“It was really a surprise for us,” said Henri Weimerskirch, a co-author on the new paper, published in Antarctic Science, and a member of the research teams in 1982 and 2016. “It’s really very depressing.”

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The research team suspects that climate change could be playing a role , as it has with other colonies of penguins in parts of Antarctica. But competition for resources, diseases and relocation may possibly have contributed to population losses.

Researchers plan to do a head count on the island but they can’t get there until late fall 2019 at the earliest, because of the cost and timing issues, said Dr. Weimerskirch, research director of the Chizé Centre for Biological Studies at the French National Centre for Scientific Research. A protected nature preserve, Pig Island isn’t easy to reach, and the animals can’t be seen from the water, because the colony is situated inland, he said.

If the count from the satellite images proves accurate, it would significantly reduce the global population of king penguins, estimated at 1.5 million to 1.7 million breeding pairs worldwide with this loss. They had not been considered endangered before, but might be, Dr. Weimerskirch said.

King penguins are second-largest in size after emperor penguins. They don’t nest, but lay one egg and parents take turns incubating the egg with an abdominal layer called a brood patch for two months. King penguins leave their young and swim south to forage for fish and squid in the waters of the Antarctic polar front, where cold, deep water mixes with more temperate seas. If they can’t reach this polar front and can’t swim back within about a week, their chicks will starve to death.