Image caption King penguins breed on more temperate islands north of Antarctica 
 
 The world's largest king penguin colony has shrunk nearly 90% since the 1980s, research suggests. 
 
 Aerial and satellite images show breeding pair numbers have fallen 88% in the last three decades, an article in the journal Antarctic Science says. 
 
 The colony lies on the France's uninhabited Île aux Cochons between Africa and Antarctica in the Indian Ocean. 
 
 Researchers say there is no clear reason for the decline. 
 
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 The paper says that only 60,000 penguin pairs remain in photos taken in 2015 and 2017, down from half a million pairs recorded on a previous conducted in the 1980s. 
 
 Second only to the emperor penguin in size, the king penguin breeds on the more temperate islands north of the Antarctic coast. 
 
 Research published in February says some of the birds populations could be at risk from climate change. 

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 Number of king penguins on the remote Île aux Cochons has fallen from 2m to 200,000, warn scientists 
 
 The planet’s largest colony of king penguins has declined by nearly 90% in three decades, researchers have warned. 
 
 The last time scientists set foot on France’s remote Île aux Cochons – roughly half way between the tip of Africa and Antarctica – the island was blanketed by 2m of the penguins, which stand about a metre tall. 
 
 But recent satellite images and photos taken from helicopters show the population has collapsed, with barely 200,000 remaining, according to a study published in Antarctic Science. 
 
 Antarctica's king penguins 'could disappear' by the end of the century Read more 
 
 Why the colony on Île aux Cochons has been so decimated remains a mystery. 
 
 “It is completely unexpected, and particularly significant since this colony represented nearly one third of the king penguins in the world,” said lead author Henri Weimerskirch, an ecologist at the Centre for Biological Studies in Chize, France, who first set eyes on the colony in 1982. 
 
 Climate change may play a role. In 1997, a particularly strong El Niño weather event warmed the southern Indian Ocean, temporarily pushing the fish and squid on which king penguins depend south, beyond their foraging range. 
 
 “This resulted in population decline and poor breeding success for all the king penguin colonies in the region,” Weimerskirch said. 
 
 El Niño events are cyclical events that occur every two to seven years, but they can be amplified by global warming, which itself produces many of the same results, albeit on a longer timescale. 
 
 Indeed, Weimerskirch and colleagues showed in an earlier study that climate change, on its current trajectory, will probably make the Crozet islands – the archipelago that contains Île aux Cochons – unviable for king penguins by mid-century. 
 
 Migration is not an option because there are no other suitable islands within striking range. 
 
 Other factors may be contributing to the decline of the Île aux Cochons colony, including overcrowding. “The larger the population, the fiercer the competition between individuals,” noted a statement from France’s National Centre for Scientific Research, which funded the study. “The repercussions of lack of food are thus amplified and can trigger an unprecedented rapid and drastic drop in numbers.” 
 
 But this so-called “density-dependent effect” can also be made worse by climate change, the study notes. 
 
 Another possible culprit is avian cholera, which has affected seabirds on nearby Marion and Amsterdam Islands, including some king penguins. But until Weimerskirch and other researchers return to Île aux Cochons – hopefully, he said, in early 2019 – they won’t know for sure. 
 
 It is also possible that invasive species such as rats, mice or cats, have found their way on to the island. The Red List of Threatened Species conservation status for king penguins is currently “least concern,” but the new data may prompt a reevaluation. 
 
 King penguins are the second-largest penguin species after the Emperor. They do not make a nest, but rather lay one egg at a time and carry it around on their feet covered with a flap of abdominal skin, called a brood patch. Parents take turns incubating the egg, switching every couple of weeks over a two-month period. 

 Known since the 1960s, the colony of king penguins (Aptenodytes patagonicus) on Île aux Cochons, 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. 
 
 The Chizé team 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. 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 Niño. This event temporarily affected the foraging capacities of another colony 100 km from Île aux Cochons, causing it to dwindle. The same process may be responsible for the fate of the Île 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 Île 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 Île aux Cochons. Field studies led by CNRS researchers, with support from the French Polar Institute (IPEV), and in close partnership with TAAF nature reserve staff, should be getting under way shortly to verify initial conclusions drawn from the satellite images. 
 
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– The last time scientists visited the remote French island of Île aux Cochons in the Indian Ocean, they found more than 2 million king penguins, including 500,000 breeding pairs. That was 1982, and it gave the island the distinction of being the largest such colony in the world. But in the first comprehensive new count since then, researchers report in Antarctic Science that only about 200,000 king penguins now inhabit the island. Of those, about 60,000 were in pairs, reports the BBC. The new numbers are based on aerial photos taken in 2015 and 2017 of the island, which sits between Antarctica and the tip of Africa, per the Guardian. For now, researchers have no clear answer on what explains the population plunge, though followup field studies could shed light. “It is completely unexpected, and particularly significant since this colony represented nearly one third of the king penguins in the world,” says lead author Henri Weimerskirch of the Centre for Biological Studies in Chize, France. One theory revolves around the weather: A particularly strong El Niño in the late 1990s warmed regional waters and might have pushed penguin prey such as fish and squid out of foraging range long enough to have devastating consequences on such a dense population, according to a news release. Those effects could have been amplified against the larger backdrop of climate change in general, notes the Guardian. Other theories include diseases such as avian cholera, or the arrival of invasive species such as rats or mice. (On the flip side, researchers found a previously unknown colony of a different type of penguin.)