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Biodiversity - Cetaceans and Pinnipeds (Whales and Seals)

P. O. Thomas1 and K. L. Laidre2

1US Marine Mammal Commission, Bethesda, MD USA
2Polar Science Center, Applied Physics Laboratory,
University of Washington, Seattle WA USA

November 23, 2011

Highlights

  • Thousands of walruses hauled out on the NW coast of Alaska by mid-August 2011, the fourth time this has been observed in the past five years. The behavior is thought to be triggered by reduced sea ice in the Chukchi Sea.
  • Observations of overlap between bowhead whales from the Baffin Bay and the Bering-Chukchi-Beaufort populations in the Northwest Passage indicate that reduced summer sea ice may facilitate exchange between the two populations.

Arctic biodiversity assessment

Over the past several years, the Arctic Report Card for Marine Mammals has presented available information on the population status of Arctic marine mammals. In 2011, the "Arctic Biodiversity Assessment - Status and Trends" (ABA) was launched by the Arctic Council's Working Group on Conservation of Arctic Flora and Fauna (CAFF) to synthesize and assess the status and trends of biological diversity in the Arctic. The report, due out in 2013, will inventory and update the status and trends of all stocks of Arctic marine mammals, and will be presented in the next Arctic Report Card.

Arctic marine mammal movements and distribution relative to ice

Pacific walruses: Diminishing Arctic sea-ice, and specifically the retreat of the pack-ice beyond the continental shelf of the Chukchi Sea, has resulted in Pacific walruses (Odobenus rosmarus divergens) hauling out on land by the thousands along the Alaskan coast of the Chukchi Sea in the summer. While fall migratory aggregations (October-November) have been seen on the Alaskan coast in the past (notably at Cape Lisburne) the summer haul-outs are new and occur primarily north of Point Lay (Garlich-Miller et al. 2011). Radio-tagging studies conducted in the Chukchi Sea by the U.S. Geological Survey since 2007 have demonstrated that walruses remain offshore over the continental shelf feeding grounds for as long as possible, hauling out on sparse marginal ice. When suitable ice disappears, some animals move west toward Russia but the majority move to land haul-outs on the Alaska coast (http://alaska.usgs.gov/science/biology/walrus/index.html). Jay et al. (2010) showed that walruses prefer to stay in particular areas of the Chukchi Sea rather than remain with specific ice floes as they drift. This indicates their dependence on shallow bottom feeding areas in the summer months. Bayesian network models, designed to integrate potential effects of changing environmental conditions and anthropogenic stressors on Pacific walruses demonstrate a clear future trend of worsening conditions for the sub-species (Jay et al. 2011). Also see the essay on Marine Ecology: Biological Responses to Changing Sea Ice and Hydrographic Conditions in the Pacific Arctic Region.

Bowhead whales: Satellite tagging studies of bowhead whales (Balaena mysticetus) shed light on how this large baleen whale species uses sea ice habitat. In the Pacific Arctic, bowhead whales undertake their spring migration, north along the Alaskan Chukchi Sea coast and eastward across the Beaufort Sea, through 80-100% ice cover (Quakenbush et al. 2010). After feeding in the Canadian Beaufort Sea they move westward in late summer, often feeding along the way, especially near Barrow, Alaska, and along the Russian Chukotka coast. http://www.adfg.alaska.gov/index.cfm?adfg=marinemammalprogram.bowheadmovements

In August 2010, two satellite tagged bowhead whales, one from Disko Bay, West Greenland and one from Alaska, entered the Northwest Passage from opposite directions and spent approximately 10 days in the same area. This is the first time distributional overlap between the two populations, which have been assumed to be separated by sea ice, has been documented. Reduced summer sea ice in the Northwest passage may remove this geographic barrier and facilitate exchange between the populations (Heide-Jørgensen et al. 2011). In East Greenland, Lydersen et al. (2011) reported the first satellite-tagged bowhead whale from the critically endangered Spitsbergen stock, and found movements consistent with the patterns described for bowhead whales in this region by the early whalers in the 16th and 17th century.

Beluga whales and narwhals: In West Greenland, a clear relationship between decreasing annual sea ice cover in Baffin Bay and increasing offshore distance of beluga (Delphinapterus leucas) sightings was established based on 30 years of aerial survey data, suggesting belugas expand their distribution westward as new areas on the banks of West Greenland open up earlier in spring with reduced sea-ice coverage (Heide-Jørgensen et al. 2011). Recent analysis of sea ice trends in autumn on narwhal (Monodon monoceros) summering grounds has documented freeze-up occurring roughly 2 to 4 weeks later since 1979. Four ice entrapments occurred in the summering areas between 2008 and 2011; however, it is not clear if the entrapments are due to random variation in narwhal residence time in summer areas, or whether there is a trend in prolonged summer residence time as narwhals adapt to a longer open water season (Laidre et al. 2011).

Tagged marine mammals as environmental sensors

Marine mammals are increasingly becoming sources of information on conditions around Arctic Seas, building oceanographic baselines and allowing observations of changing conditions beyond what is practical with ship-borne or aerially-deployed instruments. In Baffin Bay, 14 narwhals equipped with satellite-linked time-depth-temperature recorders provided wintertime temperature data for Baffin Bay. These animal-borne instruments confirmed earlier measurements of warming winter water temperatures in Baffin Bay associated with a warming West Greenland Current (Laidre et al. 2010). Hooded seals (Cristophora cristata) carrying similar instruments provided detailed oceanographic data for a significant portion of the northeast Atlantic Ocean, which have allowed scientists to study how freshwater moves from the Arctic Ocean into the North Atlantic through the Fram Strait (Fig. ME5) (Lydersen et al. 2010). For more information on ocean temperature and freshwater content, see the essay on Ocean.

Fig. 5 -- Tagged hooded seals provided water temperature profiles

Fig. ME5. Tagged hooded seals provided water temperature profiles from an area of over 3 million km2 in the Greenland and Norwegian Seas, from south of the Faroe Islands almost to the North Pole, and from the surface to over 1,000 m depth (Kovacs et al., 2011). Water temperatures recorded are from -2 °C (dark blue) to +9 °C (red).

Marine mammals and changes in prey

Changing sea-ice formation, extent and concentration are the most visible manifestations of climate change in the Arctic. The impacts of these changes on the timing and location of primary and secondary production are just now starting to be understood (see the essay on Arctic Ocean Primary Productivity). Uncertainty about how changes in production will affect trophic structure leads to concerns whether Arctic marine mammal species will continue to find adequate food and be able to compete with more temperate species in a warmer, more seasonally ice-free environment (Kovacs et al. 2010). Early loss of sea-ice over the continental shelf is predicted to reduce the productivity of the benthic communities marine mammals such as walruses and gray whales (Eschrichtius robustus) feed upon (see the essay on Marine Ecology: Biological Responses to Changing Sea Ice and Hydrographic Conditions in the Pacific Arctic Region).

Patterns of seasonal ice cover likely influence the availability of bowhead prey by affecting light penetration and wind-driven upwelling (Ashjian et al. 2010). Extreme sea-ice retreats in the Pacific Arctic sector (see the essay on Sea Ice) may actually improve foraging opportunities for bowheads (Moore et al. 2010, Tremblay et al. 2011). As temperature changes lead to changing prey species assemblages, some hypothesize that lipid-rich arctic prey species will be replaced by leaner temperate species, reducing the ability of the most arctic-adapted marine mammals (e.g., bowheads, narwhals, ringed seals) to replenish essential blubber stores in the short summer season (Kovacs et al. 2010).

In winter, narwhals feed intensively in dense pack ice over the continental slope in central Baffin Bay and Davis Strait, selecting areas of nearly 100% ice cover in order to remain above the highest densities of their preferred Greenland halibut prey. Narwhal winter distribution appears to be dictated by optimal foraging areas rather than sea ice density or the presence of open water, and a recent study has documented densities of over 70 narwhals/km2 in preferred pack ice foraging areas with <3% open water (Laidre and Heide-Jørgensen 2011).

References

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Garlich-Miller, J. L., J. G. MacCracken, J. Snyder, R. Meehan, M. J. Myers, J. M. Wilder, E. Lance, and A. Matz, 2011: Status review of the Pacific walrus (Odobenus rosmarus divergens). U.S. Fish and Wildlife Service, Marine Mammals Management, January 2011, Anchorage, AK, 155 pp.

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Heide-Jørgensen M. P., K. L. Laidre, D. Borchers, T. A. Marques, H. Stern, and M. J. Simon, 2010: The effect of sea ice loss on beluga whales (Delphinapterus leucas) in West Greenland. Polar Research, 29, 198-208.

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