varying management severity.

Atlantic haddock (Melanogrammus aeglefinus) embryos bind dispersed crude oil droplets to the eggshell and are consequently highly susceptible to toxicity from spilled oil. We established thresholds for developmental toxicity and identified any potential long-term or latent adverse effects that could impair the growth and survival of individuals. Embryos were exposed to oil for eight days (10, 80 and 300 μg oil/L, equivalent to 0.1, 0.8 and 3.0 μg TPAH/L). Acute and delayed mortality were observed at embryonic, larval, and juvenile stages with IC50 = 2.2, 0.39, and 0.27 μg TPAH/L, respectively. Exposure to 0.1 μg TPAH/L had no negative effect on growth or survival. However, yolk sac larvae showed significant reduction in the outgrowth (ballooning) of the cardiac ventricle in the absence of other extracardiac morphological defects. Due to this propensity for latent sublethal developmental toxicity, we recommend an effect threshold of 0.1 μg TPAH/L for risk assessment models.

Laboratory experiments provide knowledge of species-specific effects thresholds that are used to parameterize impact assessment models of oil contamination on marine ecosystems. Such experiments typically place individuals of species and life stages in tanks with different contaminant concentrations. Exposure concentrations are usually fixed, and the individuals experience a shock treatment being moved from clean water directly into contaminated water and then back to clean water. In this study, we use a coupled numerical model that simulates ocean currents and state, oil dispersal and fate, and early life stages of fish to quantify oil exposure histories, specifically addressing oil spill scenarios of high rates and long durations. By including uptake modelling we also investigate the potential of buffering transient high peaks in exposure. Our simulation results are the basis for a recommendation on the design of laboratory experiments to improve impact assessment model development and parameterization. We recommend an exposure profile with three main phases: i) a gradual increase in concentration, ii) a transient peak that is well above the subsequent level, and iii) a plateau of fixed concentration lasting ∼3 days. In addition, a fourth phase with a slow decrease may be added.

Impacts of climate change on ocean productivity sustaining world fisheries are predominantly negative but vary greatly among regions. We assessed how 39 fisheries resources-ranging from data-poor to data-rich stocks-in the North East Atlantic are most likely affected under the intermediate climate emission scenario RCP4.5 towards 2050. This region is one of the most productive waters in the world but subjected to pronounced climate change, especially in the northernmost part. In this climate impact assessment, we applied a hybrid solution combining expert opinions (scorings)-supported by an extensive literature review-with mechanistic approaches, considering stocks in three different large marine ecosystems, the North, Norwegian and Barents Seas. This approach enabled calculation of the directional effect as a function of climate exposure and sensitivity attributes (life-history schedules), focusing on local stocks (conspecifics) across latitudes rather than the species in general. The resulting synopsis (50-82 degrees N) contributes substantially to global assessments of major fisheries (FAO, The State of World Fisheries and Aquaculture, 2020), complementing related studies off northeast United States (35-45 degrees N) (Hare et al., PLoS One, 2016, 11, e0146756) and Portugal (37-42 degrees N) (Bueno-Pardo et al., Scientific Reports, 2021, 11, 2958). Contrary to prevailing fisheries forecasts elsewhere, we found that most assessed stocks respond positively. However, the underlying, extensive environmental clines implied that North East Atlantic stocks will develop entirely different depending upon the encountered stressors: cold-temperate stocks at the southern and Arctic stocks at the northern fringes appeared severely negatively impacted, whereas warm-temperate stocks expanding from south were found to do well along with cold-temperate stocks currently inhabiting below-optimal temperatures in the northern subregion.

Patchiness, defined as spatial heterogeneity in distribution of organisms, is a common phenomenon in zooplankton including ichtyoplankton. In heterogeneous landscapes, depending on the scale of prey and predatory distributions, individuals in patches may experience distinct differences in the survival rate compared to individuals distributed more homogeneously outside patches. In this study, we focused on drifting eggs and larvae of Northeast Arctic (NEA) cod, one of the largest exploited fish stock in the world. The eggs and larvae are largely distributed along the north-western coast of Norway and northern Russia. We ask to what degree individuals are located in patches contribute to the species recruitment. For this purpose, we developed a patch recognition method to detect the existence of patches in particle tracking simulations using a connected-component labeling algorithm. We then assessed the contribution of individuals in detected patches to the total recruitment. Our results showed that depending on year, day of year, and resolution scale for detection of patches, recruits present in patches can vary between 0.6% and 38.7% with an average of 20.4% of total recruitment. The percentage decreased with increasing day of year in the drifting season but increased with decreasing patch resolution scale, down to the finest investigated scale of 8 km. On the basis of these results, we advise field recruitment studies of NEA cod to at least resolve an 8-km spatial scale to capture effects of spatial heterogeneity in the survival rate on the species recruitment.

The successful recruitment of Northeast Arctic (NEA) cod is thought to depend on sufficient and suitable prey for the newly hatched larvae, in particular the nauplii stages of the lipid-rich calanoid copepod species Calanus finmarchicus. The role of spatial and temporal variations in prey availability in combination with temperature and other factors in influencing growth and survival of cod larvae is, however, incompletely understood. By combining an individual based model for NEA cod larvae at the Norwegian coast with a high-resolution ocean model and a nutrient-phytoplankton-zooplankton-detritus model providing 18 years of daily environmental conditions and prey availability we assessed larval growth and survival until they settle in their feeding habitat in the Barents Sea in early fall. We find on average a two-week delay from the peak timing of first-feeding cod larvae to the peak in prey availability. In warm years, more larvae experience food limitation than in normal years. The positive effects of high temperature on growth, survival and ultimately recruitment are nonetheless larger than the negative effects of food limitation. Food limitation mainly affects larvae spawned in southern areas or late in the spawning season as these larvae experience the highest temperatures and have the highest energy requirements. Our findings highlight the spatial and temporal differences in mechanisms that regulate growth and survival of early life stages of NEA cod and suggest that spatially resolved data may be essential for understanding match-mismatch dynamics.

We simulate the combined natural and pollutant-induced survival of early life stages of NEA cod and haddock, and the impact on the adult populations in response to the time of a major oil spill in a single year. Our sim-ulations reveal how dynamic ocean processes, controlling both oil transport and fate and the frequency of in-teractions of oil with drifting fish eggs and larvae, mediate the magnitude of population losses due to an oil spill. The largest impacts on fish early life stages occurred for spills initiated in Feb-Mar, concomitant with the initial rise in marine productivity and the earliest phase of the spawning season. The reproductive health of the adult fish populations was maintained in all scenarios. The study demonstrates the application of a simulation system that provides managers with information for the planning of development activities and for the protection of fisheries resources from potential impacts.

Arctic benthic ecosystems are facing high-speed environmental changes, such as decreased sea ice coverage, increased temperature and precipitations, as well as the invasion by non-indigenous species. Few sub-arctic fjords have the particularity to have an inner-most part forming a basin in which water remains very cold. Those fjords may offer a refugee for cold-water arctic species as well as a small-scale “laboratory” of the changes that arctic assemblages located at higher latitudes might face soon. The Porsangerfjord in Northern Norway is a sub-arctic fjord with an inner arctic part and face red king crabs Paralithodes camtchasticus invasion since the end of the 1990s. It offers a case study of the dynamics of arctic ecosystems facing multiple stressors, i.e., climate change and invasive species. Based on a time series of megabenthic invertebrates and bentho-demersal fishes over 2007–2019, a complex multivariate analysis (STATICO) was used to identify the trends in the relationship between taxa and the environment. We showed the main environmental changes in the fjord were the freshening of the water, the increase of the seabed current, and the decrease of the maximum sea ice extent. A strong along-fjord gradient was visible for both benthic and fish assemblages. Species richness and Shannon diversity of fishes significantly increased into the fjord, due to the arrival of warm-water species over time that overlapped with cold-water species that have seen their biomass significantly reduced. No significant decrease in the biomass of the cold-water benthic species was visible, which could indicate an efficient refugee effect of the inner fjord. Yet, this refugee effect could be unbalanced by the red king crab invasion as it is a predator of several arctic species. In the Porsangerfjord, fish species thus respond to climate change while megabenthic assemblages are more threatened by invasive species.

The cold-adapted polar cod Boreogadus saida, a key species in Arctic ecosystems, is vulnerable to global warming and ice retreat. In this study, 1257 individuals sampled in 17 locations within the latitudinal range of 75–81°N from Svalbard to East Siberian Sea were genotyped with a dedicated suite of 116 single-nucleotide polymorphic loci (SNP). The overall pattern of isolation by distance (IBD) found was driven by the two easternmost samples (East Siberian Sea and Laptev Sea), whereas no differentiation was registered in the area between the Kara Sea and Svalbard. Eleven SNP under strong linkage disequilibrium, nine of which could be annotated to chromosome 2 in Atlantic cod, defined two genetic groups of distinct size, with the major cluster containing seven-fold larger number of individuals than the minor. No underlying geographic basis was evident, as both clusters were detected throughout all sampling sites in relatively similar proportions ( i.e. individuals in the minor cluster ranging between 4 and 19% on the location basis). Similarly, females and males were also evenly distributed between clusters and age groups. A differentiation was, however, found regarding size at age: individuals belonging to the major cluster were significantly longer in the second year. This study contributes to increasing the population genetic knowledge of this species and suggests that an appropriate management should be ensured to safeguard its diversity.

Mortality rates in the early life-history stages of fishes are generally high yet identifying the causes remain unclear. Faltering recruitment rates of Atlantic herring ( Clupea harengus ) in the Norwegian Sea indicate a need to identify which mortality factors influence larval herring survival. Previous research suggests that increased predation pressure by Atlantic mackerel ( Scomber scombrus) may contribute to the disconnect between spawning stock biomass and recruitment. To quantify the contribution of predation pressure by Atlantic mackerel to herring larval mortality, two research cruises were conducted within a probable “hot spot” (67–72° N) for intensified mackerel predation based on particle drift simulations. Mackerel stomach contents were analysed for herring larvae content using droplet digital polymerase chain reaction (ddPCR) with a quantitative molecular detection assay specific for herring. The ddPCR results demonstrate clear predation by mackerel on herring larvae and also suggest that the alternative use of visual examination may give misleading results. Our results show that mackerel should be considered a potentially important predator on herring larvae. The quantitative molecular assay presented here shows great promise as an efficient and specific tool to correctly identify and quantify predation pressure on early life-history stages of fishes.