WELLINGTON, Aug. 12 -- Penguins might be flightless, but they still manage to swim thousands of kilometers every winter, New Zealand scientists said Friday.

Tracking of almost 100 penguins in the Southern Ocean revealed the secret of where they went over the dark Antarctic winter, said researchers with the government's National Institute of Water and Atmospheric Research (NIWA).

Tags on the sub-Antarctic rockhopper and Snares penguins showed they travelled more than 15,000 kilometers at sea between April and October each year, said NIWA seabird ecologist Dr David Thompson.

"If they are constantly moving, this averages out at about 100 km a day, but you also have to add on to that the distances covered vertically as the birds dive to capture food," Thompson said in a statement.

The tagging program targeted rockhopper penguins at Campbell Island and Snares crested penguins, endemic to Snares Island.

While the Snares penguin population was relatively stable, rockhoppers at Campbell Island had declined by at least 21 percent since 1984, leaving just over 33,000 breeding pairs on the island.

The island was once the world's largest breeding colony of rockhoppers, the smallest of all penguin species, but between 1942 and 1984 the population dropped by about 94 percent.

It was not known what caused such a sharp decrease, but one theory was that changes in the penguins' diet might be responsible.

Thompson said they had "an extreme lifestyle."

"They come to land to breed and when they finish that, go back out to sea where they feed up for a month. Then they come back to land to sit and moult their feathers. During that period they don't eat at all," he said.

"Having basically starved themselves, they go back out to sea in poor condition. They've grown a whole new set of feathers so their plumage is fantastic, but it's quite demanding so they're really scrawny," said Thompson.

"We think winter is pretty important and that there is almost certainly something going on in the ocean causing the population to decline."

The Snares penguin headed exclusively west towards Australia, while the rockhoppers went east and covered a wider section of the ocean.

"We don't know what they're feeding on when they're away; we don't know if the amount of food available was more or less comparable to five or 10 years ago so this is very preliminary," said Thompson.

He planned to repeat the project and include other species, such as the erect crested penguin of the Antipodes Islands.

"The extra species will give us more information on how they relate to each other when they go away. It may be that they use different space, or it may be that populations of different islands get together at sea," he said.

"Prior to this study, we didn't have a clue where rockhoppers went in the winter, but the spaces they use in the ocean might be really important -- not only for them, but for scientists to better understand what is causing the population decline."