A Humboldt penguin known only from the Southern Hemisphere but recently found thousands of miles from home likely was a stowaway on a fishing ship, say scientists.

The seemingly peripatetic penguin turned up in July 2002 when fisherman Guy Demmert netted an atypical batch of salmon off the coast of southeast Alaska. There among the salmon was the Humboldt penguin that somehow had strayed a nearly impossible distance from where the species lives.

When sightings of penguins in northern waters occur, including Demmert's find, they present a puzzle of sorts for scientists: All of Earth's 17 penguin species live primarily in the Southern Hemisphere. The Galápagos penguin is the only one with a habitat that extends somewhat north of the equator. So how would any of these flightless birds manage to voyage deep into the Northern Hemisphere?

A marathon swim (or waddle) is unlikely, partly because the birds are ill-equipped for the range of climate conditions found along the way, according to two University of Washington biologists, Dee Boersma and Amy Van Buren, who conducted research on errant penguins that is detailed in the June issue of the Wilson Journal of Ornithology.

Zoo escapees? Not likely either, they say. While penguins were routinely shipped from the south to zoos in North America in the past, international regulations that prohibited such trade were adopted in 1972, so all shipments ceased.

Since then, zoos in the United States have bred penguins in captivity, which includes transporting the individuals from zoo to zoo to prevent inbreeding. Precautions are taken during the transfer, making penguin escapees infrequent, the biologists say.

"Most zoos use some kind of flipper tag, so a penguin that escaped from a zoo should be able to be identified pretty easily," Van Buren said.

The most plausible explanation is that the tourist penguins, including Demmert's discovery, were hauled aboard fishing boats in southern waters and stowed away as pets while the vessels traveled northward. Along the way, the crew might release the birds into unfamiliar waters, Boersma and Van Buren wrote.

"The crews keep the penguins as pets on board the boat. They're appealing," Van Buren said. “People keep them around because they're so cute."

Additional sightings of penguins in northern waters have included:

In 1944, a Humboldt penguin was reported off British Columbia’s Queen Charlotte Islands.In 1975, a penguin was spotted near Long Beach in Washington.In 1978, up to three Humboldts were seen off the coast of Vancouver Island, British Columbia.In 1985, a penguin was reported off the coast of Washington.Video: Living with PenguinsWhy Don’t Penguin Feet Freeze on Ice?Top 10 Amazing Animal Abilities