CAPE TOWN — Hundreds of African penguins bask in the sun and frolic in the waves at Boulders Beach in Cape Town as tourists snap pictures from the boardwalk that traverses their breeding ground.

The flightless seabirds’ seemingly idyllic existence belies the fact that their species is under threat. Bird numbers at the main breeding colonies on South Africa’s west coast have plunged 90 percent since 2004, mainly because of a shortage of fish.

“Food is the problem,” University of Exeter researcher Richard Sherley, who published a study on the penguins this month, said. “The outlook on the west coast is quite gloomy.”

The African penguin breeds only on 25 islands and at four mainland sites in South Africa and Namibia, and government data shows the number of breeding pairs has plummeted to fewer than 25,000, from about 1 million in the 1920s. The birds also are a tourist draw.

The study led by Sherley, which was published in the Royal Society Publishing journal Biology Letters, found while chick survival rose 18 percent after a three-year fishing moratorium around Robben Island, that wasn’t sufficient to offset adult bird mortality.

Another investigation found sardine and anchovy shoals have migrated south and east, probably in response to climate change and fishing.

Conservation options include imposing further fishing restrictions and trying to encourage juvenile birds to establish new colonies along South Africa’s southern coast.

While African penguins should be able to survive for 30 years in the wild, they are probably living only about 10 years on average, according to Sherley. Fewer chicks are also being born.

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