LAGO DE SAN IGNACIO, Baja California - The season of migration has come again to the warm blue waters off the coast of Mexico. Mother gray whales are nursing their newborn calves, plumping them up for the 6,000-mile trip next month to summer feeding grounds in the Arctic.

This migration, one of the longest of any of the world's wild mammals, has gone on for thousands of years. Increasingly, the watery voyage raises questions about how the changing climate is affecting species that live in the Arctic, the part of the world transforming most dramatically from humanity's greenhouse gas emissions.

As ocean and atmospheric temperatures rise, the gray whales - and other Arctic dwellers like the walrus, polar bear, ice seal and Arctic fox -- are making their way in an unknown warming world. Their habitat and food supply are shifting as a result of warmer waters and shrinking sea ice.

But little is known of that food chain. Even less is known of how it will shift as the climate changes. And that represents a worrisome gap in basic science, say scientists meeting at a State of the Arctic conference in Miami last week.

The teeming waters are among the richest in the world yet the least studied because of difficulties overcoming months of dark days and impassable frozen seas. Arctic scientists say they've just begun to document the polar cap's biological diversity. They don't know how the animals are responding to global warming, where they're feeding, how their icy habitat has been affected or how the ecosystem's food web has changed.

The researchers want to fill crucial data gaps so that they can advise how best to safeguard the wild Arctic. Protection is crucial, they say, as the Northwest Passage begins to open year round and increasing access offers new chances for development. Nations, including the United States, are clamoring to exploit oil and gas resources, rich fish supplies and tourist and commercial vessel trade.

The pack ice is melting earlier and forming later. In the last three years, the sea ice's extent - the ocean area in which a defined minimum of sea ice can be found -- was at its lowest in the 30-year satellite record.

Plankton, crustaceans and fish, all food for wildlife, reproduce at the dynamic edge of the sea ice, where it floats over shallow near-shore waters. When that edge moves off the continental shelf into deep open ocean waters, the productivity drops off and the marine organisms that feed larger wildlife are out of reach, scientists say.

And that's a problem for the gray whales swimming today in the haven of Baja's waters.

The whale's favorite fatty marine crustacean, the amphipod, has declined in the Bering Sea feeding grounds over the last 30 years as currents in the North Pacific Ocean warmed and sea ice gradually melted and thinned. Whales with their babies are forced to swim through the Bering Strait and fan out in the Arctic Ocean searching for a substitute food supply. They're heading in greater numbers to the Chukchi and Beaufort seas in the Arctic Ocean and even congregating off Barrow, scientists say.

In recent years, say scientists at the National Oceanic and Atmospheric Administration, these baleen whales that typically sift out little crustaceans from the bottom are now eating mysid shrimp and even krill in ocean waters. They have to eat tons more of them to make up the lost mass of fatty amphipods. "It's like replacing steak with vegetables," said one researcher.

Whale experts are hopeful. The grays are opportunistic feeders, they say, which makes them better candidates to adapt to the changing climate. Their ancestors date back more than one million years and have already survived extreme geologic changes.

But adaptation takes time, and the whale's population has already dipped from roughly 26,000 to 17,000. Scientists link the swift transformation of the Bering Sea ecosystem to the decline, and don't know yet if eating a wider diet is enough to stabilize the population.

Putting around San Ignacio Lagoon in a seven-person skiff, biologist Steven L. Swartz greeted passing whales with "Hola, ballena." Swartz co-directs the Ecosystem Science Program at the lagoon with Jorge Urban Ramirez, a professor at the Universidad Autonoma de Baja California Sur.

Researchers census the lagoon's whales from February to April, building a photographic identification record. The lagoon is one of three Baja stopping places for the population. Others are Scammon's Lagoon and Magdalena Bay. On any given day, about 250 gray whales swam in the lagoon, "spy hopping" in the air to look at the small boats before approaching alongside or beneath. The daily count includes 20 pairs of mothers and nursing newborns only weeks old, which are the last to leave for the north in April.

"It's very important to see what's happening up there in the Arctic," said Swartz. "The gray whale's old prey fields are gone. I want to know where they can find food."

If they can't find enough food, they can't make the arduous journey south to Baja California, Swartz said. "They get skinny and die."

It's happened before. In El Nino year of 1997, scientists recorded the warmest water temperatures ever in the eastern Bering Sea. A small phytoplankton replaced the normal summer phytoplankton, profoundly affecting the rest of the food chain, according to NOAA scientists. Zooplankton couldn't eat the smaller phytoplankton, and their numbers declined. As a result, seabirds starved and salmon runs declined.

Hundreds of gray whales stranded dead along beaches from Mexico to Alaska on the northern migration in 1999 and 2000. Whale scientists counted 200 dead in Mexico and observed that one in 10 looked emaciated. One theory is that the whales were in poor condition from the poor food supply when they came down to Baja to mate and give birth. Then, they just couldn't make the trip back to the Arctic food supply.

Other aspects of the Arctic - sea-ice extent, temperature fluxes - are well studied. Now several leading Arctic institutes, including the University of Alaska at Fairbanks, Woods Hole Oceanographic Institution, University of Maryland and other academies in the eight Arctic nations, have been asking that biological research be added to the long-conducted physical research.

One suggestion is the creation of a collaborative network, called the Distributed Biological Observatory. Research groups from the eight nations in the Arctic Council would set up environmental sampling stations throughout the Arctic and sub-Arctic. Whenever a research vessel was near a station, it would stop and gather biological information. That way, the gaps in seasonal information could be filled. Passing vessels could record such characteristics as abundance and distribution of phytoplankton all the way up to fish, marine mammals and seabirds.

"That would give us a time-series and would allow us to better interpret what's changing and how fast it's changing," said Doug DeMaster, research science director for NOAA Fisheries in Juneau. "This is a fragile ecosystem that needs to be protected but is also going to be used."

Environmental groups and scientists have been pressing for more research before industrial development forges ahead. "The trouble is we don't yet know how to develop in ways that are very, very safe," said Melanie Smith, a biologist with the National Audubon Society in Anchorage. Her group, along with Oceana, has released this week an Arctic atlas of the Chukchi and Beaufort seas.

"I do know that the Exxon Valdez spill was very difficult to clean up in an area that was ice free. We don't yet know how to clean up oil spills in broken pack ice," said Smith.

"We need to understand where the key habitats are located, the places that are essential to ecosystems. If they were severely impacted, there would be a ripple effect," she said.

The U.S. Interior Department has sold leases for offshore oil and gas development in the Arctic Ocean throughout the Bush administration. Some of the leases are under court challenge, and Interior is expected to make an announcement on their fate this month. However, exploration could begin this summer in both the Chukchi and Beaufort seas.

Last year 400 scientists sent a letter to President Barack Obama asking for more study before industrial activity goes ahead, noting that the "Arctic Ocean is one of the least-understood regions on Earth."

In October, Congress approved a conference report attached to the U.S. Department of Interior's appropriations bill that cited expanded leasing in the Arctic Ocean, including seismic testing and exploratory drilling, and called for independent scientific analysis before the agency proceeds. Congress noted that Arctic waters host "a rich diversity of wildlife and fish resources and are critical to the survival of the subsistence culture of the Inupiat people of Arctic Alaska." The independent analysis, it said, should assess the health, biodiversity and functioning of Arctic ecosystems, including the impacts of industrial activities and of climate change.

In response, the U.S. Geological Survey began a study on changing Arctic ecosystems to better understand the consequences of lost permafrost and sea ice habitats, and the Interior Department established a Climate Science Center at the University of Alaska to specifically address Arctic issues.

Back in San Ignacio Lagoon, most of the single females and males have left by now for the long journey north. The mothers and babies remain, packing pounds on the young, preparing for the uncertainty that lies ahead.

Jane Kay is a San Francisco environmental writer. is a nonprofit news service that covers climate change.