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2 Biology C f the world's 12 living families and approximately 250 species of turtles, only two families, together comprising eight species, are marine. The eight species have several common characteristics, including relatively nonretractile extremities, extensively roofed skulls, and limbs converted to paddle-like flippers with one or two claws and little independent movement of the digits. All are large turtles, with adult body weights of 35-500 kg, and all show various adaptations to the marine environment, such as large salt glands to excrete the excess salt ingested with seawater and food. The seven species of the family Cheloniidae, the hard-shelled turtles (as opposed to the leatherback in the family Dermochelyidae), have widely divergent and often specialized feeding habits: for example, the green turtle is an herbivore, and the hawksbill subsists largely on sponges. Reproductive behavior patterns are similar among the species, but some interesting variations are known. Each female lays about 100 eggs in a sand-covered cavity above the beach high-tide line, and, after an incubation period of about 2 months, hatchlings emerge usually at night. For most and probably all species, the sex of the hatchlings depends on incubation temperature. Historically, all sea turtles have been valuable to humans. The leatherback, although widely reputed to be inedible, is killed extensively 21
22 Decline of the Sea Turtles FIGURE 2-1 Sea turtles found in U.S. coastal waters. Source: Modified from Ross et al., 1989. Kemp's ridIey hawksbill ~ "~_ ~ green loggerhead 1 ~ ~ / ~ . . ~ \~ O ~t I' ' ~ ~3 Ott o leadierback / off r - Of ° O cat 1 ~ -aft 1' 5~9 / O 0/ 1 Gym
23 Biology for its meat and its eggs are eagerly sought as food. The green turtle is famous for "turtle soup" and steaks, and the hawksbill's "tortoiseshell" has been used for centuries in making ornamental articles. The olive ridley has been used for leather in recent decades and has served as a source of both flesh and eggs for human consumption. The loggerhead, whose shell lacks decorative appeal, is sought in some areas for its flesh and eggs. This chapter describes five of the eight species of sea turtles (Figure 2- 1) each in terms of its distribution, its population and habitats, its food habits, its reproduction and growth, and major threats to its survival. The species are discussed here in the order of their apparent need for immedi- ate protection in U.S. waters. KEMP'S RIDLEY General Description The Kemp's ridley is a small sea turtle, with adult females measuring 62-70 cm in straight carapace (upper shell) length (SCL) and weighing 35- 45 kg. Adults are olive green above and yellowish below. The Kemp's ridley is slightly larger and heavier, is lighter in color, and has a lower and wider carapace than its congener, the olive ridley. Its head is large, with strongly ridged, powerful, and massive jaws. The carapace almost always has five pairs of costar scutes (scales) and usually five vertebral scutes. The hatchlings are dark gray, weigh about 17 g, and are approximately 44 mm in carapace length. The committee's interim report dealt with this species (Appendix B). Population Distribution and Habitats Foraging Arec s Although most Kemp's ridleys are found in the Gulf of Mexico (Hilde- brand, 1982), they also occur along the Atlantic coast as far north as Long Island and Vineyard Sound, Massachusetts. Drifting hatchlings and young juveniles from the western gulf gyres apparently enter the eastern gulf loop current and are carried via the Florida current into the Gulf Stream and up the east coast (Carr, 1980; Collard, 19871. Hendrickson (1980) and Carr (1980) speculated that these young turtles were "waifs" and possibly lost to the population. The numbers returning from the northern excur- sion are unknown. Juvenile Kemp's ridleys tagged in the Cape Canaveral region move north with warming water and then south as water tempera
24 Decline of the Sea Turtles lures drop in the winter; that pattern suggests that the turtles have the migratory capability to move back into the Gulf of Mexico (Henwood and Ogren, 19871. Nevertheless, only when some of the many Kemp's ridleys now being tagged along the east coast are found in the gulf will their recruitment back into the breeding population be certain. Adults are found almost entirely in the Gulf of Mexico, where tag returns from cooperative shrimp fishermen from the United States and Mexico suggest an approximately equal distribution between the northern gulf and the southern gulf (Pritchard and Marquez M., 1973; Marquez M., in prep.~. Satellite-tracked females migrating north and south of the nest- ing beach at Rancho Nuevo remained in nearshore waters less than 50 m deep and spent less than an hour each day at the surface (Bytes, 1989), an observation that reinforces the belief that the Kemp's ridley is largely a benthic species. In the northern Gulf of Mexico, juveniles are most common between Texas and Florida (Ogren, 19891. There is no unequivocal evidence of juveniles in the southern gulf. Declining water temperatures apparently induce juveniles to move from shallower coastal areas presumably to deeper, warmer waters (pers. comm., I. Rudloe, Gulf Specimens Marine Laboratory, Panacea, Florida, 19891. Hatchlings spend many months as surface pelagic drifters (Carr, 1980; 1986a). How long they stay in this habitat, what they eat while there, and how they get back to the coastal regions are all unknown, although Col- lard (1987) summarized open-water observations of the species in the gulf. The life history of the Kemp's ridley might be easier to elucidate than that of other sea turtles, because it has a more restricted distribution and nesting location in the semi-enclosed Gulf of Mexico. Rudloe (pers. comm., Gulf Specimens Marine Lab, Panacea, Florida, 1989) suggested that during the postpelagic stages, the body size of Kemp's ridleys is positively correlated with water depth. In Louisiana, northwest Florida, and New York, the smallest juveniles are found in shal- low water of bays or lagoons, often foraging in less than a meter of water (Ogren; 19891. Larger juveniles and adults probably forage in open gulf waters. Bytes (1988) radio-tracked juvenile Kemp's ridleys in Chesapeake Bay, where he reported that they used the estuary for summer feeding, but dif- fered in habitat preference and behavior from loggerheads, which he also tracked: "The loggerheads . . . fed primarily on horseshoe crabs, Limulus polyphemus. The [Kemp's] ridleys, in contrast, occupied shallower forag- ing areas over extensive seagrass beds (Zostera marina and R2~ppia mar- itima), did not range as far with the tide and fed mostly on blue crabs (Callinectes sapidus). Strong site tenacity was displayed by both species
25 Biology once foraging areas were established." P. Shaver and D. Plotkin (pers. comm., Padre Island National Seashore, 1989) believed that loggerheads and Kemp's ridleys partitioned food resources in Texas: the ridleys for- age in shallower water and take the relatively fast blue and spotted crabs, whereas loggerheads are in deeper water and feed on slow-moving crabs. Marine areas within several kilometers of the nesting beach at Rancho Nuevo, Mexico, constitute important internesting habitat for Kemp's rid- leys. Satellite and radio-tracking studies have shown that the Kemp's rid- ley can wander many kilometers in Mexican waters from the nesting beach between nesting periods. Some mating occurs in March and April near the Rancho Nuevo nesting beach. Persistent reports of large num- bers of Kemp's ridleys just south of the Mexico-U.S. border before the nesting season also indicate that social and mating aggregations might occur many kilometers from the nesting beach. More observations are needed regarding this poorly known aspect of Kemp's ridley biology. Nesting Arec s The nesting beach at Rancho Nuevo is the primary terrestrial habitat for Kemp's ridleys, at about latitude 23°N on the Gulf of Mexico. Until recently it was a fairly steep sand-covered beach. During hurricane Gilbert in 1988, the beach was scoured, and that left a mixture of gravel, sand, and rock rubble. In 1989, females returned to Rancho Nuevo as in the past, but the primary nesting area was extended from the usual 15 km of beach an additional 15 km or more northward (pers. comm., I. Woody, USFWS, 19891. Only rarely has any substantial nesting been observed at any other beach (such as at Tecolutla, Veracruz; see Ross et al., 1989, for other scattered nesting sites). Nesting on the beach at Ran- cho Nuevo is clearly crucial to the species survival. Food Habits Hatchlings move quickly through the surf zone and into the pelagic zone of the Gulf of Mexico. Their feeding habits have not been observed in the wild, but it is presumed that they eat swimming and floating animal matter in the epipelagic zone. Juveniles, subadults, and adults feed on various species of crabs and other invertebrates (Dobie et al., 19613. In the northeastern United States, where juveniles are found, crabs of several species are common in stomach contents, whereas in the Gulf of Mexico, the blue crab is the most common item. Stranded dead Kemp's ridleys often have fish parts,
26 Decline of the Sea Turtles shrimp, and small gastropods in their guts, even though they appear to be too slow to catch these animals in the wild. Perhaps they learn to feed on the bycatch dumped overboard from trawlers (Shoop and Ruck- deschel, 1982; Manzella et al., 19889. Surprisingly, Kemp's ridleys raised in the laboratory on nonliving food will commonly capture and feed on live crabs as soon as crabs are provided. Food habits of adults in the southern Gulf of Mexico are not well documented. Reproduction and Growth Reproduction of the Kemp's ridley is different from that of other U.S. sea turtle species in four important ways. First, it nests in an aggregated fashion; many females gather in the sea near the nesting beach and then emerge to nest in a loosely synchronized manner over several hours in what is known as an "arribada" or "arribazon" pattern. An important amateur movie made by Andres Herrera in 1947 documented an arribada of approximately 40,000 females nesting on one day at the Rancho Nuevo beach (Carr, 1963; Hildebrand, 19639. Second, Kemp's ridleys nest during the daytime, whereas the other species nest at night. Solitary nesters, arribada groups, and even most captive reared females nest exclusively during the daytime. Third (and unique for sea turtles), almost all nesting occurs at one site a site in the state of Tamaulipas, Mexico, near Rancho Nuevo. Exceptions are occasional nests in Texas, a single recent nest in Florida, and a potentially important but irregular nesting area near Teco- lutla, Veracruz. Fourth, most females nest annually. The most reliable index of Kemp's ridley population size has been the annual count of nesting adult females at Rancho Nuevo. The number of nesting females there decreased from an estimated 40,000 (in a single day) in 1947 to an estimate of about 650 throughout the nesting season in 1988; the latter number was based on the total of 842 nests found (Ross et al., 1989; Appendix B). In 1989, even including the newly found exten- sion of the nesting beach some 15 km to the north, the total number of nests found (784) signaled a further decline of this species (pers. comm., J. Woody, USFWS, 1989~. As with other sea turtles, both males and females migrate toward the nesting area, and courtship and mating probably occur during several weeks before the female emerges to nest (Owens, 19801. Studies of cap- tive animals indicate that a single mating receptivity period is regulated by the female and occurs about 4 weeks before the first nest is dug (Rostal et al., 1988~. After the mating, fertilized eggs are stored in the oviduct until nesting. Nesting is usually restricted to April, May, and June-and occa- sionally July, if a cool spring delays the onset of reproduction.
27 Biology A female deposits one to four clutches per season, laying an average of about 105 eggs per clutch (Marquez M., in prep.~. Most females nest annually, based on the return of tagged animals, but they can also skip a year. Because nesting occurs over only about 45 minutes and because many turtles might nest simultaneously over several kilometers of beach, it has been difficult to tag or check for previous tags on every nesting female. Data on the nesting biology of the Kemp's ridley are, therefore, still incomplete. With the limited data available, the number of adult females in the world population can be estimated from the equation: Pnf = ~ - Pnf Nf where Pnf = Nt Nf Pnf = total population of adult females total number of nests per year average number of nests per reproductively active female proportion of females that nest in a given year Observers who have worked closely with Kemp's ridleys argue that the actual number of nests per year per female is not 1.3, as suggested by Marquez M. et al. (1981), but may be about 2.3 per year (Pritchard, 19901. If this proves to be true, nesting females are far fewer than was previously thought about 350, rather than 620 per year on average from 1978 to 1988. A firm estimate of Pnfis still not available. The incubation time of kemp's ridley eggs averages 50-55 days (Ross et al., 19891. Growth rates of wild hatchlings are unknown, and the small- est wild juveniles (about 20 cm SCL) found in the northern Gulf of Mexi- co are of unknown age. In captivity, on a carefully prepared high-protein diet, they can grow to 20 cm in 10-18 months (Klima and McVey, 19821. However, it might take 2 years or longer for Kemp's ridleys to reach that size in the pelagic zone. Standora et al. (1989) tracked and recaptured three juvenile Kemp's ridleys in Long Island Sound. They averaged about 6 kg in weight and gained 548 g/month during the summer. Animals hatched in captivity, released, and then recaptured after 2 years or more grew at rates that suggested that the turtles could reach adult size in 6 or 7 years. Marquez M. (in prep.) noted that many females continue to grow slowly after reaching maturity. Age, size structure, and sex ratios of the population are poorly known. Recent stranding records and the work of Ogren (1989) and collaborators suggest an increase in recruitment of small juveniles into the coastal habi- tats of the species in recent years. Danton and Prescott (1988) found a
28 Decline of the Sea Turtles male-to-female ratio of 20:28 in 48 stranded dead juvenile Kemp's ridleys (mean SCL = 27.1 cm) from Cape Cod, Massachusetts. Although the sam- ple size is small, it does indicate that the ratio is not strongly skewed. Sex ratios for other sizes and places have not been determined for Kemp's ridleys. Sex of a developing Kemp's ridley is dependent on the tempera- ture of egg incubation, on the basis of work with headstarted animals (Shaver et al., 19881. At higher incubation temperatures, more females are produced. Major Threats to Survival At various stages of their life cycle, Kemp's ridleys can be adversely affected by a number of activities and substances. These potentially include cold-stunning; human and nonhuman predation of eggs in nests; predation of hatchlings and/or older turtles by crabs, birds, fish, and mammals, including humans from foreign nations; ingestion of plastics; industrial pollutants; diseases; exploratory oil and gas drilling; dredging; explosive removal of oil platforms; and incidental capture in shrimping and other fishing gear. The relative impacts of these mortality factors are discussed in Chapter 6. LOGGERHEAD General Description Adult and subadult loggerheads have reddish-brown carapaces and dull brown to yellowish plastrons (lower shells). The thick, bony cara- pace is covered by nonimbricate horny scutes, including five pairs of costars, 11 or 12 pairs of marginals, and five vertebrals. Adult loggerheads in the southeastern United States have a mean SCL of about 92 cm and a mean body weight of about 113 kg, but adults elsewhere are usually smaller (Tongaland, Hughes, 1975; Colombia, Kaufmann, 1975; Greece, Margaritoulis, 19821. They rarely exceed 122 cm SCL and 227 kg. The brown hatchlings weigh about 20 g and are 45 mm long. Population Distribution and Habihts Foraging Areas The geographic distribution of loggerheads includes the subtropical (and occasionally tropical) waters and continental shelves and estuaries
29 Biology along the margins of the Atlantic, Pacific, and Indian Oceans. It is rare or absent far from mainland shores. In the Western Hemisphere, it ranges as far north as Newfoundland (Squires, 1954) and as far south as Argentina (Frazier, 1984) and Chile (Frazier and Salas, 19821. Nesting Areas ~ T . · · . . 1 · ~ 1 ~ ~1_ _ __ ~ _ ~ _ _ ~~ ~ ~ 1 Nesting is concentrated In the north ano south temperate zones ana subtropics with a general avoidance of tropical beaches in Central Ameri- ca, northern South America, and the Old World. The largest known nest- ing aggregation was reported on Masirah and the Kuria Muria Islands of Oman (Ross and Bar~vani, 1982), and a nesting assemblage has been noted recently on the Caribbean coast of Quintana Roo (pers. comm., R. Gil, Quintana Roo, Mexico, 19891. In the western Atlantic, most nesting occurs on Florida beaches, with approximately 90% in Brevard, Indian River, St. Lucie, Martin, Palm Beach, and Broward counties. Nesting also occurs regularly in Georgia, South and North Carolina, and along the gulf coast of Florida. Aerial beach surveys in 1983 estimated that 58,016 nests were dug along the southeastern United States (Murphy and Hopkins, 1984) and provided the best estimate of population size. Assuming a mean of 4.1 nests per female, approximately 14,150 females nested on the southeast coast in 1983 (Murphy and Hopkins, 19841. Those nests constitute about 30% of the known worldwide nesting by loggerheads and clearly rank the southeastern U.S. aggregation as the second largest in the world, only the Oman assemblage being larger (Ross, 19821. Recently, Witherington and Ehrhart (1989a) concluded that the stock of loggerheads represented by adult females that nest in the southeastern U.S. is declining. Evidence of a decline came from the current best esti- mates of adult females nesting each year (Murphy and Hopkins, 1984), published life tables and population models (Richardson and Richardson, 1982; Frazer, 1983b; Crouse et al., 1987), observed mortality rates in the southeastern United States, and observed population declines in South Carolina (pers. comm., S. Murphy, S.C. Wildlife and Marine Resources, 1989) and Georgia (pers. comm., J.I. Richardson, University of Georgia, 19891. Adult females generally select high-energy beaches on barrier strands adjacent to continental land masses for nesting. Steeply sloped beaches with gradually sloped offshore approaches are favored (Provancha and Ehrhart, 19871. After hatching and leaving the beach, hatchlings apparent- ly swim directly offshore and eventually associate with sargassum and debris in pelagic drift lines that result from current convergences (Carr, 1986a; 19871. The evidence suggests that posthatchlings that become a part of the sargassum raft community remain there as juveniles, ride cur
Jo Decline of the Sea Turtles rent gyres for possibly several years, and grow to 40-50 cm SCL. They then abandon the pelagic habitat, moving into the nearshore and estua- rine waters along continental margins, and use those areas as the devel- opmental habitat for the subadult stage. In such places as the Indian River Lagoon, Florida, the subadults are separated from the adults, whose foraging areas are apparently hundreds of kilometers away. Nothing is known about the transition from subadult to adult foraging areas, but it seems clear that adults can use a variety of habitats including the Atlantic continental shelf. Remote recoveries of females tagged in Florida indicate that many migrate to the Gulf of Mexico, often to the turbid, detritus- laden, muddy-bottom bays and bayous of the northern gulf coast (Meylan et al., 19831. Others apparently occupy the clear waters of the Bahamas and Antilles, with sandy bottoms, reefs, and shoals that constitute a totally different type of habitat. Nothing is known of the periods of time that loggerheads spend in these disparate habitats or of their propensity to move from one to another. Food Habits Although the list of food items used by loggerheads is long and includes invertebrates from eight phyla (Dodd, 1988), subadult and adult loggerheads are primarily predators of benthic mollusks and crustaceans. Coelenterates and cephalopod mollusks are especially favored by logger- heads in the pelagic stage (van Nierop and den Hartog, 19841. Posthatch- ling loggerheads evidently ingest macroplankton associated with "weed lines," especially gastropods in the sargassum raft community as well as fragments of crustaceans and sargassum (Carr and Meylan, 19801. Logger- heads sometimes scavenge fish or fish parts or incidentally ingest fish (Brongersma, 19729. Reproduction and Growth It has been assumed for some time that, at least for Florida logger- heads, males migrate with females from distant foraging areas to the waters off nesting beaches, where courtship and mating take place. Mat- ing takes place in late March to early June (Caldwell, 1959; Caldwell et al., 1959a; Fritts et al., 19831. Although a few adult males might remain off the Florida coast throughout the year (Henwood, 1987), most of them apparently depart by about mid-June. Females mate before the nesting season during a single receptive period and then lay multiple clutches in nests dug in the beaches throughout some portion of the nesting season
31 Biology (Caldwell et al., 1959b). Mean clutch size varies from about 100 to 126 along the southeastern United States coast. In the southeastern United States adult females begin to nest as early as the last week of April; nesting reaches a peak in June and July and contin- ues until early September. Loggerheads nest one to seven times per sea- son (Talbert et al., 1980; Lenarz et al., 1981; Richardson and Richardson, 19821; the mean is believed to be approximately 4.1 (Murphy and Hop- kins, 19841. The internesting interval is about 14 days. Loggerheads are nocturnal nesters, with infrequent exceptions (Fritts and Hoffman, 1982; Witherington, 19861. Good descriptive accounts of loggerhead nesting behavior have been given by Carr (1952), Litwin (1978), and Caldwell et al. (1959a). Remigration intervals of two and three years are most common in loggerheads, but the number can vary from one to six years (Richardson et al., 1978; Bjorndal et al., 19831. Natural incubation periods for United States loggerheads are about 54 days in Florida (Davis and Whiting, 1977; Witherington, 1986), about 63 days in Georgia (Kraemer, 1979), and about 61 days in North Carolina (Ferris, 19869. The length of the incubation period is inversely related to nest temperature (McGehee, 1979), and the sex of loggerhead hatchlings also depends on temperature (Yntema and Mrosovsky, 1980; 19821. Hatching success has been reported at 73% and 55% in South Carolina (Caldwell, 1959) and 56% in Florida (Withering/on, 19861. Growth rates of captive posthatchling and juvenile loggerheads have been reported (e.g., Witham and Futch, 1977), but no data are available on these stages in the wild. In captivity, young loggerheads can grow to about 63 cm SCL and 37 kg in 4.5 years (Parker, 19261. In wild subadults, linear growth rates vary from 1.5 cm/year in Australia (Limpus, 1979) to 5.9 cm/year in Florida (Mendonca, 19811. Growth rates of larger sub- adults decrease with increasing carapace length. Frazer and Ehrhart (1985) estimated age at maturity as 12-30 years. Hatchlings engage in a "swimming frenzy" for about 20 hours after they enter the sea, and that frenzy takes them 22-28 km offshore (Salmon and Wyneken, 19879. They become associated with sargassum rafts or debris at current rips and other surface water convergences and begin the juve- nile life stage (Carr, 1986b). After perhaps 3-5 years circumnavigating the Atlantic in current gyros (Carr, 1986a) or after reaching 45 cm SCL, they abandon the pelagic environment and migrate to nearshore and estuarine waters along the eastern United States, the Gulf of Mexico, and the Bahamas to begin their subadult stage. Henwood (1987) reported a ten- dency for subadults of the Port Canaveral aggregation to disperse more widely in the spring and early summer. Chesapeake Bay subadults exhib- it a variety of movements between waters of different temperature and salinity (Killingly and Lutcavage, 19831. Recoveries of females tagged
32 Decline of the Sea Turtles while nesting on the Florida east coast suggest that they dispersed widely to foraging areas in the Gulf of Mexico, in Cuba, elsewhere in the Greater Antilles, and in the Bahamas (Meylan et al., 19831. Those females appar- ently remigrate hundreds of kilometers at multiyear intervals to nest on the preferred, high-energy nesting beaches of eastern Florida. Much less is known about migrations of Georgia, South Carolina, and North Carolina nesters outside the nesting season, because of the dearth of reported tag recoveries. Females from Georgia dispersed along the Atlantic seaboard and did not appear in tropical waters outside the United States (Bell and Richardson, 19781. Major Threak to Survival Loggerheads are subject to numerous threats to their survival, including egg-collecting, raccoon predation on nests and eggs, and a variety of human activities such as beachfront development, increases in artificial illumination and disturbance, and incidental capture in shrimping and other fishing gear. They are also subject to effects of oil-platform removal, dredging, ingestion of plastics, and boat collisions. The relative impacts of these mortality factors are discussed in Chapter 6. GREEN TURTLE General Description The green turtle is the largest hard-shelled sea turtle. Adults have a carapace varying in color from black to gray to greenish or brown, often with bold streaks or spots, and a yellowish white plastron. Populations around the world differ greatly in adult size and weight; those in Florida average 101.5 cm SCL and 136.2 kg body weight (Witherington and Ehrhart, 1989a). Characteristics that distinguish them from other sea turtles are their small rounded head, smooth carapace, and four pairs of costar scutes. Hatchlings weigh approximately _~ =, about 50 mm long, and the ventral surface is white. Population Distribution and Habited ~ , , 2S it. their black carapace is Foraging Areas The circumglobal distribution in tropical and subtropical waters has been described by Groombridge (19821. In U.S. Atlantic waters, green
33 Biology turtles occur around the U.S. Virgin Islands and Puerto Rico and from Texas to Massachusetts. Important feeding areas for green turtles in Flori- da include the Indian River, Florida Bay, Homossassa Bay, Crystal River, and Cedar Key. Those areas and the Texas coast (Aransas Bay, Matagorda Bay, and Laguna Madre) figured heavily in the commercial fishery for green turtles at the end of the last century (Hildebrand, 1982; Doughty, 1984). Green turtles occupy three habitat types: high-energy beaches, con- vergence zones in the pelagic habitat, and benthic feeding grounds in rel- atively shallow, protected waters. Hatchlings leave the beach and appar- ently move into convergence zones in the open ocean (Carr, 1986a). When they reach 20-25 cm SCL, they leave the pelagic habitat and enter benthic feeding grounds. The foraging habitats are most commonly pas- tures of seagrasses or algae, but small green turtles are also found over coral reefs, worm reefs, and rocky bottoms. Some feeding grounds sup- port only particular size classes of green turtles; the turtles apparently move among these developmental feeding grounds. Other feeding areas, such as Miskito Cays, Nicaragua, support a complete size range of green turtles from 20 cm to breeding adults. Coral reefs and rocky outcrops near feeding pastures often are used as resting areas. The navigation feats of the green turtle are well known, but poorly understood. Hatchlings and adult females on the nesting beach use phot- ic cues to orient toward the ocean (Ehrenfeld, 1968; Mrosovsky and Kingsmill, 19851. Unknown are the cues used in pelagic-stage move- ments, in movements among foraging grounds, or in migrations between foraging grounds and the nesting beach. ~ _ Because green turtles feed in marine pastures In quiet, ~ow-energy areas and nest on high-energy beaches, their feeding and nesting habitats are, of necessity, some dis- tance apart. Green turtles that nest on Ascension Island forage along the coast of Brazil, well over 1,000 km away (Carr, 19751. The location of the foraging grounds of green turtles that nest in Florida is not known, and individuals foraging in Florida waters might not be part of the nesting population there. It has been generally accepted, but not proved, that green turtles return to nest on their natal beach. Green turtles do exhibit strong site fidelity in successive nesting seasons. Meylan (1982) has reviewed information on turtle movements based on tag returns. Nesting Areas Females deposit egg clutches on high-energy beaches, usually on islands, where a deep nest cavity is dug above the highwater line. Major green turtle nesting activity occurs on Ascension Island, Aves Island, in Costa Rica, and in Surinam. In U.S. Atlantic waters, green turtles nest in small numbers in the U.S. Virgin Islands and in Puerto Rico and in some
34 Decline of the Sea Turtles what larger numbers in Florida, particularly in Brevard, Indian River, St. Lucie, Martin, Palm Beach, and Broward counties. Food Habits Posthatchling, pelagic-stage green turtles are presumably omnivorous, but dietary data are lacking. When green turtles shift to benthic feeding grounds, they prefer to feed on seagrasses and macroalgae. Details of diet and nutrition of green turtles have been reviewed by Mortimer (1982a) and Bjorndal (19851. Reproduction and Growth Green turtles mate in the water off the nesting beaches. Evidence is accumulating that males might migrate to the nesting beach every year (Balazs, 19831. Females emerge at night to deposit eggs; the nesting pro- cess takes about 2 hours. Descriptions of their behavior have been reviewed by Ehrhart (19821. The females deposit one to seven clutches in a breeding season at intervals of 12-14 days. The average number of clutches is usually stated as two to three (Carr et al., 1978), but might be more. Mean clutch size is usually 110-115 eggs, but it varies among pop- ulations. The average egg count reported for 130 Florida clutches was 136 (Witherington and Ehrhart, 1989a). Only occasionally do females produce clutches in successive years; usually 2 years or more pass between breeding seasons. Hatching success of undisturbed nests is usually high, but predators destroy a high percentage of nests on some beaches (Stancyk, 19821. Many nests are also destroyed by tidal inundation and erosion. As with some other species, hatchling sex depends on incubation temperature (Standora and Spotila, 19851. Hirth (1980), Ehrhart (1982), and Bjorndal and Carr (1989) have reviewed the reproductive biology of green turtles. The numbers of recorded nestings in Florida were 736 in 1985, 350 in 1986, 866 in 1987, and 446 in 1988 (Conley and Hoffman, 1987; unpub- lished data, Florida Department of Natural Resources). It is impossible to assess trends in the nesting population from these data because the length of beach surveyed varied among years: 616 km in 1986, 832 km in 1987, and 971 km in 1988 (unpublished data, Florida Department of Natural Resources, 19881. Green turtles grow slowly. Rates of pelagic-stage green turtles have not been measured under natural conditions, but growth rates have been measured on the benthic feeding grounds. In the southern Bahamas, they
35 Biology grew from an SCL of 30 cm to an SCL of 75 cm in 17 years, and linear growth rate decreased with increasing carapace length (Bjorndal and Bolten, 19881. Estimates of age at sexual maturity range from 20 to 50 years (Balazs, 1982; Frazer and Ehrhart, 19851. Major Threats to Survival Over much of its range, the green turtle has been severely depleted because of high demand for both eggs and meat as human food. Exploitation has been intense on both nesting beaches and foraging grounds, and cannot be reversed quickly, because the green turtle takes several decades to reach maturity. Degradation of nesting and feeding habitats are also serious problems. HAWKSBlU General Description Adult Hawksbills are easily recognized by their thick carapace scutes, often with radiating streaks of brown and black on an amber back- ground, and a strongly serrated posterior margin of the carapace. Their common name is derived from the narrow head and tapering "beak." Except for Kemp's ridley, the hawksbill is the smallest of the five species, with an SCL less than 95 cm. A sample of 121 nesting females from sever- al localities around the Caribbean averaged 81 cm SCL (range, 62.5-91.4 cm) (Witzell, 19831. Hatchlings are brown to nearly black. Population Distribution and Habihts Foraging Areas Hawksbills typically forage near rock or reef habitats in clear shallow tropical waters (Witzell, 19831. That habitat is preferred for feeding on encrusting organisms, particularly some sponges. Hawksbills observed off the shore of Antigua (pers. comm., T. Fuller, Antigua, 1989) and Mona Island, Puerto Rico (Kontos, 1985) appear to be associated with benthic feeding territories, with the deeper territories used by the larger animals. Hawksbills associate with a variety of reef structural types from vertical underwater cliffs to gorgonian flats. Adults usually are not found in shal- low marine habitats (less than 20 m deep) near land, whereas small juve
1 36 Decline of the Sea Turtles niles are never far from the shallowest coral reefs. Much of the Caribbean down to 100 m or even more might provide foraging habitat for adults, because sponges grow well to these depths. Hawksbills are found throughout the Caribbean and are commonly observed in the Florida Keys, in the Bahamas, and in the southwestern Gulf of Mexico. They are not reported as frequently from shallow coastal systems with soft bottoms and high turbidity, such as the eastern U.S. coast north of Cape Canaveral. However, small juvenile Hawksbills have been caught in shallow nearshore areas of the Guianas, characterized by very muddy water, and adults have nested on adjacent beaches. Offshore behavior of Hawksbills is poorly understood. Adults (singly or in mated pairs) and large juveniles are commonly seen in all seasons well off the shore of Antigua and Barbuda in water up to 100 m deep (pers. comm., I. Fuller, Antigua, 19891. Presumably, these animals are for- aging, their presence suggesting an ability to dive to considerable depths to feed on live bottom-sponges. During the pelagic phase, hatchlings presumably associate with sargas- sum rafts in the Caribbean. Young individuals first appear as foraging res- idents of shallow reef systems when they reach 15-25 cm SCL. Hawksbills might be much more sedentary than other members of the family Che- loniidae (Witzell, 1983), but long-range tag returns indicate that hawks- bills can move hundreds of kilometers between their nesting beaches and foraging areas (Nietschmann, 1981; Parmenter, 1983; Bjorndal et al., 19851. When a young hawksbill changes from a pelagic feeder to a ben- thic-reef feeder, it apparently uses a foraging territory that it stays in until it shifts its foraging territory, probably moving from shallow to deep water as it becomes capable of deeper dives. Whether a neophyte breeder returns to the proximity of its natal origin is unknown. Understanding of neonate movements at sea is speculative. Prevailing winds and currents would carry Antillean hatchlings into a Caribbean sar- gassum gyro, with some transportation possible on currents north along the Yucatan coast to the western Gulf of Mexico. There is no evidence that Caribbean hawksbill hatchlings use the North Atlantic Sargasso Sea and its associated gyro, as U.S. Atlantic loggerheads apparently do. In the Azores, where many young loggerheads are found, juvenile Hawksbills are not known. Nesting Arec s Hawksbills nest on tropical islands and sparsely inhabited tropical con- tinental shores around the world. Eastern Atlantic nesting records are from only a few African locations and associated offshore islands (Brongersma, 19821. Western Atlantic nesting records extend from Brazil
37 Biology to Florida's southern Atlantic coast and include the islands and continental coastline of the Caribbean and the southwestern Gulf of Mexico (Campeche). Substantial nesting might occur on the continent and the offshore keys around the Caribbean and Lesser Antilles (Witzell, 1983; Pritchard and Trebbau, 19849. Although the hawksbill is often described as a dispersed nester (Pritchard and Trebbau, 1984), small nesting concen- trations do exist on Antigua, for example. However, nesting generally is distributed at low densities across much of the Caribbean. Nesting within U.S. waters follows the same pattern as in the Carib- bean at large. Scattered nesting can occur on almost any beach of the U.S. Virgin Islands (Boulon, 1983), Puerto Rico (including Vieques (Pritchard and Stubbs, 1982) and the Culebra group (Meylan, 1989), or southern Florida (Lund, 1985; McMurtray and Richardson, 19851. Higher nesting concentrations are found on remote islands, such as Mona Island off Puerto Rico (Thurston and Wiewandt, 1975; Olson, 1985; Kontos, 1988; Tambiah, 1989) and Buck Island in the Virgin Islands (Hillis and Mackay, 1989a). Nesting habitat varies from high energy ocean beaches shared with green turtles (Carr and Stancyk, 1975) to tiny pocket beaches several meters wide contained in the crevices of cliff walls. A typical nesting habi- tat is a low-energy sand beach with woody vegetation, such as seagrape or saltshrub near the water line. Some active nesting beaches have no exposed sand, but have woody vegetation growing to the water's edge. In contrast, hawksbills at Sandy Point, St. Croix, regularly traverse 30 m of open sand to reach an acceptable nesting habitat (pers. comm., K. Eckert, University of Georgia, 19891. A portion of the nesting beach in Antigua with vegetation set 30 m back from the water's edge is rarely used (pers. comm., T. Richardson, University of Georgia, 1989), but turtles nest regu- larly on either side where the vegetation is closer to the water. Food Habits Until recently, hawksbills were considered to be generalists, feeding on a wide variety of marine invertebrates and algae (Carr and Stancyk, 1975; Witzell, 19831. But Meylan (1988) showed that hawksbills specialize on sponges, selecting just a few genera throughout the Caribbean. Much of the other material in hawksbill stomachs was apparently ingested coinci- dentally while the animals were feeding on sponges. Neonates in captivi- ty appear to do well on a diet of sargassum (Pritchard and Trebbau, 19841.
38 Decline of the Sea Turtles Reproduction and Growth The predominant nesting months for hawksbills in Puerto Rico and the U.S. Virgin Islands are June to November, although some nesting can be documented for every month of the year (Witzell, 19831. Adult females can make their first appearance at a nesting beach any time from.June to September. If a population contains only a few animals, females that use a particular nesting beach might arrive rather irregularly, causing the apparent nesting season to vary widely from year to year. Such events might explain the differences in nesting seasons observed on Buck Island in the Virgin Islands over the last 10 years (Hillis and Mackay, 1989a). The modal number of nests per female during a single season in Antigua is five; individuals nest four to six times (Corliss et al., 19891. Estimates of clutches per year (Witzell, 1983) less than the Antigua num- ber possibly result from inadequate beach coverage, as has been docu- mented for other sea turtles (Tucker, 1989a). The interval between consecutive clutches averages 14 days in Antigua (Corliss et al., 1989) and Mona Island (Kontos, 1988), 16 days at Tor- tuguero, Costa Rica (Bjorndal et al., 1985), and 18.5 days in Nicaragua (Witzell, 19831. The modal remigration interval of nesting hawksbills is 3 years at Tor- tuguero, Costa Rica (Carr and Stancyk, 19751. An intensive survey of nest- ing hawksbills in Antigua produced no records of annual remigration, but 17 of 23 nesting turtles in 1989 had been tagged at the same beach in 1987 (Corliss et al., 19891. These preliminary results suggest a dominant 2-year remigration intermural. Hawksbill nesting behavior has been well documented (Witzell, 1983; Pritchard and Trebbau, 19841. Individuals usually take one or more hours to complete the sequence. Clutch size varies greatly from site to site (Witzell, 1983), but the average for eastern Caribbean animals is close to 150 eggs (Corliss et al., 19891; one clutch of 215 eggs was recorded. The mean incubation time to emergence of hatchlings in Antigua was 61 days in 1987 and 68 days in 1988, with a range of 20 days around the mean (Corliss et al., 19891. Hatching success measured for several beaches averaged close to 80% (Witzell, 1983; Corliss et al., 19891. Temperature-modulated sex ratios have not been documented in hawksbills, but are assumed to exist as in other sea turtles. Pritchard and Trebbau (1984) reviewed information on the growth rates of captive hawksbills. Hatchlings in captivity with saturation feeding reached a carapace length of about 20 cm SCL in 1 year and 35 cm SCL in 2 years. Hatchlings in captivity can reach 50 cm SCL in 4 to 5 years. Age to maturity is not known and has not been calculated for hawksbills.
39 Biology Little is known about hawkshill reproduction in the continental United States because the observed number of nests each year in Florida could have been made by as few as 1-5 females. Major Threak to Survive! The hawksbill is considered endangered throughout its world range primarily because of widespread harvest of turtles for the international trade in tortoiseshell products, polished shells, and stuffed turtles. Killing, specimens of almost any size for their valuable scutes is widespread. Additional killing of juvenile hawksbills for trade in stuffed specimens raises mortality to catastrophic levels. The diffuse nesting habits of the hawksbill retake systematic exploitation of the nesting females difficult, but also makes them hard to protect. Even when a nesting turtle escapes to the sea, the eggs commonly are taken by humans. In addition, the hawksbill is edible and is even the preferred turtle species in a few areas. In some parts of its range, especially in the Indian Ocean, an occasional hawksbill is highly poisonous. LEATHERBACK General Description The leatherback is the largest of all living sea turtles, attaining a length of 150-170 cm SCL and a weight that occasionally reaches 500 kg (rarely 900 kg). Its shell is unique in being covered with a continuous layer of thin, black, often white-spotted skin, instead of keratinized scutes. The carapace is raised into a series of seven longitudinal ridges. Other dis- tinctive features are the absence of claws, the absence of scales (except in hatchlings and very young animals), the long forelimbs (1 m), and the reduced skeleton. Many bones that are present in the shells of other tur- tles are absent in the leatherback (Pritchard, 19791. Population Distribution and Habitats Foraging Areas The leatherback is sometimes seen in coastal waters, but is essentially pelagic and dives to great depths. It is frequently encountered outside the tropics, even in latitudes approaching polar waters. For example, it is
40 Decline of the Sea Turtles often reported in the waters of New England and the Maritime Provinces of Canada, possibly as far north as Baffin Island. In the southern hemi- sphere, records exist from Tasmania and the southern tip of New Zealand. Nesting Arec s Leatherbacks nest almost entirely in the tropics, with extra-tropical nesting essentially confined to low-density nesting (about 20-30 turtles each year) in Florida and in South Africa. Nesting is usually colonial. The largest colonies use continental, rather than insular, beaches. In the west- ern Caribbean, nesting is frequent from northern Costa Rica to Colombia and in eastern French Guiana and western Surinam. Some nesting also occurs along the central Brazilian coast, and important colonies are found in northwestern Guyana and in Trinidad. In the Antilles, most nesting occurs in the Dominican Republic and on islands close to Puerto Rico, including Culebra and St. Croix (U.S. Virgin Islands). The St. Croix popu- lation is the largest, best-studied one in the United States. A few nests are recorded each year on many of the islands of the Caribbean. Leatherback nesting beaches have some common characteristics. The absence of a fringing reef appears to be important; most beaches have high-energy wave action and a steep ascent. They also have deep, rock- free sand and are adjacent to deep oceanic water. In the Guianas, adja- cent waters are relatively shallow, but the presence of abundant mud and the absence of rocks or coral apparently make these beaches acceptable for nesting. Food Habits Leatherbacks are primarily water-column feeders, rather than benthic feeders. Many species of coelenterates, especially jellyfish, have been found in their stomachs. They have numerous adaptations of the head and mouth for their diet. Their jaws are sharp-edged and scissor-like in action, and their throat musculature is highly developed to generate a powerful inflow of water as the prey is taken. In addition, the esopha- gus, which might be nearly 2 m long, is lined with thousands of sharp flexible spines, which are also found in other sea turtles. Because the spines are directed toward the stomach, when the water taken in with prey is expelled, the spines retain the food.
41 Biology Reproduction and Growth Leatherbacks can travel great distances between feeding and nesting areas, and migrations of tagged animals from nesting grounds in the southern Caribbean or the Guianas to the waters of New York or New England have been recorded. One postnesting female moved from the Guianas to West Africa within a few months. However, such demanding migrations do not appear to be undertaken annually, and almost all recorded remigrations of leatherbacks to their nesting grounds have been 2 or 3 years after initial tagging. Up to 10 nestings per season per female have been recorded, with a typical leatherback internesting interval of 10 days. Leatherback eggs are large, about 6 cm in diameter, but are not as numerous as those of other sea turtles. In the Atlantic, a typical nest includes 80-90 normal eggs but in the eastern Pacific, usually fewer than 60. Nests contain different numbers of yolkless, undersized eggs. Eggs hatch after about 65 days. Hatching success can approach 100% in an undisturbed natural nest, but on many beaches many eggs are lost to erosion a result of the high energy of the beaches favored by leatherbacks and the limited ability of such heavy and cumbersome ani- mals to travel far inland to deposit their eggs. Eggs can be transferred to hatcheries, but they need even more careful handling than those of other sea turtles, if viability is to be maintained during the transfer. Major Threats to Survival The products of the leatherback rarely, if ever, are featured in interna- tional commerce. The common belief that this species is inedible is unfounded; intense slaughter of nesting females occurs in many areas, such as Guyana, Trinidad, Colombia, and the Pacific coast of Mexico. Even in areas where the adults are rarely killed, egg collecting might be intense. Ingestion of plastics could be an important mortality factor. OLIVE RlDIEY The olive ridley (Lepidoc1'elys olivacea), although probably the most numerous sea turtle worldwide, is very rare in U.S. waters, and its status and future are not in the main, a direct United States responsibility. Details of its biology and reproduction can be found in Pritchard (19791.