In March 2000 a huge piece of the Ross Ice Shelf, south of New Zealand, broke off and drifted westward to Ross Island. The iceberg, known as B15A, has remained grounded there since July 2001. Its presence changed the lives of the region’s emperor penguins. Some 1200 emperor pairs gather at Cape Crozier at the eastern end of Ross Island in April and May to breed.
Some time last winter, B15A shifted slightly south just far enough to crunch up the icy breeding platform of the emperor penguins. In December 2001, American scientists found ice ridges up to 30 feet high with penguin eggs on top. They collected carcasses of about a dozen male emperors that had starved to death. Ice ridges had probably prevented them from getting out of the colony area and made it impossible for females to return. It’s not known whether any penguins managed to leave the colony before disaster struck, whether any of them moved to another site or how many adults died, but it is known that not a single chick survived. If most males were trapped by the ice, the colony’s survival would be in serious doubt.
In a season of very heavy ice conditions, B15A may also have prevented sea ice in the region from breaking out. In spring 2001, Adélie penguins trying to return to their Ross Island breeding grounds were faced with a prolonged journey across sea ice and walls of jumbled ice. By November only a fraction of the 136 000 pairs usually breeding at Cape Crozier had reached the colony.
Winds of over 100 knots on 14 December 2001 blew out some 20 miles of fast ice, allowing many penguins to reach the colony. But it was two months too late. If these birds lay their eggs after 20 November, there is not enough time to rear their chicks over summer. The storm blew about 500 adult penguins and their eggs off their nests, killing the birds. Others were buried under as much as 4m of snow, from which most never emerged. After the storm only two percent of the penguins in this colony had chicks.
Other Ross Island Adélie colonies suffered similar fates. Penguins eventually returned but too late to breed successfully. In one of the Cape Royds sub-colonies, where in a good year up to 150 chicks are reared, there were only nine youngsters this season.
Mother Nature has performed an exclusion experiment that no ethics committee would ever approve, but it has provided invaluable data on the resilience of these penguins. One year of near total breeding failure tends to increase the rate of adult survival: potential parents do not have to invest energy into chick rearing activities and are exempted from the crucial decision as to when to stop feeding their chicks and start fattening up for their moult. But a continuation of the Ross Sea ice situation bringing several years of poor breeding success would threaten the survival of the world’s southernmost Adélie penguin colonies.
The good news is that the Ross Sea colonies on Franklin Island and farther north appear to have had a successful year with many Adélie penguin parents feeding two chicks.
Barbara Wienecke, seabird ecologist, AAD