18.104.22.168 Likely general effects of a warming ocean climate
Changing climatic conditions in Arctic and sub-Arctic oceans are driving changes in the biodiversity, distribution and productivity of marine biota, most obviously through the reduction of sea ice. As the sea-ice edge moves northward, the distribution of crustaceans (copepods and amphipods), adapted for life at the sea-ice edge, and fish such as polar cod (Boreogadus saida), which forage on them, will shift accordingly and their abundance diminish (Sakshaug et al., 1994). This reduction is likely to seriously impact other predators, e.g., seals, sea birds and polar bears (Ursus maritimus), dependent on sea ice for feeding and breeding (see Chapter 4, Box 4.3; Sakshaug et al., 1994) as well as humans depending on them (Loeng et al., 2005; Vilhjálmsson et al., 2005).
Thinning and reduced coverage of Arctic sea ice are likely to substantially alter ecosystems that are in close association with sea ice (Loeng et al., 2005). Polar cod, an important member of these ecosystems, is a prime food source for many marine mammals. Ringed seals, which are dependent on sea ice for breeding, moulting and resting, feed on ice amphipods and cod. Premature break-up of ice may not only lead to high mortality of seal pups but also produce behavioural changes in seal populations (Loeng et al., 2005). Polar bears, a top predator, are highly dependent on both sea ice and ringed seals (see Chapter 4, Box 4.3). Initially, the loss of sea ice and the subsequent deleterious effects are likely to occur at the southern distribution limit of polar bears, where early melt and late freezing of sea ice extend the period when the bears are restricted to land and only limited feeding can occur. Recently, the condition of adult bears has declined in the Hudson Bay region and first-year cubs come ashore in poor condition (Stirling et al., 1999; Derocher et al., 2004; Stirling and Parkinson, 2006). As a proportion of the population, the number of cubs has fallen as a result of the early break-up of sea ice. Loss of sea ice may also adversely affect other Arctic marine mammals, such as walrus (Odobenus rosmarus) that use sea ice as a resting platform and which occupy a narrow ecological range with restricted mobility. Similarly, the narwhal (Monodon monoceros) and the bowhead whale (Balaena mysticetus) are dependent on sea-ice organisms for feeding and polynyas for breathing (Loeng et al., 2005). The early melting of sea ice may lead to an increasing mismatch in the timing of these sea-ice organisms and secondary production that severely affects the populations of sea mammals (Loeng et al., 2005).
However, with an increase in open water, primary and secondary production south of the ice edge will increase and this will benefit almost all of the most important commercial fish stocks in Arctic and sub-Arctic seas; for example, cod (Gadus morhua) and herring (Clupea harengus) in the North Atlantic and walleye pollock (Theragra chalcogramma) in the Bering Sea; species that currently comprise about 70% of the total catch in these areas. However, some coldwater species (e.g., northern shrimp (Pandalus borealis) and king crab (Paralithoides spp.) may lose habitat (Vilhjálmsson et al., 2005).