Eddies are really the local weather in the ocean, is how I think of it -- and the weather is always important to whoever is there doing something.

The buoyant, warm eddies ride as much as a foot (30 centimeters) higher than the surrounding more salty seawater but can be difficult to detect in the vastness of the ocean. However they can appear clearly from the vantage point of satellites that measure sea surface heights, popping up like pressure zones in the atmosphere.

"Its like putting on a special pair of glasses," said William Crawford of Fisheries and Oceans Canada at the Institute of Ocean Sciences.

Crawford and his colleagues have used data gathered by the Franco-American satellite TOPEX/Poseidon, launched in 1992, to track large eddies known as "Sitka" and "Haida" as they form along the Alaskan Panhandle and west coast of Canada before drifting west.

Throughout the region, fish are as much a big business as they are a way of life for the native populations. In Alaska, for example, the 1999 salmon catch was worth $371 million. In that near-record year, Alaskan fishermen caught nearly a billion pounds (400 million kilograms) of the fish, among them prized Chinook, coho and sockeye.

But catches can vary from year to year and, more significantly, from region to region. Alaska might be swimming in fish one year, while farther south, fishermen in the Pacific Northwest pull up empty nets. A decade later, the situation might flip-flop.

While overfishing and dams that seal off streams and rivers to spawning salmon contribute to the problem, spaceborne tools like TOPEX and the European ERS-2 show that large-scale climatic variations may play a role as well.

"The upshot of that is there are changes that occur in the oceans climatic system over decades and there will be periods where conditions favor salmon and periods where they do not," said Gordon Kruse, a marine fisheries scientist with the Alaska Department of Fish and Game.

Crawfords group found that in strong El Nino years like 1998 large eddies were being formed. The mechanism, Crawford said, pumped the eddies -- which can contain as much water as all of Lake Michigan -- into regions of the ocean that had been nutrient-deficient.

By tracing the formation and migration of the swirling eddies, scientists said they can get a better understanding on their impact on the health of the ocean, including its salmon and other fish populations.

"They provide like a little community in which biological production can thrive," said Bill Patzert, a research oceanographer at NASAs Jet Propulsion Laboratory, of the eddies. "Its like an aquaculture community."

Similar tracking work goes on in the Gulf of Mexico and in Hawaii, where scientists are incorporating satellite data in the study of lobster and swordfish populations. They are also important for humans when it comes to placing offshore oil and gas drilling rigs.

"Eddies are really the local weather in the ocean, is how I think of it," said Bob Leben, a satellite oceanographer at the University of Colorado, Boulder, "and the weather is always important to whoever is there doing something."

Commercial fishermen have taken note of the use of satellites too. Over the last decade, companies have sprung up that use NASA and other satellite data to create maps pinpointing those areas of the ocean where there are strong gradients in temperature and current -- like alongside eddies -- where fish tend to congregate.

"Its part of a continuing revolution," Patzert said of the increased use of satellite data to manage very down-to-Earth issues. "The fisheries people, it allows them to follow these eddies -- and their biological communities -- for days, weeks, months and even years."