By PETER C. H. PRITCHARD

As the best recognized of the sea turtle species, the green turtle is an icon of popular culture today. Their image is seen on car window decals and T-shirts; they appear on the covers of tourism brochures, books, and magazines; and they even star in children’s movies. Once viewed primarily as a resource to be exploited, green turtles are now the centerpiece of a global conservation movement. The more we have learned about the green turtle, the more our opinion of them and their value has changed, and continues to change today.

A green turtle at sunset on the Great Barrier Reef, Australia. © Jordan Robins / Coral Reef Image Bank

The green turtle, Chelonia mydas, carries the name “Chelonia,” which the Greeks used to encompass the entire creation of turtles, terrapins, and tortoises, and which is still used today by scientists to include all of the shelled reptiles. “Green turtle” is the common English name for the species, although hatchlings are black above and white below, and adults are varying shades of brown-yellow to black, often with decorative spots and streaks. Their name refers to the color of the fat, or “calipee.” The species is famous as gourmet food for the privileged population on both sides of the Atlantic Ocean, and it is subsistence fare for indigenous or underprivileged coastal dwellers in many parts of the tropics.

The green is the largest of the hard-shelled marine turtles, weighing up to 395 kilograms (870 pounds), and manages to be both elegant in the water and muscular on land. It is sufficiently heavy that it cannot— or does not—walk on land with an alternating gait, but rather heaves itself forward with a series of pushes involving all of its limbs. It is the only herbivorous marine turtle species and, appropriately, has both the complex jaw surfaces for biting off and chewing marine foliage and the short, rounded snout typical of vegetarian reptiles.

A History of Consumption

For much longer than humans have been studying green turtles, they have been eating them. Some societies have concentrated on egg collection, others on catching turtles themselves on land or at sea for direct consumption. This use for consumption, above all else, has been the main factor in the green turtle’s global decline. As a result, most nations now prohibit the practice, though some societies still tolerate a level of take; both need to be carefully supervised.

Several efforts have been undertaken to raise green turtles in captivity. During the 1960s and 1970s, there was much discussion among turtle conservationists about the rights and wrongs of green turtle farming. The discussions about the world’s biggest sea turtle farm, located on Grand Cayman Island and incorporated in 1968, brought to bear significant academic and entrepreneurial brainpower. Initially, the farm imported eggs from Costa Rica, Guyana, and Suriname, and it brought in adult turtles to provide breeding stock. Techniques for raising green turtles in captivity were developed, and the farm made itself available to scientists studying various aspects of green turtle biology. The setbacks were many, including loss of turtles during hurricanes. However, each setback was overcome, and eventually, turtles were produced in industrial quantities.

Turtle farms were a divisive subject for many years, even to the extent that longstanding friendships were broken. The proponents offered to saturate the market for the shells, meat, and calipee to take the pressure off wild turtles, while opponents argued that expanding the market for farm-raised turtles would exacerbate demand and put added pressure on wild stocks, which, they insisted, tasted better anyway.

A solution, of sorts, to the divisiveness arose when the U.S. Endangered Species Act was passed in 1973 and green turtles were listed as an endangered species in 1978. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) also prohibited international trade in sea turtles at about the same time (see the article on page 42 for more information). The Cayman Turtle Farm continued to exist, but the products could not be legally brought into the United States. Eventually, the farm was purchased by the Cayman government, mainly for preservation as a tourist attraction with, by Cayman standards, a substantial payroll that should not be jeopardized. Today, the farm still exists, under the name Bo’sun’s Bay, and still has thousands of green turtles, as well as tropical fish, gift shops, aviaries, restaurants, and even Kemp’s ridleys and loggerhead-hawksbill hybrids. The green turtles breed regularly, and every week, a few turtles are sacrificed for sale to Cayman Island restaurants and hotels, out of reach of both the U.S. and the CITES authorities.

Today, many nations with responsibility for management of green turtle stocks have undergone similar transformations away from consumption. They have undertaken a range of conservation activities, primarily focused on both identifying the stakeholders who depend to varying degrees on green turtles as a resource and working out cooperative management programs.

Green turtles fitted with satellite tags leave the beach in Gangkou Sea Turtle National Nature Reserve, southern China. © Brian J. Hutchinson

A History of Research

Unlike ridleys or loggerheads, the green has been studied for centuries, beginning with early observations on their physical appearance and behavior and evolving into much more elaborate studies that include the use of techniques such as DNA analysis, population modeling, and satellite tracking. Ships’ records and logs dating back to Christopher Columbus and his contemporaries in the 15th century reference the enormous numbers of sea turtles they saw upon their arrival to the “New World.” Perhaps these pre-colonization statistics are an indication of how modern Americans have decimated turtle stocks since Columbus’s time. Or perhaps the observations of vast flotillas of turtles were a result of the understandable exaggeration that can creep into such accounts.

Some of the first scientific observations of green turtles were made more than two centuries ago by the French naturalist and aristocrat Comte de Lacépède. He reported in his famous Histoire naturelle du poissons that, in Cuba, he had found some green turtles that indeed had green flesh, whereas others were black inside and still others were yellow. Lacépède also noted that green turtles nested more than once and might produce as many as 300 eggs in a season. Other early sources of surprisingly accurate information about green turtles may be found in the writings of the Dutch governor of Curaçao about a century ago. And visitors to Palau (Micronesia) may see some carved, colorful “storyboards” in the villages, a favorite of which offers the story of how humans first realized that female marine turtles return to nest a second time after an interval of about two weeks:

It was the same turtle! This single experiment demonstrated both that sea turtles show philopatry (literally “love of place,” describing the tendency of an individual to return to his or her home or place of birth) and that their nesting is influenced by semilunar cycles. These behaviors have since been studied extensively and are foundational concepts of modern sea turtle biology.

Other similarly accidental observations of green turtles uncovered behaviors that were later studied and proven with greater rigor. For example, the ability of green turtles to perform remarkable feats of navigation was first made clear many years ago when a Cayman Island turtle fisherman caught a load of green turtles at a particular spot in the Mosquito Cays of Nicaragua. He took them back to Grand Cayman. But before they could be sent on to Key West, Florida, a buyer from Jamaica arrived and took the whole shipment back to Kingston, where the turtles were all washed out of their corral in a storm. One of those turtles, identified by its unique brand, was re-caught months later at the exact spot in Nicaragua where it had been first caught. The journey back to its old sleeping rock was at least 684 kilometers (425 miles), perhaps much more.

Beyond these early observations, relatively little was known about the green turtle until the mid-20th century. At that time, Dr. Archie Carr (1909–1987) began his studies on green turtles at Tortuguero, Costa Rica. In his younger days, Carr had been a keen consumer of the green turtle, but an adulthood dedicated to studying and working with sea turtles shifted his value system. He subsequently advocated for the complete protection of sea turtles, and he believed that it is morally wrong to kill nesting females. He became the green turtle’s most celebrated enthusiast, declaring it “the most valuable reptile in the world,” not in reference to its meat, fat, and other consumables, but rather to its aesthetic value and value for non-consumptive uses such as ecotourism and scientific study.

Most of Carr’s research took place at Tortuguero, where the extraordinary migrations of the adult green turtles have been thoroughly documented for half a century in a strictly low-tech fashion—cattle ear tags on the turtles, small rewards for capture, and a large wall map of recoveries delineated with colored pins. Though relatively simple, this research revolutionized our understanding of the green turtle’s natural history and laid the foundation for modern sea turtle biology.

The rationale for the turtles’ migrations appears mainly to be that a good nesting beach needs high-energy wave action, whereas the best feeding occurs a great distance away on calm waters with good seagrass flats. The favored feeding grounds of Tortuguero turtles turned out to be the Caribbean waters of Nicaragua, and the volume of turtles captured for food in those waters—supplemented by hunting in Colombia, Honduras, Panama, and Venezuela—continues to give rise to great concern. Yet somehow this massive-scale take has not resulted in the disappearance of the nesting colony at Tortuguero. Perhaps the key feature is that the subsistence harvest in those countries includes all size classes, not just adult females, which have greater value to the population. In addition, Costa Rica has a national park at Tortuguero, with reasonably good enforcement of turtle and egg protection.

Marine turtles have great dispersal power in the oceans of the world, and therefore, the isolation that causes speciation is not present. Indeed, only seven or eight species of sea turtle are considered valid today, compared with more than 40 tortoise species and more than 250 freshwater turtle species. But within that handful of sea turtle species, significant diversity exists. Green turtles in different parts of the world may be drastically different in size. For example, in the Atlantic system, the Tortuguero adult females may be considered to be of medium size—about 113 kilograms (250 pounds). However, in the western Caribbean, once in a while a huge green turtle is spotted that the locals call a “wind turtle.” They have no idea where it comes from. But outside the Caribbean proper, in Guyana and Suriname, and also in Ascension Island and Trindade (Brazil), the turtles are twice as big as those in Tortuguero, and they eat seaweeds rather than seagrasses. Gourmets report that they taste different as well.

In the Pacific Ocean, specifically the Galapagos Islands and western Mexico, the locals find two rather drastically different turtles that broadly fall within the green turtle type. In the Galápagos, they recognize the black turtle and the yellow turtle, and in Mexico, they have the caguama prieta (black turtle) and the tortuga blanca (white turtle). The black phenotype is truly black, and even the adults are quite small and have certain key features—a narrowing of the posterior part of the carapace, a gray plastron, and an extremely long tail in adult males. They feed on algae, and they don’t taste good. Thetortuga blanca is similar to mainstream Chelonia mydas in the western Pacific Ocean (where they nest) or in the Atlantic Ocean.

Turtle scientists are divided as to whether the two types are different species. Geneticists argue against it. Field biologists may not have an opinion until they see the two types together. The Japanese now list Chelonia agassizii—the black turtle—as part of their fauna because they have seen the turtles in their own waters, and both forms appear together in Papua New Guinea as well. A couple of years ago, an Australian research team diving for green turtles happened to catch a black turtle, with an immediate response from all aboard: “That’s not one of ours!” Seeing is believing. We are probably watching speciation in action; it takes time (see the photo montage for more information on the diversity of green turtle phenotypes).

The green turtle’s physical appearance is so variable that for as long humans have been observing this species, they have been redefining it. On the scientific side, this has principally consisted of divisions into subspecies, of which there have be… — The green turtle’s physical appearance is so variable that for as long humans have been observing this species, they have been redefining it. On the scientific side, this has principally consisted of divisions into subspecies, of which there have been four—Chelonia mydas agassizii, C. m. carrinegra, C. m. japonica, and C. m. mydas. Informally, those who study green turtles often refer to different types, such as the standard green turtle, the black turtle (East Pacific form), or the yellow turtle (or Galapagos yellow). While some continue to disagree about the green turtle’s classification, the general consensus is that there is just one species, Chelonia mydas. Yet, the fact remains that no two green turtles are exactly the same. Within the species, there are several unique forms that appear regularly, as well as brilliant variations observed in just a few individuals. To capture the magnificent variation in green turtles for this issue, we turned to the global SWOT Team and our friends at www.seaturtle.org, and were rewarded with images from all corners of the world. The images on this page are just a fraction of the more than 250 that were submitted, all of which can be seen at www.SeaTurtleStatus.org. The carapaces (shells) shown here are from images of live turtles that were edited to exclude all else. They are not shown to scale.

Screen Shot 2019-06-03 at 1.58.12 PM copy.jpg — (1) Fisheries bycatch; Ilha do Cardoso, São Paulo, Brazil, Atlantic Ocean. © Shany M. Nagaoka ; (2) Research capture; Juan de Nova Island, Indian Ocean. © Kélonia ; (3) Research capture; Europa Island, Indian Ocean. © Kélonia ; (4) Research capture; Europa Island, Indian Ocean. © Kélonia ; (5) Fisheries bycatch; Japan, Sea of Japan. © Kei Okamoto ; (6) Research capture; Turks and Caicos Islands, Caribbean Sea. © Thomas Stringell ; (7) Research capture; Punta Abreojos, Baja California Sur, Mexico, Pacific Ocean. © John Wang (UH-JIMAR) / Ocean Discover y Institute (ODI); (8) Research capture; Europa Island, Indian Ocean. © Kélonia ; (9) Cold stunned; Lower Laguna Madre, Texas, USA, Gulf of Mexico. Photo courtesy Donna Shaver ; (10) Fisheries bycatch; Ryukyu Islands, Japan, East China Sea. © Kei Okamoto ; (11) Research capture; Juan de Nova Island, Indian Ocean. © Kélonia ; (12) Research capture; Isla Pardito, Baja California Sur, Mexico, Sea of Cortez. © Jeffrey Seminoff ; (13) Underwater;
St. John, U.S. Virgin Islands, Caribbean Sea. © C. S. Rogers ; (14) Research capture; Turks and Caicos Islands, Caribbean Sea. © Thomas Stringell ; (15) Fisheries bycatch; Cananéia, São Paulo, Brazil, Atlantic Ocean. © Shany M. Nagaoka

To make sense of the great diversity within each sea turtle species, the Marine Turtle Specialist Group has recently begun defining Regional Management Units for each species by integrating nesting sites, genetics, and tag return and migration data. The effort identified 17 Regional Management Units for green turtles—the most among all sea turtle species—which further highlights their wide variation. At a finer scale, studies of maternally inherited mitochondrial DNA (mtDNA) have identified more than 34 different stocks worldwide, as shown in the map on pages 32–33.

Shifting Values and Recovering Stocks

Today, we are witnessing a substantial resurgence of these “valuable reptiles” in many areas. The green turtle was once so rare on Florida beaches that even a single nesting was worthy of being described in a scientific note. Nowadays, there are thousands of green turtle nests in Florida during the good seasons, and healthy populations of greens can be found in other places where once they were heavily exploited, such as Ascension Island in the Atlantic Ocean and Hawaii in the Pacific Ocean. Just as Archie Carr changed from consumer to conservationist, and as the Cayman Island turtle farm changed from generating revenue through consumption to generating revenue through tourism, global policies and public opinion over the past half century have gradually made a value shift in relation to the green turtle: toward protection and away from consumptive use.

Over time, human perspectives of green turtles have broadened from seeing them mainly as a resource to be exploited to seeing them as an ecosystem engineer, an indicator of ocean processes, and, ultimately, as fellow creatures worth conserving. As this development has happened, we have recognized the negative impacts of our actions on green turtle numbers, and we have responded in ways that have resulted in incredible bounce-back in several green turtle populations around the world. Like an acquaintance becoming a good friend, we have come to know the green turtle better over the years and now appreciate it for its many benefits, not solely for its yummy taste. Perhaps one day the shift will be complete, and people everywhere will consider sea turtle protection a moral imperative.

SWOT Feature Maps: Global Biogeography of Green Turtles

The maps below display the global biogeography of green turtles (Chelonia mydas). With these, SWOT has now completed global maps for all seven sea turtle species. SWOT has grown significantly since we began our mapping efforts; the leatherback map in SWOT Report, Vol. I (2006) displayed 203 nesting sites, whereas the green turtle nesting map on the following pages shows more than 1,100 sites, and the combined SWOT database now has data from more than 2,000 nesting sites for all the world’s sea turtles.

Although green turtles have an extremely broad geographic range, as shown in the nesting map below and global distribution inset, they also exhibit high levels of population structuring (many genetically distinct populations) and regional-scale movement patterns, as shown in the genetic stock and telemetry map.

In the nesting map, relative annual nesting abundances are displayed by site for the most recent available year(s) or season(s) of data. Raw count data are reported in the data citations on pages 51–60, but are displayed on the maps in generalized bins (e.g., 1–10 clutches, 11–100 clutches, and so on) to facilitate interpretation. We have also distinguished between data that were reported before or after 2006 (i.e., more or less than five years ago) to aid interpretation of data accuracy (older data may be less accurate). Nesting abundances are reported in number of clutches. We converted data that were reported in units other than clutches using regionally appropriate values, or, if regional conversion factors were unavailable, we used global average conversion values (see page 60 for more information and references). Altogether, the nesting map displays 1,167 nesting sites from more than 200 different data providers and published sources. Please see the SWOT Data Contributors section for details.

In the satellite telemetry and genetic stocks map, we have included the green turtle’s global distribution (bottom left) based on multiple data types, including telemetry, tag-returns, strandings, and sightings, as well as satellite telemetry data (displayed as number of turtle locations in a given area) and known genetic stocks (based on mitochondrial DNA). Altogether, this map shows 34 distinct genetic stocks and data from more than 200 satellite-tracked turtles. Original data sources are cited in the SWOT Data Contributors section (see citations here).

Small numbers on the nesting map are data record numbers (which correspond to the citations), while numbers in larger, bold font indicate map insets. In the satellite telemetry map, data include adult females and males, as well as juvenile turtles, and each genetic stock is represented by a different letter code. In some cases, multiple sites refer to the same letter code, meaning that the sites belong to the same stock. By spatially synthesizing several types of biological information, these SWOT maps are the most comprehensive presentation of biogeographical information collected on green turtles to date.