Beyond the Beach
by Wendee Holtcamp
| Were Archie Carr here today, he would be pleased. Unraveling the mysterious oceanic lives of sea turtles--their diving behavior, their migrations, their "lost year" after hatching--was his lifelong dream. Carr was the pioneer of sea turtle biology, and undeniably the best friend sea turtles ever had. He was a scientist in love with his work, and it showed in his writing and his ground-breaking research. In his last days, he asked those following in his footsteps to look more to the sea than to the shore, to learn more about where the turtles spend 99 percent of their lives. In the ten years since his death, rapid advances in genetics and telemetry have vastly enhanced our understanding of sea turtles' oceanic existence. And two southern California-based husband-and-wife scientist teams lead the way. |
We look for turtle heads bobbing to the surface for a breath in the murky water, but so far we have no luck. The water near the effluent channel stays 20 degrees F warmer than the adjacent ocean, offering a year-round haven for around 50 turtles. The bubbling effect results from water being forced from the pipe at a rate faster than the relatively still ocean, and likely improves aeration of the water in the inlet. "The power plant is providing this environment that really seems to be good for the turtles," says Donna. "Their growth rates here are similar to Caribbean growth rates, which is very unusual, because the water off San Diego is so cold."
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Donna directs Ocean Planet Research Institute, a non-profit organization which she and Peter founded, and which the U.S. Fish and Wildlife Service contracted to study the San Diego Bay sea turtles. She is soft-spoken and friendly, thin and fair, and is seven months pregnant with her first child. Peter, a geneticist at the National Marine Fisheries Service (NMFS) in nearby La Jolla, has begun cataloging the genetic makeup of all the world's populations of nesting sea turtles--a gargantuan task. From this database, he hopes to pinpoint the nesting beaches from which the San Diego Bay turtles originate.
Since 1989, the Duttons have used a combination of genetics, telemetry, and traditional biology to study the Bay turtles. This is the only known spot on the west coast where sea turtles gather, at a power plant of all places. The turtles' presence here has led to a U.S. Fish and Wildlife Service proposal that South San Diego Bay be designated a National Wildlife Refuge. But what will happen, the Duttons muse, if the plant should close down?
The turtles were first seen year-round in the effluent channel soon after SDG&E began operation in the early 1960s. Ten years later, a San Diego State graduate student, Margie Stinson, documented them for her master's thesis. At least one turtle that Stinson flipper-tagged in the 1970s is still present in the Bay. "This turtle, as far as we know, has never left," says Donna. "And she's big, she's huge! When Margie weighed her back in 1980, she weighed almost 400 pounds." Since then, no one's been able to lift her out of the water to weigh her.
| Rapid advances in genetics and telemetry have vastly enhanced our understanding of sea turtles' oceanic existence. |
In 1992, the Duttons attached ultrasonic transmitters to seven of the turtles to follow their movements. They learned that the turtles feed throughout the Bay on eelgrass, staying close to the warm effluent channel during cold winters and meandering further away during summers. But as far as they can tell, none have left the Bay to nest, perhaps because many are still juveniles. The nearest known nesting beach is in Baja California, but there is some concern that the turtles may never want to leave.
Peter's genetic detective work suggests the turtles originate from the Mexican Pacific, though specific nesting beaches are as yet unknown. Meanwhile, new untagged young and adult turtles keep showing up.
How turtles discover the effluent channel is something of a mystery. "One of the theories," says Donna, "is that fishermen off the coast of Mexico would catch hatchlings in their nets, just incidentally with the fish, and when they brought the whole haul back to San Diego, they would just dump everything overboard that they didn't want." A simpler explanation is that turtles discover the warm-water inlet by happenstance.
"It's not unusual to have turtles migrating up the coast of California," says Donna. In fact, green sea turtles roam as far north as Oregon during summer months, but if they get caught in cold water they can get confused, which could impair their navigation. "If they get stuck here in the winter," she says, "then they're in trouble--unless they find a little pocket like this."
But finding another little pocket like this is not likely. "It's the only place I know of in the world where turtles have a warm water refugia that they can go to where they can control their body temperature," says renowned sea turtle biologist Scott Eckert of Hubbs-Sea World Research Institute in San Diego. Scott and his wife, Karen, who is executive director of the Wider Caribbean Sea Turtle Conservation Network, were encouraged by Jim Richardson, a student of Archie Carr's, to study turtles. Says Scott, "I dropped chemistry like a hot potato."
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Scott specializes in leatherback turtle (Dermochelys coriacea) biology, using cutting-edge technology to monitor their oceanic movements. The Eckerts have worked with the Mexican Pacific leatherback population for more than ten years. They have witnessed firsthand a dramatic downward spiral. Since 1986, leatherback nests have declined from 6,500 to 100 at Mexiquillo, Mexico's primary leatherback nesting beach, and the entire coastal nesting population has declined from 70,000 to less than 2,500. "The bottom line," says Scott, "is that the numbers are abysmal."
Though sea turtles spend only one percent of their lives on land and the rest at sea, perhaps 90 percent of what we know about them has been learned from their relatively easily studied on-land nesting and hatching behavior. Sea turtle hatchlings incubate within pliable eggs that their mother carefully laid within a sandy, flask-shaped nest. When ready, the hatchlings erupt from the sand and scamper toward the sea. The males will never come ashore again, but the females, once they reach sexual maturity two to four decades later, return to shore to complete the cycle and bury their shelled progeny beneath the sand.
Where hatchlings go once they scuttle out to sea greatly puzzled Archie Carr. Some remain close to their nesting beaches for a few days, but the next time anyone has reported seeing them, they are already dinner-plate-sized, and estimated to be two to five years old. They probably spend much of this time "hanging out" in an open ocean sargassum bed.
Scientists have gained remarkable insight into young hatchlings' lives by studying their behavior in captivity. Roger Mellgren and Martha Mann from the University of Texas at Arlington compared the behavior of green, loggerhead, and hawksbill sea turtle hatchlings within large tanks. Green turtles continuously swam back and forth across the open water, avoiding artificial weed beds. The loggerhead and hawksbill hatchlings congregated within the weeds, and would remain immobile for long periods of time. If scientists could follow the hatchlings' oceanic movements over their first few years, they could determine if similar findings hold in the wild. Unfortunately, no one has yet devised a tracking device that will stay attached to the tiny, fast-growing hatchlings.
Tracking adult sea turtles across the vast ocean has proved more fruitful. At the time of Carr's death in 1987, telemetry was still in its infancy. Today, this technique allows scientists to follow an animal's movements by means of sonic, VHF, or satellite transmissions. A device is attached to the animal which relays its location and other data back to the scientists. While sonic and radio telemetry require the scientist be within a few miles of the animal to receive the signal, satellite telemetry devices can relay information from anywhere on the globe, which works much better for far-ranging and elusive sea turtles.
Scientists can also track how deep the turtles dive, how long they stay underwater, their swimming speed, their heart rates, and how long they're spending at any particular depth. In recent leatherback-tracking experiments, using swim-speed recorders, time-depth recorders (TDR), and location devices simultaneously, Scott discovered something entirely new about their migration behavior.
Previously, he had assumed that when leatherbacks were near the ocean's surface for extended periods of time, they were basking (thermoregulating) in the sun. It wasn't until he combined TDR data, which conveys how long the turtle stays at a particular depth, with swim speed, that he learned that they are actually moving, swimming, at shallow depths, to a specific destination. Then it all made sense, says Scott. "If you look at the classic literature on how [aquatic] animals move through their environment, you find that the [most energy-efficient] place to swim is 1.5 times your body diameter below the surface...and that's exactly what these animals are doing," he says.
Leatherbacks are notoriously difficult to track by satellite because of their rubbery shell, which resists the glue used to attach transmitters. Scott developed a leatherback harness that secures the space-age equipment to the turtles without impairing the animal, affecting their natural behavior, or attracting predators. Instead of gluing the satellite-pack directly to the shell, the harness fits over the leatherback like a belt and suspenders.
As Scott points out, if you're a behavioral scientist, and you're fooling with the animals' behavior, you're not getting good data. "But it's also a moral issue," he says. "Endangered species or no, as a scientist...it behooves you to get that information in a way that keeps your ethical balance in check." The harnesses Scott designed fall off the turtle naturally within three or four years.
Right now, Scott has seven of the Mexican Pacific leatherbacks harnessed with satellite-packs, and for the first time he is watching their long-distance migrations. The early results are surprising. All seven leatherbacks are heading south across the equator. "Most large vertebrates don't migrate across the equator as it represents sort of a desert zone for a lot of these temperate animals," says Scott. "I think they're going to circumnavigate the entire Pacific Ocean!"
Leatherbacks, like other sea turtles, have an uncanny ability to find their way in a seemingly unchanging ocean. According to a recently published Cornell University study, leatherbacks may actually use corridors within the Pacific to find their way. Stephen Morreale and colleagues tracked the oceanic migrations of two different leatherbacks each year, from 1992 through 1995. All eight leatherbacks followed a nearly identical path, leaving their Costa Rican nesting beach and heading straight toward the Galapagos Islands, and beyond. Because leatherbacks feed on jellyfish, unlike other turtle species, they do not have specific foraging grounds.
At one time, Morreale thought the most likely direction indicator was the Cocos Ridge which runs from Costa Rica to the Galapagos. But how would the turtles locate a ridge over a half-mile down when their dives were mainly in the top 1,000 feet of the water column? Moreover, the turtles continued several hundred miles beyond where the ridge ends.
What fascinates Scott about Morreale's finding is that the turtles really seem to know where they are at all times. "These animals know where they want to be and how to get there," he says. "To us, it's a homogeneous environment that has no street signs, yet they're going just like they have regular highway lines."
When it comes to deciphering the clues sea turtles use to navigate the high seas, progress is being made slowly. In 1985, scientists discovered a pocket of magnetite in sea turtles' brains, which led University of North Carolina marine biologist Kenneth Lohmann to study the possibility of a geomagnetic sense in sea turtles. In Lohmann's experiments, he placed loggerhead hatchlings in a large circular tank, and shined a dim light from the east, simulating an east-facing beach, then put out the lights. The hatchlings all continued to swim east, as expected. Then, Lohmann changed the direction of "magnetic east" to west using a magnetic coil system that surrounded the tank. The turtles began swimming west, indicating they sensed the change in the magnetic pole.
Olfaction also appears to play a role in helping the turtles find their way. Most sea turtles, like salmon, are thought to return to their natal beaches when they nest. Scientists theorize that they use their keen sense of smell to home in on the beach when they return, as each beach, with its distinct mineral composition, has a slightly different odor.
Whether female sea turtles actually do lay their eggs at the beach where they hatched puzzled Archie Carr. The alternative theory is that first-time nesters simply follow other females who have nested in previous years (a behavior known as social facilitation). Today, the answer lies within Peter Dutton's grasp.
If females always return to their natal beach, says Peter, turtles from each separate nesting beach will have distinct genetic "signatures" which evolved over time. Peter is creating a massive database of turtle nesting beaches and their accompanying genetic signatures. His findings so far support the notion that adults do return to the beach where they were born, with leatherbacks not so hidebound as other species. Perhaps this flexibility has allowed leatherbacks, the most ancient of the sea turtle species, to survive as long as they have. The findings have important management and conservation implications for all sea turtle species. "If nesting populations are isolated, and if one gets wiped out," says Peter, "it won't be replaced by recruits from another population."
Until recently, keeping track of which turtles nested where was only possible by attaching numbered metal tags to the front flippers, but the tags come off within one to two years. So Donna and Peter pioneered two new identification methods for leatherback turtles: PIT (Passive Integrated Transponder) tags and photo-identification. "A basic piece of information one needs for conservation management is knowing the actual population size, and up to now we haven't even been able to get that accurately," says Peter.
Researchers have still not found a way to insert a tag into a hatchling. Although the PIT tags are only the size of a grain of rice, the newborn's shoulder muscle is only three times that. Not only would insertion damage the muscle but, as the turtle grew, the tag would soon be deeply embedded.
Leatherback individuals can be distinguished by a pink spot on the top of their heads, so Donna painstakingly sorted through nine years' worth of photos of leatherbacks nesting on the Caribbean island of St. Croix, until she was able to identify each individual. As they feared, their previous estimate of the nesting population was inflated, and had to be revised downward nearly 20 percent. Only 244 individuals are now thought to have nested on St. Croix from 1984 through 1995.
While photo-identification requires a researcher skilled in deciphering pink spot variations, PIT tags, on the other hand, work as easily as grocery-store scanners. Encoded with a unique id, scientists embed the tiny chip within the animal's muscle. Once in place, the turtle can be identified by waving the scanner toward the animal. "PIT tags are designed to last the life of the turtle," says Donna. So far they have been working very well.
Photo-id and PIT tagging work great for live turtles, but what about identifying turtles which have been killed? People send Peter bits of blood and skin from turtles that are badly mutilated by boat propellers, or taken by poachers. His genetic database allows him to find a turtle's origins from a tiny bit of flesh. In fact, if the turtle had previously nested on St. Croix, he can tell you exactly which individual it was from its genetic "fingerprint."
One of Peter's latest projects involves identifying the origins of sea turtles caught in the Pacific high-seas longline fisheries, an initiative of the nmfs Southwest Fisheries Science Center. So far it appears that the loggerheads being caught by the longline fisheries originate from Japan, and the leatherbacks from both East Pacific (Mexico/Costa Rica) and West Pacific (Malaysia/Irian Jaya/Solomon Islands) nesting beaches. "One country may be exploiting a resource at sea that another is working hard to protect on land," says Peter. Sorting out which countries' nesting stocks are being impacted by the high-seas fisheries is the first step in an ambitious program that will emphasize the need for international cooperation.
Archie Carr's scientific curiosity and love of sea turtles has spread beyond his life, and into the lives of the scientists who search for answers to questions Carr first posed. In the past few years, technology has given new vision to the Duttons and the Eckerts, along with many other sea turtle biologists throughout the world. We are finally developing an accurate picture of sea turtle numbers, grim though they may be. "This kind of technology is improving our ability to go back to being the Charles Darwin-type scientists that we all want to be." says Scott Eckert. "And that is the observer."
Wendee Holtcamp writes about ecology and adventure travel for Discovery Channel, E Magazine,
and WildBird from her home on Luce Bayou in Texas.
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