Possible impact of decreasing Arctic pack ice on the higher trophic levels: Seabirds and marine mammals

Three main aspects of the possible impacts of retreating pack ice and changing ice coverage in the European Arctic on the higher trophic levels are discussed in this chapter: 1. Seabirds are depending on their colonies on land during the breeding season; the main species, e.g. little auk Alle alle, feed in mixed Polar/ Arctic Water in order to bring back food (polar zooplankton) to their chicks on the nest. In case ice coverage is very low, distance between colony and feeding grounds might become too long for such daily trips, and little auks might have to interrupt breeding and massively leave their colonies before the end of the reproduction cycle (e.g. on Jan Mayen in July 2005). On a longer term, if ice coverage was decreasing too much in future years, the whole Spitzbergen population might be affected, as well as other seabird species feeding mainly at the ice edge. 2. Stocks of large whales were close to depletion in the north-eastern Atlantic due to massive over-exploitation by whaling, and to their separation from the more abundant North Pacific stocks by high ice coverage of both the North-East and North-West Passages. In the frame of our long-term study of the at-sea distribution of seabirds and marine mammals in Arctic seas from 1979 on, resulting in more than 10,000 half-an-hour transect counts, an important increase of the north-eastern Atlantic stocks was detected from 2007 on, the year with lowest ice coverage, mainly for bowhead Balaena mysticus, blue whale Balaenoptera musculus, humpback whale Megaptera novaeangliae and fin whale Balaenoptera physalus. Such a sudden and important increase cannot be attributed to population growth, but to a probable inflow from the more abundant Pacific stocks, the decrease of ice coverage allowing the opening of the Passages. 3. On the other hand, polar bear Ursus maritimus and its prey: pinnipeds - mainly harp seal Pagophilus (Phoca) groenlandica - seem not to show any important decline of their population, with a few local exceptions. Moreover, encountered bears regularly concerned mother with cub(s), which also reflect the good health status of the population. The reason is probably that they are not bound to pack ice as such, but to the Outer Marginal Ice Zone (OMIZ): they are almost absent in the Closed Pack Ice (CPI). They seem to adapt and follow the OMIZ, even when its position is influenced by changes in ice coverage. These data collected on board icebreaking RV Polarstern are discussed in this chapter, in function of changes in ice coverage, in the extent of the main water masses and thus in the position of the ice edge and fronts