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

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Chapter 2 · Status of the natural and human environments

25 Krill biomass, million tonnes

more northerly distribution in warm years (Gjøsæter et al., 1998; Carscadden et al., 2013; Ingvaldsen and Gjøsæter, 2013). However, the size of the stock also plays a role with a less northerly distribution being the norm when the stock is low, presumably because of lower food demand (Ingvaldsen and Gjøsæter, 2013). Juvenile herring of strong year-classes of the Norwegian spring spawning herring stock grow up in the southern Barents Sea. They leave after three to four years to join the adult stock in the Norwegian Sea (Krysov and Røttingen, 2011). Polar cod spawn in association with sea ice and young age classes of this small fish species tend to remain close to sea ice, often living in interstitial spaces within the ice, which provides some protection against predators. The polar cod stock has shown large fluctuations in abundance; fromhigh levels during the early 1970s to a dramatic decline in the 1980s, followed by a recovery during the 1990s and then high levels in the early-mid 2000s. Since 2007, the stock size has again decreased, apparently driven by poor recruitment related to warming and associated reductions in sea ice and the area containing Arctic Water (ICES, 2014b; Eriksen et al., 2015). Expansion of Atlantic cod ( Gadus morhua ) into the northern Barents Sea has also played a role, leading to increased spatial overlap between the two species and increased predation pressure from Atlantic cod on polar cod. All three planktivorous fishes are or have been harvested; capelin being the most important commercially (Figure 2.8). The harvest of Barents Sea polar cod has been very limited since the 1970s. The herring fishery targets adult fish, which are actually taken outside the Barents Sea. The most important commercial species among the benthic- feeding and fish-feeding species include Atlantic cod, haddock ( Melanogrammus aeglefinus ), saithe ( Pollachius virens ) and Greenland halibut ( Reinhardtius hippoglossoides ). It is well established that climate variability is a major factor causing large variability in recruitment to the commercial fish stocks in the Barents Sea, expressed as alternating strong and weak year classes (Sætersdal and Loeng, 1987; Ottersen and Loeng, 2000). Strong and weak year classes drive fluctuations in the stocks, and strong year classes in particular have marked‘snowballing’ effects as the cohort develops over time, with impacts on prey and predators throughout food webs. Recruitment of Atlantic cod and haddock (as well as herring) is positively related to high inflows of Atlantic water and the accompanying higher temperatures in the Barents Sea (Sætersdal and Loeng, 1987; Ottersen and Loeng, 2000). During the last decade the cod stock has coveredmost of the Barents Sea shelf in autumn (August-September) and has also expanded northward during winter (Johansen et al., 2013; Prokhorova 2013; also see Figure 2.9). The cod distribution area increased from 2004 to 2013, expanding into the northern and northeastern part of the Barents Sea. In recent years a major part of the stock has been found on the northern shelf (north of 78°N) with some cod moving to the shelf edge at the rim of the Arctic Ocean at around 82°N. Increased temperature from sub-zero to positive may have removed a threshold barrier,now allowing cod to enter this northern area (Lind and Ingvaldsen, 2012).The northward expansion during the main feeding season in late summer

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Figure 2.7Mean biomass of krill recorded during joint Norwegian-Russian autumn surveys by trawl sampling in the upper 60 m of the Barents Sea. Based on Eriksen and Dalpadado (2011) with updates for 2010–2015 (Institute of Marine Research, Norway, unpubl. data).

2.2.2.2 Fish and other harvested resources More than 200 species of fish have been registered in the Barents Sea, although less than half are caught regularly (Stiansen and Filin,2008; Dolgov et al.,2011a;Wienerroither et al.,2011).Some species complete all phases of their lifecycle within the Barents Sea, while others feed in the Barents Sea but spawn elsewhere. Johannesen et al. (2012b) described six fish communities in the Barents Sea that were separated along depth and temperature gradients. Based on their geographical distribution and physiological adaptations, 166 of the fish species registered in the Barents Sea have been classified into zoogeographical groups (Andriashev and Chernova, 1995); 25% are Arctic or Arcto-boreal, half are boreal (or mainly boreal) and the rest are widely distributed or south-boreal species. However, shifts in distribution over recent decades and changing temperatures at depth are blurring the distinction among these assigned groupings. There has already been a marked ‘borealization’ of the fish community within the Barents Sea (Fossheim et al., 2015, see also Chapter 6). From a trophic perspective there are three main groups of fish in the Barents Sea that each share fundamental life-history and habitat characteristics: species feeding on plankton, species feeding on benthos, and species feeding on other fish (Dolgov et al., 2011a). Planktivorous fish dominate in terms of biomass,but not in terms of the number of species (Dolgov et al., 2011b). Among the planktivorous species, capelin, polar cod ( Boreogadus saida ) and juvenile herring ( Clupea harengus ) are most abundant, although their biomass varies greatly from year to year . The three species have broadly divided the sea area among them with capelin in the north, herring in the south, and polar cod mainly in the east, although this species is also of key ecological importance within Svalbard.All three species are important to top trophic predators within their respective ranges . The events and conditions driving capelin cycles are clearly linked to climate variability but in a complex manner involving biological interactions with, for example, variable abundance of juvenile herring, zooplankton prey, and levels of cod predation. 0-group capelin are distributed further north in warm years (Eriksen et al., 2012). The distribution of immature capelin on their feeding migration in autumn is related to temperature conditions and this age group has a