In the 1980s, some two million of the penguins lived on the Ile aux Cochons, or Pig Island, which is part of France’s territory in the southern Indian Ocean.
But now, a review of aerial and satellite imagery published in Antarctic Science shows their numbers have steadily declined since the late 1990s and that there are now only about 200,000 remaining.
“You never see a large colony declining at such a rate,” said Henri Weimerskirch, co-author of the study and leader of a long-term project monitoring seabirds in the southern oceans.
“It was probably one of the two largest penguin colonies in the world. It’s very depressing to think that about one third of the king penguin population in the world has disappeared.”
King penguins, which stand about a metre tall and are second in size to emperor penguins, live on temperate islands several hundred kilometres from Antarctica.
Researchers say there is no clear reason for the decline, particularly given penguin populations on neighbouring Crozet and Kerguelen Islands are stable or even growing.
“That’s why we were so surprised,” Weimerskirch said.
Climate change may play a role
El Nino weather events, which can be amplified by climate change, warmed the waters of the southern Indian Ocean in 1997, temporarily pushing the fish and squid on which king penguins depend beyond foraging range.
“It affected the king penguin colonies on Crozet and Kerguelen, but those colonies recovered relatively rapidly, after five or six years,” Weimerskirch said.
“We think the colony on Cochon has also been affected by this warming, but it has not recovered”.
While a reduction of food resources can be more devastating for a large colony than a small one, these factors alone do not explain why the Pig Island population is still declining.
Scientists say they need to organise an expedition to the island to determine what other factors may be at play.
“We have to check whether diseases are present on the island, for example, or introduced predators,” Weimerskirch said.
“Mice and cats have changed their behaviour over the past 20 years in the southern ocean, and are more of a problem than they were before.”
Organising an expedition may take time due to weather conditions and the availability of vessels.
Concern for future of species
In the meantime, environmental monitors may be revising the Red List of Threatened Species conservation status of king penguins.
“The king penguin was considered ‘least concerned’, because all the populations were known to be either increasing or stable,” Weimerskirch said.
“One third of the total king penguin population has disappeared. […] Probably the status of the species will be changed with this disappearance of the main colony.”