5 December, 2003
White Island rises from the ice towards the southern end of Erebus Bay. With the exception of a tidal crack on its northern side, the ice around it has been shown to be over 15m thick. The crack has a few openings that allow seals to move between the ice surface and the ocean, but the openings are clogged with platelet ice that makes access to the water difficult. In addition, the island and its tidal crack are at least 20km from the ice edge during the summer. So why would any seals live here, and how did they get here in the first place?
Seal researcher Thomas Gelatt studied these seals as part of a PhD research project. He hypothesized that the small population of seals that call White Island home has been isolated from other seals since 1947. That year marks the most extensive break-out of ice ever recorded in Erebus Bay. That event would have placed the ice edge close enough to White Island to allow seals to travel under the ice to its crack. Historically, none of the explorers of the early 1900s noted any seals at White Island. The first description of seals at White Island wasn't until 1958, with additional sightings in the 1960s.
Seals at White Island have been tagged and censused annually since 1991. Since then, there has been no evidence of any emigration from that population-no seals tagged at White Island have been seen anywhere else in Erebus Bay. In addition, there have been no untagged adults sighted at White Island since 1992. At its peak, the population had 26 seals older than pups. With no emigration or immigration, there has been no introduction of new genetic material to the White Island seals. Genetic studies of those seals have shown similarity with the other seals of Erebus Bay, while documenting inbreeding within the island's population.
Today we conducted the second census of the season at White Island. The first was done about 2 1/2 weeks ago, with 4 adults sighted and one new pup tagged. Our helicopter flight took us along the north edge of the island, flying low enough to see any black seal-like dots that might be lying along the crack. We made two stops, the first for a lone seal and the second for a small group of seals that included the pup that was tagged on the previous visit. The grand total for seals was 6, including the pup. Last year a total of 9 seals were seen, including 2 pups. This year's total is 8. The population is decreasing noticeably with each year.
What the future holds for these seals is anyone's guess. Perhaps there will be another major ice break-out event that will bring the edge close to White Island and allow mixing with other seal groups. Without immigration the population will grow increasingly inbred. Currently, pup survival rates are far lower than those found in other seal colonies. The pups have a hard time getting through the thick platelet ice and returning to the surface for air. Larger pups are probably better able to plow through that thick ice. Adults seem to have a good rate of survival. If a pup gets through its first few years, the probability is good that it will survive for many more. This population provides a unique natural laboratory for studying an isolated group of animals.
After the census we flew back to camp along the coastline over our study area. We checked the area to see if there were any significant groups of seals that had been missed during our censuses of the past few weeks. In fact, there was a small group at North Base that has gone undetected-but they are so far back within the glacier and ice cracks that it would be very difficult to get to them. It was great to get an aerial view of the study area, and even wilder to look down and see all those little black spots on the snow at each of the seal colonies.
The White Island seals
Isolated from others
What will happen next?
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