Adaptation Actions for a Changing Arctic: Perspectives from the Barents Area

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Chapter 6 · Impact analysis and consequences of change

may lead to a permanently reduced polar cod stock in the Barents Sea with consequences for the ecology of the northern and southeastern Barents Sea. With further warming, the Barents Sea will continue to be a favorable habitat for commercially important cod (see Chapter 2; Fossheim et al., 2015). The stock will probably not be able to increase further due to restrictions in space and productivity. It is likely that there will continue to be large fluctuations in the ecosystem, as is now being seen with the ongoing collapse of the capelin stock and which is likely to affect the cod stock as well as other species in the ecosystem. How the ecosystemdynamics will develop is difficult to predict, however, due to the complexity of climate forcing and food web interactions. The northern expansion of cod is a prime example of the borealization of the Barents Sea ecosystemunder warming (Fossheim et al., 2015) (see also Box 6.1). As is the case for the fish communities (Fossheim et al., 2015), continued warming is expected to lead to a further borealization of megabenthos (and probably also benthic infauna) with an increase in boreal species and a decrease in Arctic species along the southwest-northeast axis. The ecological processes thought to drive the observed changes are likely to promote the borealization of Arctic marine communities in the coming years (Kortsch et al., 2012). Climate change is expected to affect all marine mammal species (see Table 2.1 for an overview) in the Barents Sea through impacts on the productivity of plankton, benthos and fish. The ice-associated species are very likely to be negatively affected by the loss of sea ice (Laidre et al., 2015), while open water species such as the large baleen whales are very likely to benefit from the warming trend. Ringed seal ( Pusa hispida ), harp seal ( Pagophilus groenlandicus ), hooded seal ( Cystophora cristata ) and bearded seal ( Erignathus barbatus ) depend on ice as a substrate for breeding, lactation, molting and resting, and are therefore particularly vulnerable to the decline in Arctic sea ice (Laidre et al., 2015). Some bearded seals follow the marginal ice zone and may therefore be negatively affected by increased migration distances and possible changes in prey composition and availability. If sea ice retreats to deep water north of Svalbard, it can no longer serve as a feeding platform for bearded seal. Reduced availability of ice habitat over the continental shelf is therefore a concern for this species (Kovacs et al., 2011). A general concern with respect to Arctic warming is the replacement of Arctic species of zooplankton and fish by less energy-rich southern species. These species may not allow sufficient accumulation of body reserves for capital breeding animals like seals (Grebmeier et al., 2006; Dalpadado et al., 2012). Owing to low abundance, crowding in haul-out areas or food limitation close to haul-outs do not currently appear to be a problem for walrus ( Odobenus rosmarus ) in the Barents Sea area in contrast to large parts of the Pacific Arctic (Laidre et al., 2008). However, continued sea-ice retreat may become a problem for Barents Sea walrus over the long term. Continued retraction of the sea ice will almost certainly lead to large reductions in the abundance of all ice breeding seals and thereby to a reduction in the prey base for polar bears ( Ursus maritimus ) (Wiig et al., 2008; Kovacs et al., 2011;

it to sustain higher predation pressure from pelagic fish such as capelin ( Mallotus villosus ) and polar cod ( Boreogadus saida ). With continued warming, krill are expected to expand their distribution and increase in the Barents Sea. The spawning habitat of Thysanoessa intermis may expand east and north with the warmer Atlantic Water in a similar manner as for C. finmarchicus , while the southwestern Barents Sea may become a regular part of the habitat for Meganyctiphanes norvegica . Predation from pelagic fish and other consumers will continue to be important, and the interaction between climate and predation will determine how the abundance and roles of the various krill species will develop in a future warmer climate (Eriksen and Dalpadado, 2011; ICES, 2015a). It is expected that the Arctic Themisto libellula (Dalpadado, 2002; Dalpadado et al., 2002, 2008), which is one of the dominant hyperiid amphipod species of the genus Themisto , will be negatively impacted by warming, and its future role in the northern part of the Barents Sea ecosystem will diminish. Jellyfish populations share the pelagic environment with many small planktivorous fishes (Brodeur et al., 2008; Eriksen, 2016), and further warming is likely to increase overlap and strengthen species interactions. Climate may affect marine fish populations through many different pathways, operating at a range of temporal and spatial scales. Climate impacts may affect fish directly, or indirectly through bottom-up or top-down processes within the food web. These direct and indirect effects can act simultaneously but with complex patterns involving non-linearity and time lags, and are not mutually exclusive (Rijnsdorp et al.,2009).Themany drivers and pathways through which climate affects marine fish stocks can oftenmake it difficult to establish unequivocal connections between climate forcing and the ecological responses of fish populations, let alone quantify them (Ottersen et al., 2004, 2010; Vilhjálmsson and Hoel, 2005). This is even more the case for exploited fish populations where the effects of fishery exploitation interact with effects of climate forcing and the two can be difficult to separate (Skjoldal, 2004; Perry et al., 2010; Planque et al., 2010). What will happen to three species of plankton-feeding fish: capelin, herring ( Clupea harengus ) and polar cod with continued warming is a key issue due to their importance in the ecosystem (see Chapter 2).The complex biological interactions involved make it difficult to develop predictions. Occupation of new spawning grounds on banks off Novaya Zemlya is a possibility that may shift the spatial distribution and ecological role of capelin in the Barents Sea ecosystem under a warmer climate (Huse and Ellingsen,2008).Norwegian spring spawning herring is expected to continue to thrive under the warming projected for the next 50 years, but with fluctuations driven by fluctuations in the future climate.The loss of sea ice may have led to a loss of spawning habitat and thus have contributed to the dramatic recent recruitment failures and stock decline. Further, the expansion of Atlantic cod ( Gadus morhua ) into the northern Barents Sea has led to increased spatial overlap between the two species and increased predation pressure from Atlantic cod on polar cod. The decline in the polar cod stock may cause structural reorganization of the Arctic food web in the future (Hop and Gjøsæter, 2013). The projected warming

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