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Biology 103
Web Reports 1997
From Serendip

TEETERING ON THE BRINK: DECLINING SEA TURTLE POPULATIONS
Christian DuComb

Rancho Nuevo, Mexico, 1947: Film evidence shows approximately 40,000 female Kemp's Ridley Sea Turtles (Lepidochelys kempi) nesting on the beach in a single day. Nineteen years later, fewer than 1,300 turtles nest in the entire season. Since that time, only a few nesting aggregations have been reported in Rancho Nuevo, none of them containing more than 2,500 turtles. Current nesting levels are estimated at less than 1% of those for 1947.

These data, from the St. Catherine's Sea Turtle Conservation Program (1), are no less than alarming. I first took an interest in turtles in April of this year during a vacation the village of Bahia de Los Angeles in Baja California Norte, Mexico. Three friends an I stayed at a beach-side camp that doubles as a research and monitoring site for turtle populations in the Sea of Cortes. We helped to tag and weigh the dozen or so loggerhead turtles (caretta caretta) held in captivity there, periodically cleaning and refilling their tanks. The awesome size and beauty of these creatures inspired me to learn more about them. Although sea turtles lay their eggs on land, they are primarily aquatic creatures. The heavies of the eight known species of turtle can weigh up to 700 pounds, making them an excellent source of meat for human predators. Turtles were a dietary staple for the indigenous populations of both Baja California and the Cayman Islands (which Columbus originally dubbed Las Tortugas.) The life expectancy of turtles remains unknown, but females are believed to reach sexual maturity at twenty to thirty years of age (in other words, they live a long time.) They mate with males in the ocean and crawl onto sandy, temperate beaches to deposit an average clutch of 3.6 eggs. Most female loggerheads, however, do not nest every year. As a result, season fluctuations in the nesting population of loggerheads and several other species of turtles do not necessarily indicate an increase or decrease in the overall population. This makes sea turtles, and the severity of their endangerment, particularly difficult to study.

When a turtle comes to shore, she uses her swimming flippers as an awkward set of legs to crawl above the high tide line to a warm patch of sand. According to a 1981 study by Stoneburner and Richardson, turtles can sense the changing temperature of the sand surface. To deposit her eggs, a turtle will dig a hole roughly the size and shape of her body. She then excavates an urn-shaped egg chamber with her rear flippers and deposits her clutch of eggs, backfills the indentation left by her body (covering her eggs in the process), and returns to the ocean. Most species of turtle, particularly the loggerhead, concentrate their nests at certain sections of the beach. Scientists have not yet discovered the impetus behind this geographic clustering. One web site I visited (1) provided a data table on nest locations and a beach grid. It invited the visitors to plot the nest clusters themselves and help develop hypotheses as to why turtles tend to concentrate their eggs on specific parts of the shoreline.

One area of turtle research that has blossomed in the past several years is the study of loggerhead migratory patterns. A turtle tagged at the monitoring station I visited in Bahia turned up a year later on a beach in Japan. Scientists have long known that sea turtles can and do travel great distances, but few suspected them capable of traversing the Pacific. Although the turtle in question may have been an anomaly, she still adds to my respect for these most amazing creatures. Unfortunately, though, a turtle may be half way to the next continent before her eggs have hatched. Because turtle nests are left untended and concealed under a layer of sand, they are especially prone to human disruption. Tampering with nests, however, is a only a small factor in the ongoing decline in turtle populations. The practice has not ceased, but Mexico recently outlawed the harvesting of turtles and their eggs. And since 1978, the importation and manufacture of turtle products in this country has been strictly prohibited.

Science has not yet determined just what, exactly, the most severe threats to the sea turtle are. But a group of turtle-watchers in Honokowai, Hawaii has developed a plausible hypothesis to explain the fibropapilloma tumor, a disease that has spread rapidly among turtles in the past few years. Their web site, Turtle Trax (3), was the most extensive, thoughtful, and well designed source on sea turtles that I've managed to locate. It caters to everyone from the most scholarly research scientist to the five year old in search of a cute turtle picture. Best of all, it is designed and maintained not by "professionals" but by a dedicated group of turtle aficionados who want to share their knowledge and observations with as wide an audience as possible. According to Turtle Trax, the nutrient level in the waters around Honokowai has increased substantially due to agricultural runoff and human sewage disposal. The Lahaina Wastewater Reclamation Facility, which stands 500 meters from the shoreline, injects sewage effluent into soil at a rate of 15-25 million liters per day. No one knows where this effluent ends up, but it's unlikely that none of it ever reaches the ocean.

The nutrient overload, whatever it's cause, encourages excessive seaweed growth. Turtles are attracted by the surplus of food and, being social creatures, a high concentration of turtles in any one place encourages yet more turtles to settle there. Sooner or later, a turtle infected with fibropapilloma is bound to arrive. Growing population density results in more contact between individual turtles, and thus encourages the spread of communicable disease. Although the cause of fibropapillomas in sea turtles is not yet known, the same kinds of tumors in other animals are the result of viral infection. Dr. Larry Herbst, Dr. Eliot Jacobson, Dr. Paul Kline, and several other researchers have implicated a viral cause for the fibropapilloma and are searching for conclusive proof.

But if a virus is the cause, how is that virus spread? When the concentration of turtles in a particular area is great enough, a symbiotic relationship develops between the turtle population and certain members of the local fish population. At Honokowai, species such as the saddleback wrass and Hawaiian spotted toby will glean the majority of their food from barnacle-encrusted turtle shells. When first approaching a turtle, the "cleaner fish" will often first bite at their eyes. The fibropapilloma tumor, interestingly enough, usually occurs in this very location. Because the tumors are infested with parasites, they are an especially rich source of food. The fish have learned this and will bite at the eyes of a turtle whether or not that turtle has a fibropapilloma. It is possible, then, that fish act as the mechanical vector for the spread of the disease. This explanation, as Turtle Trax is careful to point out, is not conclusive. It is a hypothesis, a summary of observations gathered by a community of divers attracted to Honokowai for its large turtle population. But if the hypothesis is true, the number of turtles afflicted with fibropapilloma will continue to rise as more fish discover the tumors as a source of food. I hope that research into the validity of the Turtle Trax hypothesis, and into other possible causes of the fibropapilloma, will proceed with haste. In todayÕs political climate, persuading Maui County to reduce the level of its wastewater discharge wonÕt be easy. Without concrete scientific evidence attesting to the plight of the sea turtle and broad-based public support, itÕs unlikely to happen at all.

I admit that this attitude is a cynical one. Since sea turtle populations are beginning to stabilize, perhaps such pessimism is unwarranted. But if we must save our efforts until a species is teetering on the brink of extinction, we may well be too late to save it. Fortunately, there are some rays of hope for the sea turtle. Although its resources are limited, the Mexican government is doing all it can to protect and study the turtles that nest on its shores. Research into and awareness of the plight of the sea turtle are on the rise; I located close to fifty turtle web sites (I was unable, of course, to visit them all), and Turtle Trax is among the top 5% of internet pages accessed worldwide. The Cayman Island Sea Turtle Farm (4) has managed to make good business out of turtle preservation. They breed several different species of sea turtle and release close to 4,000 turtles into the Caribbean every year. They also sell 3,000 turtles per annum for domestic consumption. Their breeding tanks are open to the public, and tourists can view turtles in every stage of development. In a world reluctant to use its resources for conservation, innovative and conscientious private enterprise may be the most pragmatic route to saving the ecosystem.

As we learn more about sea turtles, I sincerely hope that we will find a way to rescue them from the clutches of extinction. The scene at Rancho Nuevo in 1947 is not likely to repeat, but I hope that on my next spring trip to Baja California, the sight of beaching turtles stumbling awkwardly across the sand will not be such a rarity.

Webliography:

1) The St. Catherine's Sea Turtle Conservation Program

2) Asupmatoma, Cabo San Lucas, Baja California Sur, Mexico

3) Turtle Trax

4) Cayman Island Sea Turtle Farm




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