The Amazing Olive Ridley Sea Turtle

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.