The last time scientists visited Ile aux Cochons in 1982, an island that is part of an archipelago in the southern Indian Ocean, the king penguin population was booming. Over 500,000 breeding pairs (around 2 million penguins total) huddled together there, making the island the largest king penguin colony in the world. But new research shows their numbers have been on a stiff decline since then—by as much as 88 percent. And scientists don’t really know why.
To get the current penguin count on Ile aux Cochons, researchers employed a combination of high-resolution satellite imaging and photos taken from helicopters. Estimates of the total number of penguins were made based on the square footage they occupied, because they huddle so close together. The researchers found that the penguins declined from 502,400 breeding pairs in February 1982 to just 59,200 in April 2017, as they noted in their study published last week in the journal Antarctic Science.
“The rate of decline is unbelievable,” Henri Weimerskirch, study author and ecologist at the University of La Rochelle in France, told Gizmodo via email.
Dee Boersma, a conservation biologist at the University of Washington who was not involved with this study, told Gizmodo she wasn’t fully convinced by the new population estimates.
“The problem is, they cite varying methods from the 1960s to the 1980s to 2017,” she said, referencing how measurements from helicopters vs. satellites can be very different, and also that measurements were taken from many different months out of the year. “It’s really difficult to know just how much the penguins have declined.”
Still, she said the evidence is overwhelming that the colony is shrinking overall. And king penguin populations on other islands in the archipelago have stayed more or less stable, which makes this particular decline both alarming and puzzling.
“The cause of the massive decline of the colony remains a mystery, and needs to be resolved,” the team wrote in the new paper. The cause could still be an active threat to the colony, so identifying it could prevent further shrinking.
The researchers offered up a few possible scenarios in their paper for what could be at hand here. For one, feral cats and mice were found on the island in the 1970s, and were almost certainly introduced by humans traveling to the island. As the cats multiplied, they may have posed a predatory threat to penguin eggs and chicks, and probably still do.
Parasites like ticks and and diseases like avian cholera may also be plaguing the penguins, either killing them off or making it hard for babies to survive.
Boersma said climate change and overfishing may also be the culprit. King penguins’ favorite food is myctophid fishes, also known as lanternfishes, which they hunt at depths of up to 300 feet. But myctophids have been overfished, and warming waters could be nudging their habitats elsewhere.
Identifying precisely what’s going on matters not just for king penguins, which are listed as “least concern” on the IUCN Red List of Threatened Species, but also for other, more vulnerable penguins. Understanding threats to penguins in remote locations, Boersma said, is critical—penguin declines are often a sign that something funky is going on with the environment there.
“Penguins are ecosystem sentinels that can tell us about how the environment is changing,” she told Gizmodo. “We need to be paying more attention to them.”