Kemp's Ridley Sea Turtle

Lepidochelys kempii

Description

Kemp's ridley, Lepidochelys kempii, and its congener, the olive ridley, are the smallest of all extant sea turtles, the weight of an adult generally being less than 100 lb (45 kg) and the straight carapace length around 26 in (65 cm). Adult Kemp's ridleys' shells are almost as wide as they are long. The coloration changes significantly during development from the gray-black dorsum and venter of hatchlings to the lighter gray-olive carapace and cream-white or yellowish plastron of adults. There are two pairs of prefrontal scales on the head, five vertebral scutes, five pairs of costal scutes, and generally 12 pairs of marginals on the carapace. In each bridge adjoining the plastron to the carapace, there are four scutes, each of which is perforated by a pore. This is the external opening of Rathke's gland, which secretes a substance of unknown but possibly pheromonal function. Males are not well described but resemble the females in size and coloration. Secondary sexual characteristics, typical of males of sea turtle species, are present in L. kempii —i.e., the longer tail, more distal vent, recurved claws, and, during breeding, a softened mid-plastron. The eggs are 1.3-1.8 in (3.4-4.5 cm) in diameter and 0.8-1.4 oz (24-40 g) in weight. Hatchlings generally range from 1.6-1.9 in (4.2-4.8 cm) in straight line carapace length, 1.3-1.7 in (3.2-4.4 cm) in width, and 0.5-0.7 oz (15-20 g) in weight. In 1984 and 1985, the National Park Service reported hatchlings from an imprinting project had mean carapace lengths of 1.71 and 1.7 in (4.35 and 4.325 cm), respectively. For 1984, hatchlings had a mean weight of 0.58 oz (16.4 g) and in 1985, the mean was 0.56 oz (15.7 g).

Behavior

Principal courtship and mating areas for Kemp's ridley are not well-known. Anecdotal information supplied by fishermen revealed that mating presumably occurs at or before the nesting season in the vicinity of the nesting beach. A mating pair of ridleys were reported in Mansfield Channel at the southern boundary of Padre Island National Seashore, Texas. Reproduction for the majority of the extant population appears to be annual. Nesting occurs from April into July, and is essentially limited to the beaches of the western Gulf of Mexico, primarily in the Mexican state of Tamaulipas. Clutches numbered 80-200 eggs, with a mean clutch size in 1978-1991 of 100.8 eggs. The hatchlings emerge after 45-58 days, depending upon the incubation conditions, especially the temperature.

Growth data for wild Kemp's ridley are sparse and confounded by imperfectly reproducible measurements, but it is unlikely that most adults grow very much after maturity. Juveniles may grow rapidly. Two individuals of Kemp's ridley at Cayman Turtle Farm fed high-protein diets began to lay eggs at five years old and at a much smaller size than seen in the wild. The age to maturity based on captive growth, recapture data, and minimum nesting size is estimated at six to seven years. The recovery team for the Kemp's ridley feels that this estimate may be too low based on growth rates for other carnivorous cheloniids, namely loggerheads, which reach maturity at 12-35 years.

Neonatal Kemp's ridley presumably feed on the available sargassum (brown algae) and associated infauna or other epipelagic species found in the Gulf of Mexico. In the postpelagic stages, the ridley is largely crab eating, with a preference for portunid crabs. From studies of stomach contents, usually of stranded dead turtles, Kemp's ridley appears to be a shallow water, benthic feeder.

Habitat

The major nesting beach where Kemp's ridley emerges in any concentration to lay eggs is on the northeastern coast of Mexico. This location is near Rancho Nuevo in southern Tamaulipas. Kemp's ridley, together with the flatback turtle of Australia, has the most restricted distribution of any sea turtle. The species occurs mainly in coastal areas of the Gulf of Mexico and the northwestern Atlantic Ocean. Occasional individuals reach European waters. There is a single record from Malta in the Mediterranean, a few from Madeira and the Moroccan coast, and a record from Bermuda. A juvenile ridley was found in the Azores in the late 1990s.

Adults of this species are usually confined to the Gulf of Mexico, although adult-sized individuals sometimes are found on the eastern seaboard of the United States. The postpelagic stages are commonly found dwelling over crab-rich sandy or muddy bottoms. Juveniles frequent bays, coastal lagoons, and river mouths. Adults are present seasonally near the Mississippi River mouth and the Campeche Banks, converging annually on the Rancho Nuevo nesting grounds. What appeared to be winter dormancy was observed in Canaveral Channel during seasonally low temperatures.

Distribution

Movements of the adult females away from the nesting beach have been recorded; postnesting adult females stayed nearshore in water of 165 ft (50 m) or less during their movements away from the beach. During the nesting season, postnesting females make slow and seemingly random movements offshore near the nesting beach for one to two days, then more rapid, longshore movements at least 6.2 mi (10 km) north or south of their last nesting site before returning to lay eggs again or leaving the area entirely. L. kempii exhibits extensive internesting movements and there may be some factors grouping turtles nesting on the same day together until the subsequent nesting emergence.

Juvenile/subadult L. kempii have been found along the eastern seaboard of the United States and in the Gulf of Mexico. Atlantic juveniles/subadults travel northward with vernal warming to feed in the productive, coastal waters of Georgia through New England, returning southward with the onset of winter to escape the cold. In the Gulf, juvenile/subadult ridleys occupy shallow, coastal regions. One researcher has suggested that in the northern Gulf they move offshore to deeper, warmer water during winter. Little is known of the movements of the post-hatching, planktonic stage within the Gulf.

Kemp's ridley is extremely rare in Florida. Two nests, however, have been reported from Pinellas County, one from Lee County in 1996, and two from Volusia County in 1996. Four false crawls were reported from Palm Beach County in 1989.

Threats

Internationally, Kemp's ridley is considered the most endangered sea turtle. Less than 50 years ago the Kemp's ridley was an abundant sea turtle in the Gulf of Mexico. Populations were able to generate a synchronized reproductive effort that resulted in an estimated 40,000 females nesting in one day on the single known nesting beach on the northeastern coast of Mexico. Such former aggregations could only have been produced by a very large adult population. Kemp's ridley has experienced one of the most dramatic declines in population numbers recorded for an animal. Dr. Archie Carr and others sought the nesting areas of Kemp's ridleys throughout the Gulf of Mexico, the Caribbean, and the southeastern United States over many years. When the Mexican nesting beach was first discovered by scientists in 1961, the population was already severely depleted. That year, Dr. Henry Hildebrand showed an amateur film he obtained in Mexico from Ing. Herrera to a meeting of herpetologists. The film revealed an estimated 40,000 female Kemp's ridleys nesting on one day in an arribada (group of females arriving to nest) at Rancho Nuevo. On May 23, 1968, the number of turtles nesting in a single arribada had declined to an estimated 5,000 females. In the years 1978-91, a single arribada rarely reached 200 females, less than 0.5% of a day's nesting in 1947.

Threats to the nesting beach for Kemp's ridley in Mexico are few, but potentially serious. Human population growth and increasing developmental pressure will ultimately result in escalating threats to the nesting beach. Only the central part of the prime nesting area is protected by Mexican presidential decree, and legislation has never been enacted to fully implement the decree. A primary concern is human encroachment and access along the entire nesting area. The wording of the Mexican decree is so vague that construction of commercial fishing facilities proceeded in 1987 immediately adjacent to the main turtle camp at Rancho Nuevo. Occasionally, plans for massive expansion of La Pesca, just to the north of the nesting area, as a fishing center, or dredging the Gulf Intercoastal Waterway from Brownsville, Texas, to Barra del Tordo, in the south part of the nesting beach, are reported. These projects would result in detrimental and possibly disastrous effects on the nesting environment if they were to be completed.

Other nesting environment threats—such as armoring, nourishment, or cleaning of the beach; motorized equipment; and non-native dune vegetation—do not currently exist. Erosion, nest depredation, and other nest loss agents are not considered problems because every nest possible is moved to protected central corrals.

A threat that comes about due to management practices at Rancho Nuevo is the problem of concentrating all of the collected nests in corrals. This concentration makes the eggs more susceptible to reduced viability from the manipulation, disease vectors, and inundation. The former two do not seem to have been factors over the time of the binational project, but inundation was a severe problem in 1980 and 1983, drowning nests and reducing the overall percentage hatch by significant margins.

Direct exploitation of Kemp's ridley eggs occurred at the Rancho Nuevo nesting beach in the 1940s through the early 1960s prior to the initiation of protection of the beach in 1966. Prior to the late 1960s, the eggs were taken out in mule trains, by truck, and by horseback.

Dredging operations affect Kemp's ridley through incidental take and by degrading the habitat. Incidental take of ridleys has been documented with hopper dredges. The National Marine Fisheries Service (NMFS) consulted with the U.S. Army Corps of Engineers in November 1991 and issued a biological opinion under section 7 of the Endangered Species Act finding that the unrestricted operation of hopper dredges from North Carolina to Cape Canaveral, Florida, jeopardized the continued existence of sea turtles, particularly Kemp's ridley. In addition to direct take, channelization of the in-shore and nearshore areas can degrade foraging and migratory habitat through spoil dumping, degraded water quality and clarity, and altered current flow.

Conservation and Recovery

In 1977 a refuge was established at the only known nesting beach and a Mexican presidential decree included the Rancho Nuevo Nesting Beach Natural Reserve as part of a system of reserves for sea turtles. In 1990 a complete ban on taking any species of sea turtle was effected by Mexican presidential decree. In addition, the Mexican government has proposed a national plan that could be a major force, if adopted and implemented, in the protection of all of the remaining sea turtle resources of Mexico.

Nesting beach protection in the vicinity of Rancho Nuevo has been significantly increased over the past two decades. The collaboration of Mexican and U. S. conservationists is now used as a model for an international multiagency effort. Protection efforts on the Rancho Nuevo nesting beach were initiated in 1966 by the Mexican government. During 1966-77, an average of 23,000 hatchlings were released annually. From 1978 to 1991, under a cooperative beach patrol effort involving personnel from both countries, the number of released hatchlings was increased to a yearly average of 54,676 individuals. For adult females, a downward trend in population numbers continued through 1985, in spite of the efforts since 1966 to stop the egg poaching and harm to the nesting females on the beach. More than one million hatchlings have been released at the nesting beach but has yet to have much effect on recruitment into the adult female portion of the population.

There has been an increase in the number of nests documented at Rancho Nuevo since 1985. The increase is in part due to wider coverage of the nesting beach by the binational protection team and in part due to increased numbers of nests laid. How much of the increase is attributable to new recruits to the nesting population versus increased efforts to patrol north and south of the reserve (after a dispersion of nesting females since Hurricane Gilbert altered large expanses of the primary nesting area) is difficult to say. Regardless of the recent apparent increase in nests laid, the view is quite different when all known nests are plotted over time since 1947.

As far as is known, no adult turtle has suffered nonhuman predation on the beach since 1966 when the Mexican program began. Because of the intensive vigilance of the binational protection team, adequate motorized beach patrols, and the presence of armed marines, poaching of adult turtles on the nesting beach has not been documented since 1980, and only occasionally is a clutch of eggs taken by humans.

Nearly all nests laid on the beach are moved the same day to fenced and guarded corrals near the camps. Hatching success has been improved in the corrals since the binational project began. The mean during 1987-91 was 72%, nearly that of undepredated in situ nests. Almost all of the nests left in situ suffer predation, primarily by coyotes, skunks, and raccoons. The few missed nests that are discovered a day or more after being laid and that are too old for safe transport to a corral are preferentially protected with plastic mesh in situ and monitored for hatching. Alternatively, if those older nests cannot be protected in situ, they are carefully transferred to a sandpacked Styrofoam box for incubation at one of the camps.

Habitat research now underway promises to provide a much improved picture of the biology of this species. Netting studies in the northern Gulf of Mexico, East Coast habitat use and tracking studies, and adult migratory and wintering studies are continuing. These studies will contribute considerably to our understanding of Kemp's ridley habitat use and requirements and thus to our ability to protect foraging and migratory habitats.

"Head-start" is the term used to describe the process whereby sea turtles are maintained in captivity for a period following hatching, so that the very high neonatal mortality may be circumvented. The animals are released when they have outgrown threats from avian and the majority of nonavian predatory species. The Kemp's ridley head-start experiment began in 1978 as part of a complex, binational agreement to undertake several conservation and research measures at Rancho Nuevo, Padre Island National Seashore, and the National Marine Fisheries Service Galveston Lab. The head-start experiment was undertaken as a last-ditch effort in the face of the alarming decline in turtles nesting at the Rancho Nuevo nesting beach. In 1977, when the project was conceived, protection of the beach lacked manpower and funds, and whether protection would continue was unclear. In fact, the major cause of mortality from human activities, shrimping, was only then being established and there were no turtle excluder devices to eliminate this mortality. Currently, protection of the nesting beach is reasonably secure and turtle exclusion device (TED) regulations are in place and being expanded in the U. S. shrimp fleets, while Mexico is embarking on a program of TED placement in their shrimp fleets. Between 1978 and 1992, about 18,000 head-started Kemp's were released. In 1992 the program was ended.

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
P.O. Box 1306
Albuquerque, New Mexico 87103-1306
Telephone: (505) 248-6911
Fax: (505) 248-6915
http://southwest.fws.gov/

References

Center for Environmental Education. 1986. Sea Turtles and Shrimp Trawlers. Center for Environmental Education, Washington, D.C.

Hendrickson, J. R. 1980. "The Ecological Strategies of Sea Turtles." American Zoologist 20 (3): 597-608.

Marquez, M. R., et al. 1981. "The Population of the Kemp's Ridley Turtle in the Gulf of Mexico." In Biology and Conservation of Sea Turtles: Proceedings of the World Conference on Sea Turtle Conservation, edited by K. Bjorndal. Smithsonian Institution Press, Washington, D.C.

Pritchard, P. C. 1980. "Report on the United States/Mexico Conservation of Kemp's Ridley Sea Turtle at Rancho Nuevo, Tamaulipas, Mexico." Contact Report #14-16-002-80-216. U. S. Fish and Wildlife Service, Washington, D.C.

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