Nor is it a remnant from failed attempts in the mid-20th century to establish penguin colonies in Norway. Within a decade all those penguins were dead, the researchers noted.
And since 1972, all U.S. zoos have bred penguins in captivity and keep close tabs on their whereabouts. No escapees coincide with the Northern Hemisphere sightings.
Nevertheless, penguins—perhaps including the one caught in 2002—have been sighted up and down the North American West Coast a handful of times since 1975.
"In this case, we think they probably first showed up in Washington [State], probably coming on tuna boats as the tuna boats came up from Peru," Boersma said.
Wayne Trivelpiece is a penguin expert with the Southwest Fisheries Science Center in La Jolla, California. He agreed that boats are the most plausible penguin transport.
Crossing the warm, food-poor tropical waters, he said, "would be a really effective barrier in keeping any of them from naturally making that migration."
Guy Demment, the fisher who found the Humboldt penguin in Alaska in 2002, set it free once he snapped its picture.
Boersma said it probably won't survive.
"They don't live forever, and they got a lot of predators there [in Alaska]," she said.
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