“THIS IS HARDER than it should be,” Barbara Block muttered as she leaned over the gasping fish. Blood dripped from her left hand where the tuna’s teeth had slashed her. Though covered from neck to toe in a neoprene wet suit, she shivered.

Block, a 44-year-old marine biologist, was in a skiff in Monterey Bay, California, offshore the facility she codirects, the Tuna Research and Conservation Center. Apearly light shone through a lifting layer of fog. In a sling in the center of the boat, a pair of four-foot-long bluefin tuna lay side by side. “Keep the hose on that one,” she told an associate, and he trained a stream of aerated water into its mouth and thus through its gills.

She picked up a tool that resembles a spear. Attached to one end was an electronic device containing a sensor and a transmitter. It looks like a wireless microphone and can beam a signal to an orbiting satellite, conveying the transmitter’s whereabouts on the globe. Block bent over the animal and stuck the “satellite tag” into the fish just below the dorsal fin. The researchers lifted a sling holding the fish and then lowered it over the side of the boat and into the water. “One, two, three!” they chanted, and pushed the tuna free. For a minute the tuna appeared to swim woozily near the surface, the tag bobbing above its back like a mechanical pilot fish, before disappearing into the depths.

In the coming months, the device would make a daily record of the tuna’s travels, then, after roughly six months, detach itself from the animal, float to the surface and upload the data to a satellite. One of the fish that Block and her coworkers tagged that November day swam some 1,300 miles over the next several months, at depths of up to 1,000 feet, and was last detected in March near Magdalena Bay, Mexico, about 200 miles north of Cabo San Lucas.

The animal played a bit part in an unprecedented global biological research project known as the Census of Marine Life, an estimated $1 billion, ten-year effort by scientists in at least 40 nations to identify, characterize and track the movements of scores of life-forms. Those include a wide range of animals other than bluefin tuna, from whales and turtles to sea-bottom bloodworms and albatrosses. Some researchers will scuba dive. Others will send robots to the ocean floor. And still others will soar above the sea’s surface in aircraft equipped with laser-based radar devices that can penetrate the ocean and monitor, among other variables, the locations of schools of fish. Researchers also plan to study historical records to gain a view of ocean life in the past.

Assessing the status of marine life around the world has gained urgency in this era of coastal pollution and alarming overfishing. Policymakers who set ocean-fishing quotas and pollution limits are working with outmoded or incomplete data, the census researchers say. They argue that the project is needed because the oceans, which make up 90 percent of the Earth’s biosphere, “are largely unexplored, and the life in them largely undescribed.”

Block’s study of Pacific bluefin is only part of the larger census project, but it highlights how the researchers hope to gain a detailed picture of not only where the creatures go but also the watery world they inhabit. “Let’s take the animals we know that can carry the electronics and get them to sample the oceans for us,” Block says. “Let’s use the organism as a way to get a window into the world it lives in—to get the organism’s-eye view.”

After releasing the tagged fish, Block and her associates motored back to the tuna research center, where a group of graduate students was matching wits with two dozen bluefin swimming around a 40-foot-diameter tank. The center is world famous for its studies of captive bluefin tuna, most of which are caught off the Southern California coast and trucked to the facility. Tuna can survive in the tanks for several years.The water in the tanks turns over every 70 minutes, and its temperature, acidity and oxygen content are carefully regulated.

The water in the tank had been drawn down to hip level, and the graduate students, wearing wet suits, maneuvered a vinyl mesh corral to capture the circling fish. Once the students caught a fish, they guided it into a sling, which was hoisted out of the water, transferred to a forklift and rushed to the dock. Block trotted alongside the animal like an emergency room physician tending to a patient on a gurney. She jumped into the skiff, and the crew raced out to the bay to release the fish.

Block is among the world’s leading experts on warm-blooded ocean fish— tuna, mackerel sharks and billfishes such as marlin. She has authored or coauthored 60 studies on their physiology, behavior, genetics and ecology and has pioneered methods for keeping tuna in captivity. In 1996, she received a MacArthur Foundation grant, and she poured most of the $250,000 socalled genius award into the research center at Monterey Bay.

Tuna are among the most economically important creatures in the sea. Worldwide, the tuna fishing and processing industry amounts to some $3 billion annually. In the United States, people consume more than 900 million pounds of canned tuna a year. It troubles experts that so little is known about the behavior of tuna, never mind less commercially valuable creatures. “We can go to Mars and the Moon,” Block says, “but we haven’t had a way to see where the ocean animals go.”

Tuna draw heat from the action of their own muscles. That gives the fish more power, according to Block’s laboratory experiments, but it also means they need lots of oxygen: water must flow ceaselessly through their gills. If they don’t swim, they die. In the wild, a bluefin grows rapidly. If it lives long enough, it might top 1,000 pounds. Yet it’s a marvel of hydrodynamics, says Randy Kochevar, a biologist at the Monterey tuna center and a colleague of Block’s. Abluefin can sprint 40 miles an hour and cruise 150 miles a day. The giant Atlantic bluefin, which Block has studied even more extensively than the Pacific variety, may survive 25 years or longer and reach 1,500 pounds.

The Pacific bluefin is one of the world’s most prized fishes. Sushi lovers pay a premium for its fatty meat. At Tokyo auctions, dealers routinely spend $10,000 for a fish. Though bluefin are caught with hook and line, in commercial operations they are typically captured with a purse seine, an enormous net that encircles a school and entraps the fish when cinched at the bottom. A large school in the eastern Pacific can consist of 2,000 fish.

Unlike other tuna species, the Pacific bluefin appears to be holding its own in the face of fishing pressure, although population data from the Pacific, which Block calls “that big blank slate,” are limited. As Chuck Farwell, codirec-tor of the Monterey tuna center, remarked, “If there were a problem, we wouldn’t know it.”

In the Atlantic Ocean, the problem is worse, but the data, thanks largely to Block and coworkers, are better. Each winter since 1996 she has chartered fishing boats to survey tuna off the North Carolina coast. She follows the animals with satellite tags or another type of sensor, known to the scientists as an archival tag, which she sews into the abdomen of a large tuna. It, too, records the fish’s movements and the ocean temperatures and depths, but furnishes researchers with more data—provided that whoever catches the fish is willing to return the device to the researchers for decoding. (It’s plastered with a return address and the promise of a $1,000 reward.)

Sitting in her office at the tuna research center, a joint venture between Stanford University and the Monterey Bay Aquarium, Block, a Stanford professor of biological sciences, pushed an archival tag across her desk. It looks like a metal cigarette lighter with a short probe and is surgically implanted in a fish’s belly. The probe pierces the skin and measures the water temperature and pressure every two minutes. The pressure indicates the depth at which the tuna is swimming. A clock and a light-sensitive diode establish the time of the setting and rising sun, which can be converted to latitude and longitude. Thus, the scientists can reckon a tuna’s position on the globe once a day within 60 miles.

Block took out a map showing the distribution of plankton in the North Pacific. Plankton, a general term for tiny drifting plant and animal life, are at or near the bottom of the food chain, and all sorts of marine creatures flourish where plankton abound. “In the center of all oceans are empty gyres— ‘deserts’ with low chlorophyll and plankton—across which the animal migrates to get to ‘hot’ zones off the continents,” she said. Color-coded, the hot zones were strips of reddish yellow and the desert a yawning blue. “If you’re a tuna, why do you risk your life crossing the desert? You wake up in the Coral Sea, and you swim to the other side. That’s a journey of 6,000 kilometers [3,720 miles]. Why? We think they somehow know the food is better that particular year in the zone of rich upwelling off Baja California, for instance.” She glanced at the ocean outside her window. “We can’t tell you where the most lucrative animal in the sea feeds or breeds,” she said, shaking her head.

The idea for the census of marine life originated with Jesse Ausubel, a program officer at the Alfred P. Sloan Foundation. Based in New York City, the foundation sponsors research and helps launch new scientific programs. In 1997, following a National Academy of Sciences report calling for increasing research on marine biodiversity, the foundation proposed a sweeping tally to be called the Census of the Fishes. But counting even a fraction of the fish in the sea, scientists hastened to point out in several brainstorming sessions, would be impossible. After all, scientists estimate there are 20,000 kinds of marine fish—including perhaps 5,000 yet to be discovered.

Sloan Foundation advisers, led by Frederick Grassle of Rutgers University, scaled back the plan, deciding to hunt for some new species and assess others. The project shifted focus again after biologists studying marine mammals and other non-fish creatures said they wanted in on the action too. So it was renamed the Census of Marine Life. All told, the enterprise will consist of 30 to 40 separate field studies, at $5 million to $25 million apiece, with the money coming largely from the U.S. and foreign governments.

The census’s senior scientist, Ronald O’Dor, a marine biologist at Dalhousie University in Halifax, Nova Scotia, who is now based at the Consortium for Oceanographic Research and Education in Washington, D.C., says the virtue of the approach is that the animals lead the way. “They cover more area than an expensive, traditional research cruise can,” he says. “You let the animals choose your sites.”

The study that Block and three other scientists are heading is called Tagging of Pacific Pelagics (TOPP), and the plan is to monitor a dozen animal species, including tuna, Humboldt squid, great white sharks and elephant seals. Some tagged creatures are likely to cross paths, the researchers say, possibly providing new insights into how the animals interact in the wild. One of the scientists, Daniel Costa of the University of California at Santa Cruz, mounts electronic sensors and transmitters onto elephant seals, which weigh up to 7,000 pounds, when they come ashore to breed. He says he looks forward to learning whether the seals’ travels overlap with tuna migration patterns. If so, that might hint at common feeding strategies that the two species were previously not known to share.

Though the marine census project doesn’t officially start until later this year, pilot studies have been under way for a couple of years. Besides Block’s tuna-tagging project in Monterey, another pilot study involves tagging squid in the Sea of Cortés. William Gilly, of Stanford’s Marine Station, has worked with fishermen in Mexico to see how willingly they would return a yellow plastic ring attached to squid they caught—a dry run before attaching electronic sensors to the animals. First, Gilly went out with the fishermen at night and, using handheld lines, caught squid, some weighing 50 pounds or more. The scientists put the plastic rings on nearly 1,000 animals and released them. Signs posted near fishing docks offered a $50 reward for each tag returned. Over a three-month period, about 80 tags were recovered. That pleased the scientists, who also were glad to learn where the squid were caught, hinting at winter migrations of squid in the Sea of Cortés.

“The census is an example of an old fashioned mentality in science,” Gilly says, meaning it addresses a basic curiosity. “We’re reopening the blinders, the way we used to look at the world, through wide exploration.” Block and others engaged in the census may shed light on the condition of some of the world’s fisheries, which are under great strain. People exploit just a few hundred fish species for food, and populations of those species have fallen precipitously, some as much as 90 percent. Some environmentalists and marine experts call the situation a crisis, but even people who dispute that view do not doubt that fishermen and fishing nations are working harder than ever for smaller catches.

Whether the fishing industry will make the best use of data collected by the sea searchers is open to question. Recent studies by Block and colleagues in the Atlantic have found that far more tuna than previously believed cross the ocean. The finding overturns a basic assumption held by not only biologists but also international policymakers, who set different fishing quotas for the eastern and western Atlantic. But it now appears that alleged overfishing in the eastern Atlantic and Mediterranean has reduced catches in the western Atlantic and North America, say some researchers, who are calling for lower fishing limits in the Atlantic. Block says she hopes that fisheries managers take steps to “make the changes necessary to ensure the future of the species.”

At the end of a long day for Block in Monterey, she was still wearing a wet suit. She climbed down into a tank at the tuna center with her graduate students and resumed showing them how to affix satellite sensors to a couple of the corralled fish. Even when she got out of the tank, Block continued coaching from a walkway above it. “Keep the fish under water,” she called, pacing back and forth. Below her, the captured bluefin were swimming round and round, never stopping, as relentless as the researcher herself.