Hawksbill Sea Turtle

Eretmochelys imbricata

Description

On the hawksbill sea turtle, carapacial scutes are often richly patterned with irregularly radiating streaks of brown and black on an amber background. The scutes of the plastron of Atlantic hawksbills are usually clear yellow, with little or no dark pigmentation. The soft skin on the hawks-bill's venter is cream or yellow and may be pinkish-orange in mature individuals. There are typically four pairs of inframarginal scales. The head is elongate and tapers sharply to a point. The lower jaw is V-shaped. The scales of the head and fore-limbs are dark brown or black and have yellow borders.

The hawksbill is a small to medium-sized marine turtle. Nesting females average about 2.9 ft (87 cm) in curved carapace length and weigh about 100 lb (45 kg), with a record weight of 280 lb (127 kg). Hatchlings in the U. S. Caribbean average about 1.7 in (4.2 cm) in straight carapace length and weigh 0.5-0.7 oz (13.5-19.5 g).

Hawksbill crawls are difficult to distinguish from those of the loggerhead turtle, and hatchlings of the two species are also very similar, making identification of nests and estimates of productivity very difficult.

Behavior

Primarily carnivorous, the hawksbill feeds near coral reefs on jellyfish, sponges, and other sessile organisms. Unlike the green turtle, the hawksbill does not migrate but occupies a relatively small home range. It breeds year-round in tropical waters, usually at two-or three-year intervals. The female comes ashore, scoops out a nest in the sand, and deposits about 160 eggs, which incubate for 50 days. Hatchlings emerge as a group and scurry perilously for the sea. While other sea turtles nest in colonies, the hawksbill prefers to nest alone, using the same nesting sites year after year.

The six-month nesting season of the hawksbill is longer than that of other sea turtles. Most nests on Buck Island Reef National Monument (St. Croix, U. S. Virgin Islands), are made from July to October. On Mona Island, Puerto Rico, the peak season is August to October. Courtship and mating apparently begin somewhat earlier, and may occur either along the migratory route or off the nesting beach. Nesting in the Caribbean is principally nocturnal, although rare daytime nesting is known. Nesting behavior follows the general sequence of that of other species of sea turtles: emerging from the sea, selecting a site, clearing the site and constructing a body pit, constructing the egg chamber, laying the eggs, filling in the egg chamber, disguising the nest site, and returning to sea. The entire process takes approximately one to three hours.

Hawksbills nest an average of 4.5 times per season at intervals of approximately 14 days. As many as 12 clutches may be produced by a single female in one season. Not all emergences or nesting attempts result in eggs being laid. On Mona Island, an average of two emergences per successful nest was calculated; one female was observed making as many as 11 digging attempts on a single emergence. The ratio of crawls to nests varies geographically depending on local conditions, making site-specific information necessary for accurate interpretation of aerial survey data. On the basis of limited information, two-and three-year remigration intervals appear to predominate; annual nesting by the hawks-bill has not been recorded in the Caribbean.

Hawksbills have strong site-fidelity for their nesting beaches, and are capable of returning to specific beach areas. The extent to which site-fidelity is expressed among and within populations, or even by individuals over time, remains to be quantified. Clutch size is directly correlated with carapace length and varies markedly throughout the range of the species. In Florida and the U. S. Caribbean, clutch size is approximately 140 eggs, and several records exist of more than 200 eggs per nest. Eggs are approximately 1.6 in (4 cm) in diameter and take about 60 days to hatch. Hatching success at nesting beaches in the United States is approximately 80%. Hawksbills are suspected to exhibit temperature-dependent sex determination, as do other sea turtles, but data are limited.

Few data are available on the growth rates of wild hawksbill turtles. Most information has come from a study involving recaptures of 32 turtles on the Great Barrier Reef. Mean growth rates ranged from 0.02 in (0.06 cm) per year for two adults, to 0.9 in (2.17 cm) per year for immature turtles with initial curved carapace lengths of 20-24 in (50-60 cm). The study concluded that hawksbills recruiting onto the reef at 14 in (35 cm) in length would begin breeding 31 years later. Because the time required for these turtles to reach 14 in (35 cm) is unknown, the actual age at sexual maturity is not known.

The few data available suggest slow growth and an advanced age at sexual maturity, as has been demonstrated for several other species of sea turtles. Rates of growth vary among different size classes and seem to decrease considerably after sexual maturity is reached.

Very little is known of the movement patterns of post-hatchling hawksbills, although their occupation of the pelagic environment is relatively well documented. Post-hatchlings in Texas waters are presumed to have been passively transported there by currents that pass along Mexico. The movement patterns of hatchlings entering the sea from U. S. beaches are unknown. Immature hawksbills show evidence of residency on specific feeding grounds, but developmental migrations may occur with changes in habitat occupation.

Immature hawksbills tagged in the U. S. Virgin Islands have been recovered in eastern Puerto Rico, the British West Indies, St. Martin, and St. Lucia, representing travel distances of 59 mi (95 km), 29 mi (46 km), 115 mi (185 km), and 400 mi (650 km), respectively. Other recaptures of immature hawks-bills have documented the long-distance travel of an 24 lb (11 kg) hawksbill from Great Inagua, Bahamas, to the Turks and Caicos Islands and the migration of a subadult hawksbill from Brazil to Dakar, Senegal, a distance of 2,290 mi (3,680 km). The purpose and regularity of migrations by immature hawksbills deserve further study.

Recoveries of tagged adult hawksbills suggest that some populations or groups within a population undertake reproductive migrations. Migrations have been documented of adult females from beaches in Costa Rica to feeding grounds in Nicaragua, and from Nicaragua feeding grounds to a beach in Jamaica. An adult male tagged on the foraging grounds in Nicaragua was recovered in Panama. The National Marine Fisheries Service and the U. S. Fish and Wildlife Service (FWS) reported the travel of a hawksbill from Isla Mujeres, Mexico, to Bani, Dominican Republic, a distance of 1,815 mi (2,925 km). There is also indirect evidence of migration by hawksbills, such as a population of immature hawksbills in the Great Barrier Reef that were found to reside at least 870 mi (1,400 km) from any regular hawksbill nesting site.

Very little is known about the diet of post-hatchling hawksbills in the pelagic environment. Eggs of pelagic fish, pelagic species of sargassum (brown algae), and various floating debris such as tar droplets, Styrofoam, and plastic have been identified. Although a wide variety of benthic organisms have been recorded from digestive tracts, sponges are the principal diet of hawksbills once they enter shallow coastal waters and begin feeding on the bottom. Quantitative studies have focused on the Caribbean, but there is evidence that spongivory is a worldwide feeding habit. It is unquestionably a highly unusual one, being shared by only about a dozen other vertebrates. A high degree of feeding selectivity is indicated by the consumption of a limited number of sponge species. Sponge predation by hawksbills may influence reef succession and diversity by freeing up space on the reef for settlement by benthic organisms. The hawksbill's highly specific diet, and its dependence on filter-feeding, hard-bottom communities make it vulnerable to deteriorating conditions on coral reefs.

Although the hawksbill turtle is rare in south Florida, it shares nesting beaches with the threatened loggerhead turtle, and the endangered green and leatherback turtles. Other federally listed species that occur in coastal dune and coastal strand habitat, and that need to be considered when managing nesting beaches, are the southeastern beach mouse and the beach jacquemontia. Beach nourishment projects, in particular, could affect these species as well as the turtles. The range of the beach mouse in south Florida is estimated to include Indian River County south to Broward County. The beach jacquemontia is found in Palm Beach County south to Miami, Dade County.

Some hawksbill nests have been discovered that are believed to be the result of hybrid crosses. Preliminary genetic testing in some of these cases has revealed the female parent was a loggerhead; tests are pending to reveal the identities of the male parent as a hawksbill or a hybrid.

Habitat

Hawksbill sea turtles use different habitats at different stages of their life cycle. Sightings, strandings, and gut-content analyses suggest that post-hatchling hawksbills occupy the pelagic environment, taking shelter in weedlines that accumulate at convergence zones. Sargassum and floating debris such as Styrofoam, tar droplets, and plastic bits—common components of weedlines—are consistently found in the stomachs of post-hatchling hawksbills that strand in Texas. Thus, it seems likely that weed-lines in the Gulf of Mexico serve as habitat for hawksbills that enter U. S. waters from nesting beaches in Mexico and Central America. Post-hatchlings from beaches in the United States are presumed to occupy weedlines in the Atlantic Ocean.

Hawksbills reenter coastal waters when they reach a carapace length of approximately 8-10 in (20-25 cm). Coral reefs are widely recognized as the resident foraging habitat of juveniles, subadults, and adults. This habitat association is undoubtedly related to their diet of sponges, organisms that need solid substrate for attachment. The ledges and caves of the reef provide shelter for resting both during the day and night. Hawksbills are found around rocky outcrops and high-energy shoals, which are optimum sites for sponge growth. Hawksbills are known to inhabit mangrove-fringed bays and estuaries, particularly along the eastern shore of continents where coral reefs are absent. In Texas, juvenile hawksbills are associated with stone jetties.

Hawksbills nest on low-and high-energy beaches in tropical oceans of the world, frequently sharing the high-energy beaches with green turtles. Both insular and mainland nesting sites are known. Hawksbills will nest on small pocket beaches and, because of their small body size and great agility, can traverse fringing reefs that limit access by other species. They exhibit a wide tolerance for nesting substrate type. Nests are typically placed under vegetation.

Distribution

The hawksbill occurs in tropical and subtropical seas of the Atlantic, Pacific, and Indian Oceans. The species is widely distributed in the Caribbean Sea and western Atlantic Ocean. Representatives of at least some life history stages regularly occur in southern Florida and the northern Gulf of Mexico, especially Texas, in the Greater and Lesser Antilles, and along the Central American mainland south to Brazil. In U. S. jurisdiction in the Caribbean Sea, hawksbills are most common in Puerto Rico and its associated islands, particularly Mona, Culebra, and Vieques, and in the U. S. Virgin Islands. In the continental United States it occurs along all of the Gulf states and along the eastern seaboard as far north as Massachusetts, with the exception of Connecticut, but sightings north of Florida are rare.

Hawksbills are observed in Florida with some regularity in the waters near the Florida Keys and on the reefs off Palm Beach County, where the warm Gulf Stream current passes close to shore. Before their numbers were reduced by overfishing, the Florida Keys were once considered the world's finest fishing grounds for hawksbill turtles.

Texas is the only other state where hawksbills are sighted with any regularity. A total of 77 observations, most involving post-hatchlings and juveniles, have been recorded there between 1972 and 1984. These small turtles are believed to originate from nesting beaches in Mexico.

Within U. S. jurisdiction in the Caribbean Sea, nesting occurs principally on beaches in Puerto Rico and the U. S. Virgin Islands. The most important sites are Mona Island and Buck Island. Nesting also occurs on other beaches of St. Croix, Culebra Island, Vieques Island, mainland Puerto Rico, St. John, and St. Thomas.

Within the continental United States, nesting is restricted to the southeastern coast of Florida, and has been reported from Broward, Dade, Martin, Monroe, Palm Beach, and Volusia Counties. Nesting by hawksbills has been recorded several times on Soldier Key, a small, mangrove-fringed islet in Biscayne Bay. The only reported nesting in Manatee County on the west coast of Florida was not adequately documented. Low levels of nesting are suspected to occur in the Marquesas and Dry Tortugas, but these areas have not been adequately surveyed.

Throughout their range, hawksbills typically nest at low densities; aggregations consist of a few dozen, at most a few hundred individuals. This is in contrast to green turtles and loggerhead turtles which nest by the thousands or tens of thousands at concentrated sites. The largest known nesting concentrations in the Caribbean are in the Yucatan Peninsula of Mexico, where approximately 800-1,000 nests are made each year between Isla Holbox and Isla Carmen. This corresponds to approximately 178-222 turtles, given an estimated average of 4.5 nests per female per season. Other important, but relatively small, nesting beaches in the Caribbean region are located in Belize, Nicaragua, Panama, Venezuela, Antigua, and the Grenadines. Hawksbills are also known to nest in Cuba, possibly in significant numbers, but population estimates are not available. With few exceptions, all of the countries in the Caribbean report less than 100 females nesting annually.

In the U. S. Caribbean, there is evidence that hawksbill nesting populations were severely reduced during the twentieth century. As of 2000, they are not believed to be declining, but neither are there signs of recovery, despite more than three decades of protection. The most recent status review of the species in the United States recognized that numerous threats still exist for U. S. populations and recommended that the hawksbill remain listed as endangered throughout its range.

Estimates of the size of nesting populations are available for only a few localities. During seven years of monitoring, an average of 160 nests were made annually on Mona Island. This corresponds to approximately 36 nesting females per year. A total of 196 nests were recorded on the island in 1990. Approximately 65-125 nests are made annually on Buck Island Reef National Monument, St. Croix, U. S. Virgin Islands. Since research began in 1988, 15-30 female hawksbills have been recorded nesting on Buck Island Reef National Monument each year.

The hawksbill sea turtle does not nest frequently or commonly in Florida. Since 1989, nesting has been reported from Broward, Dade, Martin, Monroe, Palm Beach, and Volusia Counties, and the number of known nests each year through 1996 varied from zero to two. Results of surveys, however, undoubtedly underestimate the actual number of nests in Florida, and it appears that hawksbills are using the more remote islands and cays of the Florida Keys, where surveys are not conducted regularly.

Threats

The hawksbill is listed as endangered by the International Union for the Conservation of Nature and Natural Resources and is listed in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). It was also listed as endangered throughout its range on June 2, 1970, under the Endangered Species Act of 1973, as amended. An exhaustive review of the worldwide conservation status of the hawksbill turtle was carried out; the conclusion was that the species is suspected or known to be declining in 38 of the 65 geopolitical units for which nesting density estimates are available. Severe declines were noted in the western Atlantic Ocean and Caribbean region, and it was reported that current nesting levels may be far lower than previously estimated. Despite protective legislation, international trade in tortoiseshell and subsistence use of meat and eggs continue unabated in many countries and pose a significant threat to the survival of the species in this region.

A variety of natural and introduced predators prey on hawksbill eggs and hatchlings. Until eradicated in 1987, mongooses were destroying up to 55% of all nests on Buck Island Reef National Monument. Prior to extensive live trapping, mongooses were destroying an estimated 24% of all turtle eggs in 1980 and 1981 on St. John, U. S. Virgin Islands. Feral hogs destroyed 44-100% of all hawksbill nests deposited outside of fenced areas on Mona Island, Puerto Rico, during 1985-87.

On Sandy Point National Wildlife Refuge, hawksbill and leatherback hatchlings are strongly attracted, especially on moonless nights, to the lights of Frederiksted, St. Croix, U. S. Virgin Islands. Another example is the Hotel Palmas del Mar parking lot lights at Humacao, Puerto Rico. These lights regularly disorient or misorient hawksbill hatchlings.

The placement of physical obstacles on nesting beaches can hamper or deter nesting attempts and interfere with incubating egg clutches and the seaward movement of hatchlings. The placement of recreational beach equipment directly above incubating egg clutches may hamper hatchlings during their emergence and can destroy eggs through direct invasion of the nest. Nesting females gravitate to dark horizons when seeking a nest site, whether the horizon be a beach forest or a cabana. Hawks-bills may nest in the shadow of a chair or umbrella on the open beach. If the structure is removed, the nest is no longer protected from direct sunlight and the nest may get too hot.

The greatest threat to hawksbills on nesting beaches in Puerto Rico, St. Thomas, and St. Croix is poaching. While on the beaches, adult females are killed for tortoiseshell. Better surveillance by law enforcement and volunteer groups is believed to be reducing the levels of take. Hawksbills that use the remote beaches on Mona and Culebra Islands are vulnerable to poaching. Hawksbills that use Pinones, a beach close to San Juan, Puerto Rico, are taken, in spite of the fact that Pinones has been given one of the largest Puerto Rico Department of Natural Resources ranger contingents deployed on any Puerto Rican beach. Although the rate of poaching may be limited on any given beach, the overall effect is an enormous drain on hawksbill populations.

In nearshore waters, hawksbills are periodically captured in the cooling water intakes of industrial facilities, such as Florida Power and Light Company's St. Lucie Power Plant on Hutchinson Island. Between March 1976, when the St. Lucie Plant opened, and November 1988, six hawksbills were captured. As of June 1, 1992, three more had been captured. All were released unharmed.

Conservation and Recovery

Because the hawksbill is rare in south Florida, there is no specific management ongoing for this species. Conservation measures to protect nesting beaches for sea turtles in general, however, will also benefit the hawksbill. The following discussion is taken from the recovery plan for the Hawksbill Turtle in the U. S. Carribean, Atlantic, and Gulf of Mexico, as examples of specific management and conservation measures being implemented for the species in the U. S. Caribbean.

The most important hawksbill conservation achievement in recent years was Japan's decision to end import of hawksbill shell by 1993 and to drop its CITES reservations on sea turtles by July 1, 1994. Because Japan is the largest importer of stuffed hawksbills and hawksbill shells in the world this decision should significantly diminish the future demand for the species.

The two most important hawksbill nesting beaches in the U. S. Caribbean are now fully protected. Buck Island Reef National Monument, St. Croix, U. S. Virgin Islands, became part of the National Park Service (NPS) in 1962. Mona Island, Puerto Rico, was established as a natural reserve under the protection of the Puerto Rico Department of Natural Resources in 1980. In addition, Isla Culebrita was transferred to Culebra National Wildlife Refuge in 1982. Sandy Point National Wildlife Refuge, St. Croix, was established in 1984.

In 1988 the NPS initiated a study of the hawks-bill nesting population at Buck Island Reef National Monument to monitor long-term trends. In 1991, the FWS collaborated with the NPS in a study of hawks-bill postnesting migrations and movements at Buck Island Reef. In 1991 the NPS also used radio and sonic telemetry to study internesting movements, and the NPS initiated nesting surveys of hawksbill beaches on St. John, U. S. Virgin Islands.

Since 1986, a nesting-behavior study has been conducted at Humacao under the auspices of the Puerto Rico Department of Natural Resources. A similar study has been initiated on Caja del Muertos. Since 1990, with U. S. Navy support, the Puerto Rico Department of Natural Resources has been tagging hawksbills on Vieques.

In the U. S. Virgin Islands, the St. Croix Environmental Association, the University of the Virgin Islands Extension Service, the Virgin Islands Department of Fish and Wildlife, the FWS, and the NPS are actively involved in circulating newsletters and information packages and in presenting slide shows and seminars. Earthwatch has supported projects in Puerto Rico and in the U. S. Virgin Islands. Projects on Sandy Point National Wildlife Refuge and Culebra National Wildlife Refuge have brought attention to sea turtle conservation and have generated local involvement and awareness. In the U. S. Virgin Islands, schoolchildren are being introduced to the problems that sea turtles face and to how people can help them. Problems associated with plastics in the ocean have also been brought to the public's attention via news releases, public service announcements, and television programs. In Puerto Rico, presentations on sea turtle biology are made at school levels from kindergarten to college. Projects on the east coast of Puerto Rico and in Culebra have involved many segments of the community, including volunteers, the Chelonia Society, the Boy Scouts, 4-H groups, and various other clubs.

Contacts

National Marine Fisheries Service
Office of Protected Resources
1315 East-West Highway
Silver Spring, Maryland 20910
Fax: (301) 713-0376
http://www.nmfs.gov/prot_res/prot_res.html

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

References

Bjorndal, K., ed. 1981. Biology and Conservation of Sea Turtles. Smithsonian Institution Press, Washington, D.C.

Carr, A. F., and S. Stancyk. 1975. "Observations on the Ecology and Survival Outlook of the Hawks-bill Turtle." Biological Conservation 8: 161-172.

National Marine Fisheries Service and U. S. Fish and Wildlife Service. 1993. "Recovery Plan for Hawksbill Turtles in the U. S. Caribbean Sea, Atlantic Ocean, and Gulf of Mexico." National Marine Fisheries Service, St. Petersburg, Florida.

U. S. Fish and Wildlife Service. 1984. "Recovery Plan for Marine Turtles." U. S. Fish and Wildlife Service, Atlanta.

Witzell, W. N. 1983. "Synopsis of Biological Data on the Hawksbill Turtle." FAO Fisheries Synopsis 137:38.