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II. HABITAT AND
DISTRIBUTION
Regional Occurrence:
The green turtle is a circumglobal species with foraging and nesting occurring
throughout the tropics.
IRL Distribution:
Chelonia mydas is found throughout the Indian
River Lagoon.
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
Adult green turtles weigh
between 110-205 kg (250-450 lbs), with lengths averaging 91-122cm (36-48 in.).
Reproduction:
Sexual maturity estimates range from 12-35 years with
Lutz and Musick (1997) reporting 25-35 years. However, Female green turtles may
typically mature at 8 to 25 years of age (J. Provancha - pers. comm.).
Copulation occurs at sea at no particular time of day or night and nest
deposition occurs predominately at night on ocean beaches with well drained sand
dunes. It is estimated that individual green turtles nest approximately three
times per season and have remigration intervals of 2.86 years. The result is
that in some years large numbers of turtles have cycles that match, therefore
yielding considerably higher nesting densities in a given location (Miller
1997).
Nests are clutches of approximately 112 eggs
requiring up to 60 days to develop and hatch. Several factors affect the sexual
determination of sea turtle hatchlings but much of it can be accounted for by
sand temperature around the nest site. Generally, the higher the temperature of
the nest environment is above the critical temperature, the more females are
produced (Mrosovsky and Yntema 1980).
IV. PHYSICAL TOLERANCES
Temperature:
The behavior and distribution of this species is obviously affected by cold
temperatures that are common in the northern IRL area during January and
February. When water temperatures within the IRL drop below 8 °C, turtles
become lethargic and float to the surface (Witherington and Ehrhart 1989).
During the winter period they roam greater distances and in unpredictable
patterns within the lagoon when compared to summer activities. The atypical
movements during winter are assumed to be attempts to leave the lagoon which,
due to its configuration, acts as a trap (Mendonca 1983).
V. COMMUNITY ECOLOGY
Trophic Mode:
The youngest green turtles are omnivorous. Among other things, their
gut contents often include ctenophores, insects, pelagic snails, and small fish
(Carr 1986). As they move into various habitats, their diets are adapted
to the new surroundings. Juvenile greens shift to herbivory, which includes the
consumption of macroalgae and seagrasses (Mendonca 1983, Green 1993, Balaz
1982).
Competitors:
Nest Predators:
Predation on nests by raccoons (Procyon lotor) and to a lesser degree,
feral pigs, cats and ghost crabs is an additional threat. The levels of
predation vary with the area and predator control methods. In other regions of
the world, predators include foxes, jaguars, ghost crabs and humans.
Habitat:
The various life stages of the green turtle utilize bays, lagoons, and the
mouths of rivers. As with other sea turtles, nesting adults migrate to ocean
beaches to deposit their eggs for incubation. Although nesting in Florida
appears to be increasing in the late 1990’s, key nesting sites are found in
Costa Rica, eastern Surinam, islands off of Queensland, Australia and Ascension
Island (Ernst and Barbour 1989).
Green turtle hatchlings from Florida emerge
from the nesting beach and enter the sea, swimming out to the Sargassum
driftlines where they find shelter and food (Carr 1986). These post-hatchling
greens apparently drift with oceanic circulation patterns toward the eastern
Atlantic where they forage and grow along the Azores and Canary Islands. In
the vicinity the IRL, green turtles use both beach and estuarine habitat
areas. The estuary provides a developmental habitat for immature greens
(post-yearling to subadults). The Mosquito Lagoon probably represents the
northern most winter range for this species. Mendonca and Ehrhart (1982)
estimated this population to be about 135 animals but later, Witherington and
Ehrhart (1989) speculated that the population was growing, based on data from
cold stunning events. During the winter of 1989, 209 greens were found stunned
in Mosquito Lagoon after a cold period (Schroeder et al. 1990).
The beaches along the IRL provide nesting
habitat for green turtles, although their numbers relative to loggerhead turtles
are quite low. Greens account for about 2 % of the sea turtle nests deposited
along IRL area beaches.
VI. SPECIAL STATUS
Special Status:
Endangered Species
Notes on Special Status:
The green turtle, listed by the USFWS as an endangered species, is
commercially valued in many countries for it's meat, eggs and calipee (the cartilaginous
portions of the shell). Threats to the species are numerous and include habitat
alteration, boat strikes, nest predation, oil pollution, entanglement, marine
debris, disease and incidental capture is fisheries. Until the 1970's, the
harvesting of turtle eggs in Brevard county was not uncommon.
As with other sea turtle species, continued
existence of this species is severely threatened by human impacts, especially
from fishing operations. Shrimp trawlers in particular (Lowe et al 1990)
cause the majority of incidental deaths of sea turtles. Nest predation is an
additional threat, particularly in areas where no predator control methods are
employed around nest
sites. Additionally, disorientation and
misorientation of newly emerged turtles is becoming an increasing threat to the
species along Florida’s coast and many other locations in the world.
Various human structures with night-time lighting have been implicated in
disorientation events (Raymond 1984, Witherington 1992) which lead turtle
hatchlings away from the ocean and into roads and other hazards.
An additional threat to this species is a
disease known as fibropapillomatosis, which is an epizootic affecting a variety
of species but has been primarily reported for the green turtle. Lesions
resulting from the disease have been documented since 1938 but have been noted
worldwide with increasing frequency since the late 1980’s (Herbst 1994). The
disease appears most frequently in juveniles and to date the etiological agent
is unproven.
Benefit in the IRL:
Aesthetic
Economic Importance:
None currently. However,
the Florida
fishery for immature green turtles was not outlawed until 1974. 19th century
reports indicate the green turtle fishery in the region began in about 1878,
with the species becoming severely diminished over a period of 17 years of heavy
exploitation. Fisheries
data for the Mosquito Lagoon in the late 1800s indicated that green turtles were
often exported from Florida, with as many as 150 greens exported in 1879.
By 1895 captures of estuarine green turtles declined dramatically
(Ehrhart 1983).
Report by: J. Dineen,
Smithsonian Marine Station
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irl_webmaster@si.edu
Page last updated: July 25, 2001
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