Distribution
Olive ridleys thrive in the tropical to warm-temperate Pacific and Indian
Oceans, but are uncommon in the Western pacific and eastern Indian Ocean. They
also inhabit the Atlantic Ocean along the west coast of Africa and northeastern
coast of South America. Although Olive Ridleys are seen as far north as Alaska
and as far south as New Zealand and Chile, their infrequent visits occur only
during unusually warm periods. While a few olive ridleys are present in the
West Indies, they do not occur in the Gulf of Mexico or in most of the North
Atlantic Ocean.
About 3-5 million years ago, when the Isthmus of Panama formed and closed
off the Atlantic from the Pacific, olive ridleys and Kemp's ridleys began
travelling down different evolutionary paths. We believe that at the time only
the ancestors of the Kemp's ridley sea turtle lived in the western Atlantic. In
the 1990s, geneticist Brian Bowen conducted studies that suggested olive
ridleys spread from the eastern Pacific to the western Pacific and Indian
Oceans. Olive ridleys in the eastern Pacific then died out apparently, perhaps
owing to climate change or loss of nesting beaches. When climatic conditions
improved about 200,000 years ago, olive ridleys expanded out of the Indian and
western Pacific Oceans and recolonized the eastern Pacific. At that time, the
genetic lineages suggest, they also moved into the southern Atlantic Ocean.
Olive ridleys can be seen at coastal waters, but scientists have reported
observing them far out to sea in drift lines of seaweed (Sargassum algae)
and floating debris along convergence zones between warm and cold water
regions.
While solitary nesting occurs on the beaches of 32 countries, arribadas
are found in only a handful. One of the arribada beaches, Playa Nancite
in Costa Rica, is protected as part of Santa Rosa National Park. Another Costa
Rican beach, Playa Ostional, is a national wildlife refuge, but it is also the
site of a village of 90 houses. Ostional's people collect olive ridley eggs in
a legal harvest. Little information is available about the effect of this
harvest on the turtle population.
In Mexico, olive ridleys nest in arribadas at Playa La Escobilla in
the state of Oaxaca. The beach is protected and studies suggest the ridley
population is actually increasing. Another large arribada beach,
Gahirmatha, is in Orissia, India. The beach is protected by the government but
increased fishing activity in the region has drowned many adult turtles.
Smaller arribadas of 2,000 - 20,000 turtles occur in other parts of
Mexico and India as well as in Nicaragua and Panama.
Because of olive ridleys' nesting behavior it is difficult to accurately
count them. They come and go constantly, literally piling up on top of one
another. Some fail to complete a nest because there is just no room on the
beach. Repeated attempts mean that they can be counted twice or three times. It
is also difficult to count solitary nesters because they visit so many
different beaches and in such small numbers. There are simply not enough
biologists to count them all (or perhaps not enough money to hire enough
biologists to count them all).
The most reliable estimates that I have seen suggest that annually there
are about 500,000 - 600,000 olive ridleys nesting at Costa Rica arribadas,
450,000 in Mexico, and 135,000 in India. There may be another 40,000 - 60,000
nesting at the smaller arribada beaches and as solitary nesters. Since
olive ridleys reproduce once every 1.7 years, on average, the total world
population of nesting females is probably about 2 million. If adult males occur
in similar numbers, there are about 4 million adults and anybody's guess as to
the number of juveniles. Thus olive ridleys are undoubtedly the most abundant
sea turtle in the world.
Unfortunately, this large number hides the fact that olive ridleys are in
serious decline. Consider that Mexico had more than 10 million olive ridleys
around 1950 and India supported about 600,000 olive ridleys as recently as
1994. Indeed, past arribadas of several hundreds to tens of thousands of
individuals have completely disappeared in Suriname and South Adaman Island off
the coast of Thailand. Ridley populations have declined precipitously in
Bangladesh, Burma (Mayanmar), Malaysia, Pakistan, and Sri Lanka.
Life History
Most olive ridleys lay 2 or 3 clutches of eggs each year they nest (2.2 on
average), numbering about 110 eggs weighing 1.4 oz. (40g) each. The clutches
are laid at intervals of 17-45 days during the nesting season. They usually
nest in the open sand but at Playa Nancite, Costa Rica, some turtles nest under
trees, a practice that tends to produce male hatchlings. Olive ridleys appear
to waddle along as they use alternate flippers to quickly ascend the beach and
leave a distinctive track about 30 inches wide. The female ridleys return to
nest every year or two.
Hatchlings weigh 0.6 oz (17g) and generally emerge from the sand at night.
The pivotal temperatures for sex determination are 86-88F (30-31C) at Playa
Nancite and about 84F (29C) at Gahirmatha, India. In Costa Rica,
temperatures of 90F (32C) and higher will produce hatchlings that are 100
female. In India, temperatures of 86F (30C) and higher produce 100 females,
It takes 50-65 days for an egg to hatch depending upon nest temperature.
On arribada beaches thousands of hatchlings swarm to the sea at
once, usually emerging at night. The beach literally spills open and erupts
with tiny turtles that quickly fill the ocean. Most biologists think that this
behavior has the effect of "swamping" predators, ensuring that many
hatchlings survive the gauntlet from the nest to the open ocean miles offshore.
The gauntlet begins while the eggs are still developing in the nests, which are
so shallow that it is easy for animals to dig them up. Only by overwhelming predators,
placing thousands of nests on the beach or by hiding nests one at a time on
widely scattered beaches, can olive ridleys hope to avoid their many predators.
At sea they face hungry fish. Here too, perhaps, the swarm of hatchings
provides safety in numbers, as a fish can take only one turtle at a time.