White Island. A small volcanic island of the Ross Archipelago in Antarctica poking up through the Ross Ice Shelf several miles south of McMurdo Station. The island is home to an isolated colony of Weddell seals that are trapped there by the Ross and McMurdo Ice Shelves, and the seals' inability to swim far enough under the ice to rejoin the main Weddell seal population in Erebus Bay.
One of the tasks of the Weddell population project is to travel to White Island during the Weddell pupping season to survey, tag, and collect genetic samples from the trapped colony. Last season they found 6 adult females and 3 pups (so 3 of the adults were moms), according to field team leader Thierry Chambert. Here is a short video of a helicopter trip to White Island during a previous season...
Weddell seals are the southernmost mammal on Earth. And this small colony of Weddells trapped at White Island has the distinction of being the most southerly of all--and closer to the South Pole than any other mammal on the planet.
The situation of this isolated Weddell colony is described in an informative and fascinating paper, "History and fate of a small isolated population of Weddell seals at White Island, Antarctica" (Gelatt et al. 2010):
"The minimum distance between the island and open water averages approximately 20 km, but varies annually depending on the extent of sea-ice break-out in McMurdo Sound. Except for a narrow crack along the island’s northwestern side (Stirling 1966), maintained by tidal action and movement of the ice shelf, the ice exceeds 15 m thick within 20 m of shore (Castellini et al. 1984, 1991). There are no other known cracks or holes in the vicinity. A small isolated population of Weddell seals breeds and pups at White Island and uses the tidal crack as its only access to the surface."1
Using historic accounts of earlier explorers and researchers, it is thought that the seals became trapped at White Island sometime back in the late 1940s to 1950s when the sea ice broke out during the Antarctic summer so far south that a small group of seals was able to make it to White Island. However, when the sea ice reformed, the seals that didn't leave the island in time were trapped there. The swim under the ice is further than they are capable of surviving. And they are also likely blocked by the Ross and McMurdo Ice Shelves, as well. Since that historic time, the sea ice has yet to break out far enough south again to free these seals.
So, how is it that a small colony of Weddell seals trapped at White Island for over half a century and isolated from the rest of the Weddell seal population in Erebus Bay has managed to survive and successfully reproduce?
Because White Island is subject to the same tidal rise and fall of the ocean as is any other island, a tidal crack in the fast ice surrounding the island is maintained year around by that tidal movement. However, because the island is located in the Ross and McMurdo Ice Shelves, the ice surrounding the island is very thick--as deep as 45 feet in places near the island. The White Island seals have access to the ocean to hunt via this very deep tidal crack. To give birth on the surface of the ice, somehow these seals make their way up to the surface of the ice shelf through this deep tidal crack. Even more mysterious is how the Weddell pups born on top of the ice shelf are able to survive the steep drop down through the deep tidal crack to reach the ocean below to learn to swim and hunt.
One thing is for certain: despite great challenges to their survival, this colony of hardy seals has persisted for well over half a century to live and pup at this isolated colony, cut off from the general population of Weddells many miles away. How long they will continue to persist is a mystery; and equally so, whether the sea ice will ever again break out far enough to free these seals so that they may rejoin the Erebus Bay colonies.
The Weddell seal population field team makes two annual visits by helocopter to the White Island Weddell colony to survey, tag, and collect genetic samples from these seals, and has been doing so for many years. While the seals are unable to mix with the general population of Erebus Bay, they are still originally of the Erebus Bay Weddell population. The Weddell population project collaborates with other Weddell research groups to make census information and genetic samples available for further study.
Geneticists have determined that, "The White Island [Weddell seal] population showed altered reproductive dynamics compared to Erebus Bay [Weddell seals], including highly skewed sex ratio, documented inbred mating events, and the oldest known reproducing Weddell seals."2
According to scientists studying the genetics of these isolated seals, "The extensive amount of data available about the White Island population provides us with an opportunity to better understand the origin and fate of such Weddell seal colonies, and, by extension, of other small and isolated population of large mammals."3 This kind of insight may also be useful in better managing threatened populations of other large mammals elsewhere in the world--populations facing habitat loss or other challenges to their survival.