Into dangerous waters - sea turtles threatened

International Wildlife, March-April, 1996 by Bernice C. Wuethrich

Conservation efforts have protected many sea turtle nesting beaches, but new threats loom on the open ocean

On a June day in 1993, two biologists sailed a wooden fishing boat into shark-infested waters off Mexico's Baja Peninsula. One of the scientists was Brian Bowen, a geneticist at the University of Florida in Gainesville who has been using DNA to study sea turtle distribution. The other was Alberto Abreu of the Mexican National University's Institute of Marine Sciences in Mazatlan, Mexico. Both were in pursuit of sea turtles, and they were out for blood. Literally. They wanted samples from loggerhead sea turtles for use in DNA studies.

The waves through which the boat plowed were tinted crimson by hordes of crabs feasting on plankton. In their turn, the crabs were eaten by sea turtles. When Bowen spotted one satiated loggerhead basking on the surface of the sea, gulping air and too buoyant to dive quickly, Abreu jumped overboard and grabbed the animal from behind, where he could avoid its dangerous jaws. "These things are definitely snappers," Bowen said.

This turtle was one of several that the two men wrestled aboard during the course of the day. From each they withdrew 10 drops of blood before lowering the reptile back into the sea. Later, Bowen and Abreu took DNA samples from the blood in an attempt to determine the origin of the tens of thousands of juvenile loggerheads that feast off Baja each summer - a major biological mystery, since loggerheads do not have a single known nesting beach on the eastern Pacific.

Abreu and Bowen are part of a cadre of biologists who are seeking long-elusive answers to questions about sea turtle migration. But in their quest, they have discovered a threat to the reptiles on the high seas: longline fishing, which biologists now suspect is the primary danger to sea turtle survival. Longlining, along with other forms of commercial fishing such as trawling, kills thousands of sea turtles yearly.

The IUCN-The World Conservation Union has labeled all seven sea turtle species - the loggerhead, green, hawksbill, leatherback, Kemp's ridley, olive ridley and Australian flatback - as either endangered, threatened or vulnerable to extinction (some biologists would add an eighth to the list, the black sea turtle; others lump it with the green sea turtle). Until recently, most efforts to protect these jeopardized animals have focused on nesting beaches. The many successes of this approach, from Australia to Brazil to Florida, now are being undermined by the longliners. "The take at sea," warns Bowen, "is in no way sustainable."

Studying sea turtle behavior is among biology's most difficult challenges, primarily because the reptiles spend virtually their entire lives in the water. Females return to land only for the few hours it takes to dig nests, lay about 100 eggs per nest and bury them. The nest is left unguarded, and the young have to dig out when they hatch. They then scuttle to the sea and disappear in the waves. Most males never come ashore again.

The first extensive studies of sea turtle distribution began in the 1950s, when scientists started putting metal tags on turtles in great numbers. Tag recoveries helped biologists to gather the first good data on sea turtle movement. In the late 1980s, biologists turned to fitting large sea turtles with radio transmitters so they could track the animals by satellite for several months at a time.

Even more recently, Brian Bowen has pioneered a high-tech tagging method that identifies unique sequences of mitochondrial DNA (mtDNA), a type of genetic material that hatchlings receive only from their mothers. Because females usually return to nest on the beaches where they hatched, turtles from a given beach are often related. Consequently, a given mtDNA sequence can be characteristic of a particular beach, allowing scientists to determine any turtle's place of origin once the mtDNA of its nest beach has been sampled.

Bowen's study in Mexico led to an eye-opening discovery regarding the travels of Pacific loggerheads. To determine where Baja's juvenile turtles come from, Bowen and Abreu compared the molecular patterns of blood mtDNA from the Baja turtles with patterns from turtles that nest in Australia and Japan. The results indicate that about 95 percent of the Baja turtles originate on Japanese beaches, with the rest probably coming from Australia. The turtles start the 12,000-kilometer (7,500-mi.) journey as 5-centimeter (2-in.) hatchlings scuttling from eggshell to sea. They follow the Kuroshio and North Pacific currents, often feeding on and around seaweed rafts until they arrive in Mexican waters several years later. There, Bowen surmises, they remain for several more years before returning to nest in Japan.

The genetic study not only confirmed an amazing migration, but highlighted the threat posed by longlines. These fishing lines are truly long: Fishing vessels put out lines measuring up to 120 kilometers (75 mi.) and hung with thousands of hooks.