Swimming With Giants

August 16, 2004

Release from: Thomas K. Grose
U.S. News and World Report

OFF THE COAST OF PLYMOUTH, ENGLAND--When hunting for big fish, small clues matter. On a sunny yet chilly day here recently, researcher David Sims is searching for basking sharks, marine beasts that can grow to 13 yards and weigh up to 5 tons. But as his boat bobs restlessly in the English Channel, Sims is also looking for crustaceans just a couple of millimeters long. Alas, the only signs of life in his shot-glass-size sample of water are microscopic orange dots, some of which may be zooplankton known as calanoid copepods. It's not a good sign. No calanoids means no basking sharks, because calanoids--obviously lots of them--are the staple of this fish's diet.

Sims is no recreational shark hunter. He is a leader of a British government effort to place electronic tags on basking sharks to help track their movements and behavior. The Basking Shark Project is part of a rapidly developing area of science: the tagging of pelagic marine life, those often huge animals, like whales, tuna, and sea turtles, that spend most or all of their lives in the open sea.

Big as they are, little is known about these creatures because they live in a murky world where humans cannot go. But advances in microelectronics and telemetry have given biologists exciting new tools with which to glimpse these behemoths in their habitats. Enthuses Stanford University researcher Barbara Block: "Now we're really getting a window into their world."

A key rationale behind tagging, says Sims's codirector, Julian Metcalf, is learning "why animals are where they are, and not somewhere else," or understanding how a deep ecosystem works, then predicting how animals would behave if that environment changed, say, from global warming. Like land-based watering holes, there seem to be oceanic "hot spots" that lure a variety of species. But how are they formed? And how do the animals find them?

Sims isn't surprised by the absence of calanoids. Most years in early summer, these waters are rich in zooplankton, and it's common to find 50 to 60 sharks in his study area each year. But this season, it's clear that the plankton, which calanoids feed on, bloomed in greater abundance elsewhere; indeed, there were recent reports of perhaps 100 basking sharks off the coast of Ireland. So, evidently, basking sharks don't return to feeding grounds purely out of fidelity or instinct; they know where to find their calanoids.

But how? Biologists have been tagging marine life since the 1960s. Earlier technologies were mostly inefficient, but now, sensors can measure such things as water and body temperature, depth, and ambient sunlight. Tiny memory chips store the data. The first electronic tags were implanted in fish. But these tags have to be retrieved, and although the retrieval rate is not bad--for bluefins, as high as 30 percent--thousands of fish need to be tagged to produce enough data.

Enter the pop-up satellite tag. This newer device doesn't need to be culled; instead, it releases itself from the animal after a set time, floats to the surface, and sends data via a geopositioning system. The retrieval rate for pop-ups is as high as 90 percent. But glitches keep scientists from getting more than about 80 percent of expected data. In "dead zones" in the Mediterranean Sea, for instance, tag signals don't reach satellites.

Still, from very few measurements scientists learn a lot about pelagic life. Some tags show not only where an animal has been but also when it ate and, at least once, when it mated. Ambient sunlight measurements can fix longitude. And latitude can be derived from light and surface temperature readings. In the case of bluefin tuna, which have a body temperature warmer than surrounding waters, researchers can tell when a fish ate. Because the sardine the tuna munched is colder than the tuna, there's an ice cream effect: a dip in the tuna's body temperature that's followed by warming as it digests the meal.

Human ignorance of pelagic life remains immense, but the knowledge base is growing. Consider the bluefin tuna. For fishing, the Atlantic's population has been managed in east-west sectors, along an imaginary line. Scientists long suspected that boundary was arbitrary, and tagging proved them correct. "The fish don't turn around when they hit that line," says University of New Hampshire researcher Molly Lutcavage. "Bluefins are regularly crisscrossing the Atlantic. There may be one vast Atlantic stock."

Far-flung. Tagging has also revealed that bluefins are capable of traveling incredible distances. One peripatetic Pacific bluefin, tagged over four years, swam from the waters off Baja California to Japan and back again, logging around 18,000 nautical miles.

Another long-distance swimmer is the Atlantic leatherback turtle. From nesting beaches in French Guiana and Suriname, sea turtles have been tracked to such far-flung locales as the west coast of Africa and north beyond the Azores. At the same time, scientists are discovering that because the Atlantic turtles don't stick to narrow migration routes--instead dispersing widely over the Atlantic--it will be difficult to set aside no-fishing zones to protect them.

Emerging technologies should help reveal even more about deep-sea life. Magnetic sensors may soon tell scientists not only when marine creatures are eating but what they're eating, by measuring how wide they open their jaws. And Block, who is with the Tagging of Pacific Pelagics project, an international effort to study migration in the northern Pacific, recently helped place the first "real time" tags on the dorsal fins of salmon and blue and mako sharks. Sharks bask to help regulate body temperature, so their tagged fins are often above water, capable of instantly transmitting their location to computer screens.

Research "volunteers." Oceanographers, who are trying to learn how the seas relate to the atmosphere and weather patterns, may bring tagging needed money and support. Tuna and mammals are excellent collectors of the data oceanographers need to help construct more-accurate profiles of the water, such as temperature, salinity, and chlorophyll levels. Block calls animal data collection the new paradigm in ocean research. "It will help our animals earn their keep."

Sims, meanwhile, after spending nine hours aboard his boat without spotting a single shark, is unperturbed. "You have to balance days like today with our real successes," he says, removing tags from the harpoons he had readied "just in case." Indeed, he has collected so much data already that he'll be plenty busy back at the lab. "Besides," Sims says, "if this were easy, everyone would be doing it."