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OCR for page 21
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
OCR for page 22
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
OCR for page 23
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
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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
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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,
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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.
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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
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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.
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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.
Representative terms from entire chapter:
sea turtles
