Wildlife : Seals
Arctic: Baby Harp seal. Early ice breakup could separate mothers and pups, increasing mortality of newborn pups.
Arctic: Ringed seal. Ringed seals give birth in snow-covered lairs or directly on the sea ice surface.
Antarctic: Weddell seal. Weddell seals are able to stay underwater for up to 80 minutes while they look for breathing holes in the sea ice.
Many different species of seal live in both Antarctica and the Arctic, and life cycles vary considerably among species. Some species depend entirely on the presence of sea ice to survive. Many give birth and nurse their pups on the ice, and they look for food near the ice edge and under the ice. Most seals never leave the ice pack, creating open breathing holes all winter, and they make lairs under snow mounds to protect newborn pups from polar bears and from the cold air above (Krajick 2001).
Six seal species live in the Arctic: harp, hooded, ringed, bearded, spotted, and ribbon. Although sea ice affects all these seal species, harp, ringed, and bearded seals have life cycles that are tightly linked to sea ice. Harp seals follow and live at the sea ice edge all year. In the spring, harp seals migrate southward, assembling in large groups to breed near the Newfoundland and Norwegian coasts. During late February and early March, thousands of harp seal females gather to deliver their babies on the White Sea ice (Pagophilus.org 2001).
Ringed and bearded seals often use holes in the ice to breathe, and they congregate along the edges of the ice. Ringed seals use fast ice and sometimes dense pack ice for giving birth. They require sufficient snow cover on the ice to construct birth lairs, and the sea ice must be stable enough in the spring to rear the pups. Some ringed seals in the Okhotsk Sea regularly give birth on the exposed sea ice surface (Pagophilus.Org 2001). Unlike the harp seal, the ringed seal does not migrate to open water in the winter.
Decreasing sea ice extent would reduce the habitat available to ringed seals. Early ice breakup could result in premature separation of mothers and pups, leading to higher death rates among newborn pups. In the southern Baltic Sea, from 1989 to 1995, a series of nearly ice-free winters led to very high pup mortality rates (Härkönen et al. 1998). If autumn and winter are fairly mild, the ice is soft and thin and disintegrates easily. As a result, newborn seal pups, which are born on the ice, do not have enough time to wean properly and may not survive. Seal pups need at least 12 days on the ice before they finish nursing (Nickerson 2002).
In 1998, ice in the western Arctic broke up three weeks earlier than usual, sending pups into the water before they had been weaned. In 2002, many seal mothers had no choice but to give birth in open water because they could not find ice, which resulted in several hundred pups drowning (Johansen, in press).
Although only four species of seal are found in Antarctica (crabeater, Weddell, leopard, and Ross), there are many more individual seals in Antarctica than in the Arctic. This is primarily due to the crabeater seal, which far outnumbers any other species; there are more crabeater seals than all other seal species combined. Despite their name, crabeaters subsist almost entirely in krill, and so they are part of the phytoplankton-based Antarctic food chain. Like Arctic seals, crabeater seals depend on pack ice for breeding and giving birth.
Although killer whales can also pose a threat, the primary predators of crabeater seals are leopard seals. Because of their large size, leopard seals cannot move easily on sea ice, and so crabeater seals often rest on floes where they are safe from predation. Clearly, a decrease in sea ice could negatively affect crabeater seal populations by increasing predation and decreasing breeding and birthing sites.
Sea ice characteristics also affect Weddell seals, the favorite among most Antarctic visitors because of their placid nature. Weddell seals spend most of their time beneath fast ice, coming onto the sea ice surface only to rest and have their pups. Modified physiology allows Weddell seals to spend up to 80 minutes underwater, sufficient time to hunt and search for cracks in the ice that they can use as breathing holes.