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WALRUSES, SEALS, AND CLIMATE CHANGE
KELLY, BRENDAN P University of Alaska Southeast.
Pinnipeds (seals, sea lions, and walruses) exhibit an unusual pattern of increasing species richness with increasing latitude that can be attributed, in part, to the critical role that sea ice has played in the group's evolution (Kelly 2001). The potential impact of climate change on ice-associated marine mammals has largely been discussed in terms of reductions in the thickness and areal extent of sea ice. We should look beyond ice thickness and extent, however, and consider the specific ecological relationships between individual species and the sea ice environment to predict likely impacts of climate change. Examples include Pacific walruses (Odobenus rosmarus divergens) and ringed seals (Phoca hispida).
Pacific walruses are benthic feeders that rest and bear their young on pack ice of the Bering and Chukchi seas. The reductions in the summer and fall ice cover observed in recent years effectively de-coupled the walruses' feeding and nursing habitats. In September 2002 and 2003, record retreats of the ice edge were observed in the Arctic Ocean. Female walruses and their young migrate in spring from the Bering Sea to the Chukchi Sea (Fay 1982) where they typically alternate resting on the ice edge with feeding on benthic prey below the ice. In recent years, however, the ice edge retreated to north of the shallow shelf so that the ice was over water too deep for walrus feeding (Kelly 2001).
Whereas the distribution and extent of sea ice relative to forage grounds is critical to walruses, the snow cover on top of the ice is the critical feature of ringed seal habitat. Ringed seals are regarded as the most ice adapted and the most numerous of the northern hemisphere pinnipeds (seals, sea lions, and walruses) (Chapskii 1940; McLaren 1958; Smith and Stirling 1975). They are found in all seasonally ice-covered seas and even some lakes in the northern hemisphere.
Ringed seals feed under heavy ice cover on fish and zooplankton and obtain air at breathing holes they maintain in the ice. Early in the winter, the breathing holes become snow covered. As snow accumulates on top of the ice, the seals continue to breathe under the snow. Where snow drifts to sufficient depths, the seals excavate lairs in the snow above their breathing holes. These subnivean lairs provide protection from extreme cold and decrease the seals' vulnerability to predation. Pups are born in the lairs in April and are nursed there for the first couple of months of their lives. The survival of pups depends on their remaining in lairs concealed from mammalian and avian predators. Ringed seals typically abandon lairs only in late spring and early summer, when the pups are weaned.
For the past 5 years, the abandonment of lairs has taken place increasingly early as spring temperature and snow melts have advanced. The transition from lair use to basking on the surface was especially early and abrupt in 2002, and by mid May all the seals had abandoned their lairs. Many pups in their natal coats were resting on the ice in the open instead of in subnivean lairs as is usual in mid May. The early snow melts we have observed are consistent with a general pattern observed in the Beaufort Sea. Premature lair abandonment by ringed seals, associated with early snow melts, likely will increase juvenile mortality rates via exposure to freeze-thaw conditions and predation.
The degree to which ringed seals can respond adaptively to earlier snowmelts will depend on several factors including the rate of climate change, the heritability of pupping dates, and the genetic variability within the population.
The area of breeding home ranges used by radio-tracked ringed seals averaged 2 km2, and we have found adult seals returning to the same or nearly the same breeding home range in successive years. This high fidelity to breeding sites suggests that rather than forming a single, panmictic population, ringed seals may exist in a series of locally adapted demes. As such, they would be expected to have limited genetic diversity and scope for adaptive change in response to climate change.
Chapskii, K. K. 1940. The ringed seal of western seas of the Soviet Arctic (The morphological characteristic, biology and hunting production). Tr. Vses. Arkt. Inst. (Leningrad) 145:1-72. (Transl. from Russian by Fish. Res. Board Can., 1971, Transl. Ser. 1665, 147pp.)
Fay, F. H. 1982. Ecology and biology of the Pacific Walrus, Odobenus rosmarus divergens Illiger. North American Fauna 74. United States Fish and Wildlife Service. Washington, D.C.
Kelly, B. P. 2001. Climate change and ice breeding pinnipeds. Pages 43-55 in G.-R. Walther, C. A. Burga and P. J. Edwards, editors. "Fingerprints" of climate change: adapted behaviour and shifting species' ranges. Kluwer Academic/Plenum Publishers, New York and London.
McLaren, I. A. 1958. The biology of the ringed seal seals (Phoca hispida Schreber) in the eastern Canadian Arctic. Fisheries Research Board of Canada. Ms. Rep. (Biology) 653. 146 pp.
Smith, T. G., and I. Stirling. 1975. The breeding habitat of the ringed seal (Phoca hispida). The birth lair and associated structures. Canadian Journal of Zoology 53:1297-1305.
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