25 MAY 2006
A new study shows that loggerhead sea turtles in the Eastern Atlantic forage in two distinct habitats, so if you want to conserve these holdovers from dinosaur-dom, you'll need to conserve two distinct areas: one on the high seas, and another near the African coast.
The first area is fished by millions of hooks on long fishing lines set for tuna and swordfish. The second is occupied by hungry people in dirt-poor nations like Mauritania and Senegal.
The new research was based on satellite tags glued to 10 loggerheads after they laid eggs on Cape Verde, an island nation off the northwest coast of Africa. As the turtles swam through the ocean for two years, they segregated into two locations. The larger turtles moved toward the shallow, coastal water, and the smaller ones stayed in the deep water east of Cape Verde, where they ranged across half-a-million square kilometers.
This movement pattern was radically different from that on the other side of the Atlantic, said Brendan Godley, of the Marine Turtle Research Group at the University of Exeter (United Kingdom). Research starting in the 1960s showed that after hatching, loggerheads "enter the Gulf Stream and spend years in the open oceans. When they get to a certain size, they move into near-shore waters and migrate along the coast in winter and summer."
That pattern became the accepted wisdom on loggerheads. "Because there are so many loggerheads, and so much interest in the U.S., the model everybody used is based on knowledge from Florida, Georgia and the Carolinas," Godley said. When the Cape Verde mothers returned to the sea, "We expected them to swim away from Cape Verde toward West Africa, and do the analogous thing. But lo and behold, seven out of 10 stayed out in the pelagic [deep water] completely."
The logic of a turtle
We wanted to know why the turtles demonstrated such an un-American pattern, but Godley says the answer is unknown. "What is cause and what is effect, we are not sure. You could say that because they are small, they stay pelagic, because smaller individuals can survive in the pelagic. Or you could say because they stayed in the pelagic zone, they did not grow so fast."
Overall, the study provides one more data point in the growing picture of how turtles migrate to survive. "It's possible that they could be using an intermittent strategy, where they breed and live in the pelagic ocean, and then get big enough to live in [coastal zone]," says Godley. "There could be a switching point, or living a completely pelagic lifestyle could be another completely valid strategy" for survival.
One thing that is clear is that the coastal waters of West Africa are a zone of huge productivity, which can be expected to feed any number of marine organisms -- even those saddled with satellite trackers.
Photo courtesy Luis-Felipe Lopez, University of Las Palmas Aegenia.
Loggerhead turtles are a remnant of the age of the dinosaurs, but an estimated 200,000 were caught in long-line fishing gear in 2000, and tens of thousands died. The new information is critical for conservation, Godley says. In contrast to the lifestyle found in the West Atlantic, "It appears that the minority of loggerheads are doing the classic adult behavior. The vast majority are in the pelagic zone, doing what we would have called 'juvenile behavior.'"
In the United States, sea turtle conservation relies on the turtle excluder device (TED), a trapdoor in a net that serves as an escape hatch for trapped turtles. Although initially hated by the fishing industry, these gadgets do work. But they apparently won't help on the other side of the Atlantic. "We used to think, based on the North American scenario, that what was caught in longlines was small juveniles," Godley says. In coastal waters, he says, "as long as the TEDs worked, they would be relatively protected, because the big threat was in the pelagic."
The study's goal was to provide information for conserving the loggerheads, but that could be difficult. Michael Coyne, a turtle expert at Duke University who also worked on the loggerhead study, said, "Given the range these reptiles can cover, an international cooperative effort in seven African states is needed to create a strategy that would protect them."
Photo Courtesy Luis-Felipe Lopez, University of Las Palmas Aegenia.
These suggestions could help the turtles survive:
Marine reserves: Dedicating parts of the ocean to reproduction has helped sustain fish productivity elsewhere.
Fishing hooks: Changing the design so they catch fish, but fewer turtles.
Fishing technology: Place the hooks at depths where the turtles don't forage. Or dye bait a color that is less attractive to turtles.
While the sample size in the current study -- 10 animals -- is small, Godley says a recent National Oceanic and Atmospheric Administration grant will fund 25 tags for loggerheads, so a wave of new data should be breaking soon.
-- David Tenenbaum
• Phenotypically Linked Dichotomy in Sea Turtle Foraging Requires Multiple Conservation Approaches, Lucy A. Hawkes, et al., Current Biology 16, 1-6, May 23, 2006.
• Home page of Cape Verde project at Seaturtle.org. Accept their terms, then view their maps.