World's largest king penguin colony has shrunk by nearly 90 per cent

The world's biggest colony of king penguins has reduced by a staggering 88 per cent over the past 35 years, according to scientists who said that the reason for collapse remains a mystery.

Known since the 1960s, the colony of king penguins (Aptenodytes patagonicus) on Ile aux Cochons, an island in the southern Indian Ocean, had the distinction of being the world's biggest colony of king penguins and second biggest colony of all penguins.

However, due to its isolation and inaccessibility, no new estimates of its size were made over the past decades.

Researchers from French National Center for Scientific Research (CNRS) and French Polar Institute (IPEV)used high-resolution satellite images to measure changes in the size of the colony since the island was last visited by a crew of scientists (1982).

At the time, the colony included 500,000 breeding pairs and consisted of over two million penguins. The numbers have now reduced to about 60,000 pairs, according to the study published in the journal Antarctic Science.

To calculate the area occupied by the colony at different times between 1960 and the present, the researchers studied changes in its contours over the years.

They found that the colony has shrunk, yielding its territory to encroaching vegetation. Photographs taken from a helicopter during the Antarctic Circumpolar Expedition confirm that the colony's penguin population has plummeted.

Data show that the decline began in the late 1990s, coinciding with a major climatic event in the Southern Ocean related to El Nino. This event temporarily affected the foraging capacities of another colony 100 km from Ile aux Cochons, causing it to dwindle.

The same process may be responsible for the fate of the Ile aux Cochons colony. Its size may also subject it to density-dependent effects. That is, the larger the population, the fiercer the competition between individuals, slowing the growth of all members of the group.

The repercussions of lack of food are thus amplified and can trigger an unprecedented rapid and drastic drop in numbers, especially following a climatic event like the one at the end of the 1990s.

Disease is another hypothesis entertained. Avian cholera is currently ravaging populations of seabirds on other islands in the Indian Ocean, like the albatross of Ile Amsterdam and the penguins of Marion Island.

Still, none of these possibilities seems to offer a satisfactory explanation for a decline of the magnitude observed on Ile aux Cochons.