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Maps of modeled bottom temperatures (BT) by year in the eastern Bering Sea. Blue denotes areas with temperatures below 2 °C (the cold pool) and red denotes temperatures above 2 °C. The black contour lines represent the modeled 20% probability of occurrence for polar cod (Boreogadus saida)
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Polar cod (Boreogadus saida) is the most abundant and ubiquitous fish species throughout the Arctic Ocean. As such, they serve an important ecosystem role linking upper and lower trophic levels and transferring energy between the benthic and pelagic realms. Our objective is to explore what limits the southern distribution of polar cod in Pacific an...
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... Our results estimate that the four-fold increase in the biomass of Atlantic cod from the late 1980 s to 2015 resulted in > 80 % reduction in the survival of polar cod to ages 3 and 4. The effect of Atlantic cod biomass on abundance of polar cod was similar in size to the effect estimated as the difference in ages 3 and 4 biomass between low and high predation regimes assessed in Dupont et al. (2021) and the effect on polar cod age 1 + abundance at high Atlantic cod biomass assessed for the Labrador Newfoundland shelf region (Marsh & Mueter, 2020). ...
... The spatial distribution of Atlantic cod has been shown to expand northward in the Barents Sea during warm years (Fall et al., 2018), with potential implications for polar cod distribution and the species stock assessment. Marsh and Mueter (2020) also showed that high temperature and high Atlantic cod biomass could restrict the distribution of polar cod to cold pool waters. High temperature and a wide spatial distribution of the Atlantic cod stock could cause polar cod to move north and out of the surveyed area, providing an alternative explanation to predation for low estimated abundance of polar cod in warm years. ...
... The projected increases in habitat suitability for capelin and the subsequent declines for Arctic cod replicate empirical data, indicating a shifting prey base in the Arctic [131,132]. However, the effect of changes in forage fish biomass (as a prey base) on higher trophic levels was not as strong as expected. ...
Climate change is rapidly reshaping species distributions in the Arctic, which could profoundly impact ecosystem structure and function. While considerable effort has focused on projecting future species distributions, assessing the impacts of range-shifting species on recipient communities and subsequent disruptions to food webs remains largely unstudied. Here, we address this gap by combining species distribution models and ecosystem models to explore the emergence of novel ecosystems in the North Water Polynya. The North Water Polynya is an open-water area between Greenland and Canada, surrounded by sea ice and one of the world’s most productive ocean ecosystems. Using existing literature and projections from species distribution models of four marine species, we develop six plausible future ecosystem scenarios for the North Water Polynya. These scenarios include changing biomass of primary producers, changing biomass and size structure of copepods, shifting abundances of forage fish species, and the establishment of killer whales. We find that the biomass of higher trophic levels show pronounced decreases in response to the decrease in pelagic primary producers, with polar bear biomass halving compared to present conditions. Changes in the copepod size structure has the largest impact on the entire ecosystem compared to the other novel ecosystem scenarios, suggesting a strong reliance of higher trophic levels on large, lipid-rich copepods. We further show that increasing capelin with a simultaneous decrease in Arctic cod biomass causes large decreases in the biomass of marine mammals such as polar bear, beluga and ringed seal. Finally, we show the establishment of killer whales as a key novel predator could have cascading top-down effects on the North Water Polynya ecosystem. The framework presented here provides an approach for exploring the emergence of novel ecosystems and highlights how climate change could disrupt a high Arctic ecosystem.
... Boreal mobile pelagic fish can relocate to follow the production and expand into newly suitable habitats, thus propagating the borealization of the lower trophic levels up the food chain, although other important factors such as recruitment and availability of suitable spawning habitat also have a strong impact on population spatial-temporal dynamics (Hollowed et al., 2013b;Eriksen et al., 2017). In contrast, some Arctic species such as polar cod are very dependent on ice to complete their life cycle, and loss of sea-ice thus drives a loss of essential habitats for those species Huserbråten et al., 2019;Gjøsaeter et al., 2020;Marsh and Mueter, 2020;Deary et al., 2021;Geoffroy et al., 2023). ...
... At the ecosystem-level, long-term projections using species distribution ensemble models suggest an increase in species richness and functional redundancy poleward with an increase in predatory taxa that will threaten Arctic species and decrease the modularity of Arctic food webs in the Bering and Chukchi seas (Alabia et al., 2020). It is predicted that the co-occurrence of boreal and arctic fish species in borealized regions will vary from 1 year to the other, with more Arctic species in cold years and more boreal species in warm years (Geoffroy et al., 2023), as seen in the Labrador and Bering seas (Marsh and Mueter, 2020). ...
Climate change is rapidly modifying biodiversity across the Arctic, driving a shift from Arctic to more boreal ecosystem characteristics. This phenomenon, known as borealization, is mainly described for certain functional groups along sub-Arctic inflow shelves (Barents and Chukchi Seas). In this review, we evaluate the spatial extent of such alterations across the Arctic, as well as their effects on ecosystem-level processes and risks. Along the inflow shelves, borealization is driven by long-term strengthened inflow of increasingly warm waters from the south and punctuated by advection and low sea ice extreme events. A growing body of literature also points to an emerging borealization of the other Arctic shelf ecosystems, through a “spillover” effect, as local changes in environmental conditions enable movement or transport of new species from inflow shelves. These modifications are leading to changes across functional groups, although many uncertainties remain regarding under-sampled groups, such as microbes, and technical challenges of consistent, regular monitoring across regions. There is also clear consensus that borealization is affecting phenology, species composition, community traits, population structure and essential habitats, species interactions, and ecosystem resilience. Non-dynamic environmental factors, such as depth and photoperiod, are thought to limit the complete borealization of the system, and may lead to intermediate, “hybrid” ecosystems in the future. We expect current borders of Arctic and boreal ecosystems to progress further northward and ultimately reach an equilibrium state with seasonal borealization. Risks to the system are difficult to estimate, as adaptive capacities of species are poorly understood. However, ice-associated species are clearly most at risk, although some might find temporary refuge in areas with a slower rate of change. We discuss the likely character of future Arctic ecosystems and highlight the uncertainties. Those changes have implications for local communities and the potential to support Blue Growth in the Arctic. Addressing these issues is necessary to assess the full scale of Arctic climate impacts and support human mitigation and adaptation strategies.
... Increased water temperature due to earlier annual sea ice retreat and increased inflow of warm Pacific waters into the Chukchi Sea have been hypothesized to enhance growth and transport of age-0 polar cod from spawning sites located in the south (Levine et al. 2021). However, changes in polar cod zooplankton prey quality and potential increases in predation and competition with boreal fish species could have detrimental effects on polar cod populations (Marsh and Mueter 2019). At this time, overall effects of environmental changes on polar cod remain uncertain. ...
The Chukchi Sea pelagic ecosystem continues to undergo dramatic oceanographic changes associated with reductions in sea ice and increasing temperatures over the last decades. Impacts of these changes on polar cod (Boreogadus saida), an ice-associated pelagic fish that constitutes a key energetic link between lower and upper trophic levels, remain uncertain. Here, we use 4 years (2016–2019) of high-resolution acoustic and oceanographic data from the Chukchi Ecosystem Observatory to characterize temporal patterns in polar cod densities and identify its environmental drivers in years with contrasting sea ice and temperature conditions. Polar cod densities were 2–16 times greater with peaks occurring 14–60 days earlier in years with early sea ice retreat and higher water temperatures (2017 and 2019). The variance to mean relationship showed a decrease in variance for larger abundances in warmer years. Increased densities occurring earlier in the summer are attributed to a combination of earlier and increased transport of polar cod eggs and larvae from spawning areas, enhanced local primary and secondary production, and increased growth rates of fish due to higher temperatures. Earlier sea ice retreat and increases in temperature could temporarily benefit polar cod production in the NE Chukchi Sea but potential changes in prey quality, mismatch between polar cod and its prey, and increased competition with boreal fish species could have detrimental effects on polar cod populations with further warming. Such effects on polar cod populations could propagate through pelagic Arctic food webs impacting higher trophic levels and human communities.
... Polar cod is of particular interest in the Arctic marine food web because they efficiently link lower trophic levels they consume as prey to top predators (e.g., mammals and seabirds) (Falk-Petersen et al., 1990;Hop & Gjøsaeter, 2013;Pedersen, 2022;Pedersen et al., 2021). Polar cod are likely to experience diet and habitat shifts in future due to continued borealization of Arctic fish communities, increased competition for resources (Eriksen et al., 2015;Fossheim et al., 2015;Renaud et al., 2012) and increased water temperature (Marsh & Mueter, 2020). ...
... Concurrent reductions in Arctic zooplankton that are key prey for juvenile polar cod (e.g., Themisto libellula) may further reduce the biomass of polar cod in the Barents Sea in the future (Dalpadado et al., 2012) and other zooplankton species (e.g., euphausiids) may become more important in the diet of polar cod (Cusa et al., 2019). The northward expansion of boreal fish species that are better acclimated to warmer water (e.g., Atlantic cod, Gadus morhua and Capelin, Mallotus villosus) may also outcompete polar cod in terms of growth (Marsh & Mueter, 2020) and feeding (McNicholl et al., 2016;Orlova et al., 2009). ...
Polar cod (Boreogadus saida) is an important trophic link within Arctic marine food webs and is likely to experience diet shifts in response to climate change. One important tool for assessing organism diet is bulk stable isotope analysis. However, key parameters necessary for interpreting the temporal context of stable isotope values are lacking, especially for Arctic species. This study provides the first experimental determination of isotopic turnover (as half-life) and Trophic Discrimination Factors (TDF) of both δ13 C and δ15 N in adult polar cod muscle. Using a diet enriched in both 13 C and 15 N, we measured isotopic turnover times of 61 and 49 days for δ13 C and δ15 N, respectively, with metabolism accounting for >94% of the total turnover. These half-life estimates are valid for adult polar cod (> 3 years) experiencing little somatic growth. We measured TDFs in our control of 2.6 ‰ and 3.9 ‰ for δ13 C and δ15 N, respectively, and we conclude that applying the commonly used TDF of ~1 ‰ for δ13 C for adult polar cod may lead to misrepresentation of dietary carbon source, while the use of 3.8 ‰ for δ15 N is appropriate. Based on these results, we recommend that studies investigating seasonal shifts in the diet of adult polar cod sample at temporal intervals of at least 60 days to account for isotopic turnover in polar cod muscle. Although isotopic equilibrium was reached by the fish in this study, it was at substantially lower isotope values than the diet. Additionally, the use of highly enriched algae in the experimental feed caused very high variability in diet isotope values which precluded accurate calculation of TDFs from the enriched fish. As a result of the challenges faced in this study, we discourage the use of highly enriched diets for similar experiments and provide recommendations to guide the design of future isotopic turnover experiments. This article is protected by copyright. All rights reserved.
... Areas containing polar fronts serve as both feeding grounds and spawning areas for polar cod (Melnikov and Chernova, 2013). There is a hypothesis that polar cod may spawn in a system of winter quasi-stationary polynyas extending from the White Sea to the Chukchi Sea along the edge of the fast ice (Melnikov and Chernova, 2013;Vestfals et al., 2019Vestfals et al., , 2021Marsh and Mueter, 2020). ...
... Polar cod is sensitive to climate change which makes this species a potential indicator of the state of the Arctic Ocean ecosystems (Mueter et al., 2016). Currently, climate warming is accompanied by a retreat of the polar front and a reduction in the area of seasonal ice (Wang et al., 2018;Baker et al., 2020b;Ovall et al., 2021), which leads to a northward shift in the boundaries of polar cod distribution (Marsh and Mueter, 2020) and a reduction in its abundance causing a restructuring of the ecosystem in the region. This is perhaps the main reason for comprehensive monitoring of the status of this species for its conservation. ...
Integrated pelagic trawl surveys were conducted in the eastern sector of the Russian Arctic (Laptev Sea, Chukchi Sea and East Siberian Sea) in August–September of 2003–2018. Data were used to further describe biology and spatial distribution of Polar cod (Boreogadus saida) in this region. Polar cod in surveyed areas are characterized by similar linear size and spatial distribution. In all surveyed seas, polar cod aggregations consisted of individuals
3–29 cm in length with the age of 0+ - 6+ years. Lower growth rates of polar cod were evident in the eastern sector compared to the Kara Sea (the western sector of Russian Arctic). The lower growth rates in the eastern sector are probably due to the significant difference in environmental conditions (mainly temperature) that directly affect polar cod metabolic rates. In the Chukchi and East Siberian seas, the main concentrations were observed within the near-bottom layer,
while in the Laptev Sea they were recorded throughout the water column. The abundance and biomass of polar cod in the Chukchi Sea in different years ranged from 514 million inds. and 0.83 thousand tons (2008) to 8.26 billion inds. and 117.5 thousand tones (2003). Respective indices for the Laptev Sea amounted to 233 thousand
tones and 12.75 billion individuals. The abundance and biomass of the East Siberian Sea polar cod were at a relatively low levels compared to other areas in the Russian Arctic (about 0.150 thousand tons and 20 million individuals).
... Estimates from the Canadian Arctic indicate that polar cod can funnel up to 75% of the carbon between zooplankton and top predators, such as seabirds and whales (Welch et al., 1992). Changes in the distribution and abundance of polar cod will therefore likely lead to broad trophic, subsistence hunting and economic impacts (Huntington et al., 2020;Huserbråten et al., 2019;Marsh and Mueter, 2020). Unfortunately, studying these and other Arctic fish species is logistically challenging outside the summer open-ice period (Geoffroy and Priou, 2020). ...
In the Arctic, winter warming and loss of sea ice pose largely unknown risks to keystone species and the marine ecosystem that they support. Young-of-the-year juvenile polar cod, Boreogadus saida, are an energy-rich forage fish that accumulate high levels of lipid in the summer but retain a relatively small body size during the winter. To address winter bioenergetics and survival, we held age-0 juveniles under simulated winter conditions (food deprived, 24-hr darkness) at a range of four constant temperatures (-1, 1, 3, 5 ℃). Our goals were to 1) determine how age-0 polar cod utilize lipid energy in muscle and liver across variable temperatures and durations of food deprivation, 2) understand temperature- and size-dependent impacts on survival and 3) provide energy loss models using multiple condition metrics that are commonly used in fisheries science (lipids, morphometric ratios, body weight). These data have relevance to projecting winter
outcomes for polar cod sampled pre-winter, when fish are more easily sampled in the field. As expected, in the absence of food, juvenile polar cod better conserved lipids and survived longer at colder temperatures. There was no negative impact of cold extremes on this pattern; for example, 50% mortality was at 170 days when polar cod were held at -1 ℃, compared to only 94 days when they were held at 5 ℃. During the first 28 days of simulated winter, polar cod preferentially catabolized triacylglycerols from muscle tissue, then depleted this storage lipid class in their muscle and liver until starvation. Mortality occurred when whole-body lipid concentrations fell below 12.4 mg. g -1 wet weight. Temperature-dependent declines in morphometric condition (hepatosomatic index and Fulton’s K) and lipid content were parameterized and developed into temperature-dependent condition loss models. Applying a laboratory-based lipid loss model to field-collected polar cod demonstrated that winter survival is highly sensitive to small changes in temperature between -1 and 1 °C when fish are in good condition at the end of the preceding summer. Alternatively, fish in poor summer condition cannot survive winter relying exclusively on stored energy reserves, and will be required to forage throughout the winter. Collectively, these results suggest that lipid-based indices offer a sensitive means of predicting overwintering success for polar cod experiencing climate-driven changes in summer and winter habitats in the Arctic.
... Boreogadus saida also appeared diminished in abundance in the southern Chukchi Sea, possibly due to competition with G. chalcogrammus, change in availability of prey, or physical marine changes. B. saida have extended well into the southeastern Bering Sea in years with an extensive cold pool (Marsh and Mueter, 2020) and was detected as far south as latitude 60 • N in the Northern Bering Sea in 2010 (Table 2). But in 2019, only 14 B. saida individuals were genetically detected in gadid samples collected south of latitude 70 • N. ...
We used genetic techniques to identify gadids (cods) to species in the Pacific Arctic during a time of substantial physical change in the marine ecosystem between 2012 and 2019. The dominant fish species in the Chukchi Sea is Arctic Cod (Boreogadus saida); however, other gadids such as Saffron Cod (Eleginus gracilis), Pacific Cod (Gadus macrocephalus) and Walleye Pollock (Gadus chalcogrammus) have been observed. Two aims in this study were to evaluate the accuracy of at sea morphological identification (which can be difficult for juveniles) with genetic species identification and to document potential variation in species composition and distribution of gadids in the Pacific Arctic in response to changing environmental conditions. Microsatellite and mtDNA genetic results revealed that most B. saida collected in the Chukchi Sea in 2012 and 2013 were correctly identified at sea. Conversely, genetic results from samples collected in 2017 and 2019 revealed a large number of G. chalcogrammus and some G. macrocephalus and E. gracilis that were initially identified at sea as B. saida. The majority of misidentification occurred between B. saida and G. chalcogrammus. This study indicates a northward shift of G. chalcogrammus and B. saida during warmer conditions. In addition, juvenile Polar Cod (A. glacialis), which is not typically found in the Chukchi Sea and was not identified at sea, was genetically detected on 3 hauls on the northern Chukchi Shelf, outside of its documented distribution. Accurate species identification, especially during a time of changing marine landscapes, is not only important for survey abundance estimates but for downstream analyses as well. This emphasizes the value of implementing strategies for correct identification of the gadid species to better capture and monitor responses to varying and likely changing conditions. Our results provide strong evidence of distributional shifts and range expansions of gadid species in the Arctic, which may be the result of changing climactic conditions.
... However, species expanding northward could exert both bottom-up (e.g., through lower prey quality) and top down (e.g., through competition and predation) effects on Arctic Char. For example, boreal zooplankton and forage fish might increase and eventually surpass Arctic prey, like Calanus glacialis and Arctic Cod (Boreogadus saida), whose range might contract and shift poleward with continued climate warming (e.g., Meredith et al., 2019;Marsh and Mueter, 2020). This would affect Arctic Char, other Arctic predators, as well as coastal communities, who depend on the availability of highquality lipids, pigments, and other nutrients fueled through Arctic marine food webs (e.g., Falk-Pedersen et al., 2000;Falk-Pedersen et al., 2007;Leu et al., 2011;Darnis et al., 2012). ...
Managing Arctic marine resources to be resilient to environmental changes requires knowledge of how climate change is affecting marine food webs and fisheries. Changes to fishery resources will have major implications for coastal Indigenous communities whose livelihoods, health, and cultures are strongly connected to fisheries. Understanding these broad social-ecological changes requires a transdisciplinary approach bringing together contrasting and complementary disciplines and ways of knowing. Here, we examine climatic proxies, ecological, and fishery indicators (stable isotopes, fish condition, and lipid content), and interviews with Inuit fishers to assess how marine ecosystem changes have influenced Arctic Char (Salvelinus alpinus) ecology and fisheries over a 30-year time period (1987–2016) in the Kitikmeot region of the Canadian Arctic. Inuit fishers reported several observations of environmental changes, including longer ice-free seasons, warmer ocean temperatures, and the arrival of new marine species. Biophysical data revealed important changes toward earlier dates of ice breakup (>12 days in some areas) and a shift in isotopic niche reflecting a changing Arctic Char diet, with increased contribution of pelagic carbon and higher trophic level prey. Fish condition was improved in years with earlier ice breakup, as observed by both Inuit fishers and biophysical indicators, while lipid content increased through time, suggesting that longer ice-free seasons may have a positive effect on Arctic Char quality as reflected by both fish condition and lipid content. Long-term impacts of continuing climate change, however, such as the northward expansion of boreal species and increasing ocean temperatures, could have negative effects on fisheries (e.g., physiological impairment in fish if temperatures exceed their thermal range). Continuous community-based monitoring that directly informs fisheries management could help communities and managers adaptively, and sustainably, manage in the face of multiple interacting changes in Arctic marine systems.
... Polar cod are not only sensitive to warming and loss of sea ice (Gjøsaeter et al., 2020;Laurel et al., 2017), but are also impacted by increased anthropogenic activity associated with the reduction of sea ice, such as increased ship traffic (Ivanova et al., 2020) and increased oil and gas activity (Nahrgang et al., 2016). In general, Arctic taxa appear to be highly sensitive to habitat changes, and polar cod in particular can serve as a sentinel species that responds quickly to changes in temperature and ice extent at the transition zone between the Subarctic and Arctic (Marsh & Mueter, 2020). ...
Warming oceans, the loss of sea ice, and changes in advection drive changes in highly productive Subarctic marine ecosystems and the borealization of Arctic marine ecosystems. Borealization refers to the northward shift or expansion of marine organisms, including commercially important fish stocks, into Arctic waters. These shifts in distribution have been particularly pronounced on the major Arctic inflow shelves, which are important gateways from the Atlantic and Pacific oceans into the Central Arctic Ocean (CAO). Climate-driven changes in the abundance and distribution of fish stocks can pose significant challenges for fisheries stock assessment and management, as some Subarctic fish stocks decline and others are displaced or expand into new areas. Shifting stocks are a challenge for resource surveys as fish move out of historically surveyed areas, for managers as changing stock and fishery interactions may lead to resource conflicts, and for international institutions as stocks cross national boundaries or expand into the CAO. To meet these challenges, researchers will have to adopt new and cost-effective strategies for monitoring, managers must be flexible to adapt to rapidly changing conditions, and enhanced international cooperation will be required to address transboundary issues and ensure the conservation and sustainable management of living marine resources in the CAO.
