Trailing the Trunkback
The most exciting insights biologists have gained into the lives of leatherbacks have come from data-gathering devices that unsuspecting females have carried with them into the deep sea. “We’ve always been accused of studying animals in a maternity ward,” laughs Eckert, referring to working with females on and near nesting beaches. “Now we can step back and find out what they spend the other 99.9 percent of their lives doing.”
Some of the earliest clues into leatherback behavior at sea, for instance, came from microprocessor-controlled time-depth recorders (TDRs). Attached to the animals’ shells, these instruments, which biologists initially designed for studies of other great divers such as seals and penguins, record dive depth and duration, ascent and descent rates, and surface times. Eckert and his wife Karen, who is also a leading leatherback researcher, first shed light on the turtle’s diving skills using TDRs on nesters departing Sandy Point, St. Croix. The females, they found, dove almost continuously, day and night, averaging ten minutes a dive and five dives an hour. They also dove more deeply during the day, often to profound depths. One turtle swam down 3,330 feet before the TDR stopped recording. While that dive remains the deepest on record, Eckert thinks leatherbacks routinely dive far deeper. After analyzing hundreds of dives, the husband-and-wife team believes the turtles are following the so-called deep scattering layer, a horizontal zone rich in jellyfish that rise to the surface at night to feed on phytoplankton but retreat during the light of day to depths below 1,800 feet, where the illumination is but 1 percent of that at the surface.
Biotelemetry, a means to remotely detect and measure movements and other conditions of wildlife fitted with electronic telemetry devices, has also helped clarify leatherback behavior on the high seas. To learn more about what Playa Grande females do in the days or weeks between nesting events, Paladino and his colleagues fitted individuals with radio and sonic transmitters and trailed them in boats as they left the beach. The radio transmitters allowed the scientists to monitor the surface activity and location of turtles up to 10 miles away. The sonic transmitters, whose sound waves travel readily through water, enabled them to record dives to depths of 1,500 feet and more.
Through this work, Paladino and his colleagues discovered, among other findings, that females awaiting another chance to nest spend their time diving and feeding at relatively shallow depths within about 40 miles of Playa Grande. At the end of a day of study, the researchers simply dove into the water to retrieve their instruments when the turtles surfaced.
For turtles heading off on their long-distance migrations, the biologists needed equipment that could operate remotely. Satellite telemetry was the answer because tracking turtles this way, Eckert points out, means that “you don’t need to chase them around.” In June 1995, he outfitted three leatherbacks on a beach in Trinidad with one-watt satellite transmitters, each bearing an onboard microprocessor that recorded dive depths and durations. Each time a turtle surfaced, the transmitter looked for one or both of the Argos polar-orbiting weather satellites, each of which passes over the equator about four times a day. When a link was made, the transmitter sent a code identifying the turtle as well as a stream of data. The satellite, in turn, used a triangulation function to determine where on the planet the signal was coming from. If the turtle remained on the surface long enough, then the satellite got a solid fix on its location, often to within 500 feet, Eckert says. The satellite relayed the information to a ground-based receiving station, which forwarded the data by email once a day to Eckert’s office computer.
For the first time, scientists were able to get a detailed look at where leatherbacks go on their ocean-spanning migrations. One of Eckert’s transmitters failed after three months, but the other two each lasted for more than a year. (Eckert designed them to give out after about 40,000 transmissions, and for the harness that carried them to fall off the turtle after about a year.) In that time, the two turtles traveled more than 10,000 miles each. After leaving Trinidad, one headed north into subarctic waters, where it lingered for two months before heading south to the Canary Islands and onto to Africa. The other swam east, veering north about 1,000 miles west of Mauritania. It wound up in the Bay of Biscay off France, then turned south, eventually approaching close to the African coast.
In the meantime, Eckert has been trying to improve his telemetric techniques. He recently outfitted nine leather-backs on the Pacific coast of Mexico with improved transmitters designed to last for 250,000 transmissions over as long as 4 years. And he is now working on a transmitter that will use the Global Positioning System (GPS) to follow his turtles. “The beauty of a GPS location is that it’s very easy to get,” he says. Typically leatherbacks stay on the surface for only a few minutes, but the transmitters on his Trinidad subjects needed the turtles to be on the surface for at least five minutes to obtain a satellite fix. The GPS unit needs less than 20 seconds. Further, during that time Eckert will be able to communicate with the transmitter. “You can reprogram it, such as tell it to turn itself off for a month if you’re getting data from areas you don’t want,” he says.
While satellite-tracking eight leatherbacks on the Pacific side, Paladino and his colleagues made an interesting discovery. Turtles leaving Playa Grande can depart anywhere along an arc of about 160 degrees. Yet all eight turtles in the study headed southwest along the subterranean Cocos Ridge to the Galapagos Islands; four continued past the Galapagos into deeper Pacific waters. The biologists believe the turtles are migrating along distinct ocean corridors, which they conservatively estimate to be about 300 miles wide. At Playa Grande, Paladino has begun to investigate how the turtles sense such routes. Normally newborn leatherbacks exiting the beach invariably head straight out to sea. But when Paladino temporarily glued magnetized needles with about twice the strength of the planet’s magnetic field to the turtles’ heads, they began wandering aimlessly. “When you disrupt their ability to sense the earth’s magnetic field,” he says, “they seem to go in a random pattern.”
Eckert considers the jury still out on whether specific pathways exist; his three Trinidad turtles, for instance, headed in three different directions. If such corridors do exist, however, they could go a long way to help protect the species on the high seas, Paladino says, for authorities can know when and where to restrict the long-line fishing operations that drown thousands of leatherbacks each year. Indeed, the new knowledge about leatherback behavior that biologists are now collecting is giving them renewed hope that the species can be pulled back from the brink of extinction. “Using satellite telemetry and other technologies that allow us to understand leatherback movements and habitats,” says Eckert, “gives us the tools we need to respond, for instance, to fishermen who say We want to build a new fishery out here.’ We can say, Fine, but here’s what you need to avoid.’”