(CNN) The world's largest colony of king penguins has shrunk by almost 90% in the last three decades, researchers said Monday.

The colony lies on the subantarctic French island of Île aux Cochons, or Pig Island, and once numbered over two million king penguins. When scientists last stepped foot on the island in 1982, it was not only the world's largest king penguin colony, but the second largest colony of all penguins.

However, when researchers from the Chizé Center for Biological Studies (CNRS) returned to survey the island in 2016 and 2017, the colony had collapsed. The total population had shrunk by 88%, down to barely 200,000 king penguins. Before, there had been half a million breeding pairs -- now, only 60,000 remain.

The remote island in the southern Indian Ocean is so inaccessible that researchers have not conducted new population estimates for decades, leaving the exact cause of the population drop a mystery. However, there are several possible factors, which are explored in the full report published last week in the Antarctic Science journal.

Photographs and satellite images taken over the decades found the colony's territory has shrunk, taken over by encroaching vegetation. The decline also began in the late 1990s, the same time a major climatic event related to the cyclical weather phenomenon El Niño took place in the Southern Ocean, potentially affecting the penguins' ability to forage for food.

The king penguin colony on Île aux Cochons in 1982, when the population was more than two million.

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