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Mass Turtle Nesting at Ostional

Last Updated: Oct 15, 2012

olive ridley sea turtle
 - Costa Rica

Every year, thousands of Olive Ridley sea turtles storm the black sand beaches of Ostional in phenomenon known as arribadas, or mass nesting. Located on Costa Rica's north Pacific coast, a short jaunt from Nosara, the Ostional Wildlife Refuge is one of the world's most important nesting sites for this endangered species. Conservation efforts are in place to protect this crucial habitat and increase survival rates for newborn hatchlings.

Olive Ridley Turtles

There are only seven species of marine turtle in the world, and four – leatherback, Pacific green, hawksbill, and Olive Ridley turtles – nest along the country's Pacific coast. Ostional is considered the second-most important nesting site for Olive Ridley sea turtles, known in Costa Rica as the "tortuga lora". Small populations of hawksbill and leatherback turtles also nest at Ostional.

Olive Ridleys, sometimes referred to as Pacific Ridley turtles, are the smallest marine turtle species, weighing in around 70-100 pounds and measuring 2-2.5 feet in length. Their diet consists mostly of crustaceans, fish, mollusks and other small sea creatures. The U.S. Federal Endangered Species Act classifies Olive Ridley sea turtles as threatened, while the International Union for Conservation of Nature (IUCN) considers the species to be critically endangered.

Nesting Arribadas

During an arribada, tens of thousands of turtles arrive on Ostional's shores, to do their biological duty. While there are no hard rules for the turtles' nesting practices, arribadas follow the 28-day lunar cycle. They begin congregating offshore several days or weeks before nesting. Then, usually during the last quarter of the moon phase, on the month's darkest nights just before the new moon, the first wave of turtles hits the beach to lay their eggs. Over the course of the next three to seven days, thousands more nest along the beaches of Ostional.

Timing an Arribada

While Olive Ridleys nest at Ostional year-round, the green season months of May to December host the largest numbers of turtles. Nesting is at its peak from September through November, with an average of 100-200 females arriving each hour. To maximize your chance of catching an arribada, schedule your visit for this peak period, and crosscheck with a moon phase calendar to arrive during a new moon.

Turtle Tours

Turtles generally nest at night, between 8:00 p.m. and 4:00 a.m., but the Ostional Wildlife Refuge is only open from 8:00 a.m. to 4:00 p.m. Because the refuge's schedule does not overlap with turtle nesting habits, a guided turtle tour is your best opportunity for spotting nesting Olive Ridleys. Flashlights and flash photography are not permitted, so if you'd like to take photos of the nesting process, it is best to schedule a private tour around dawn (usually about 5:30 a.m.), when lingering turtles still dot the beach. You can arrange guided tours through the Ostional Development Association, or ADIO: 2682-0470.

Volunteer Opportunities

If you'll be in the Ostional area for an extended period of time, and have at least two weeks to spare, the Ostional Wildlife Refuge accepts volunteers year-round. A $20 per day donation covers food and lodging, and volunteers assist with data recording, egg counting, beach maintenance and other tasks integral to turtle conservation. Contact the Ostional Wildlife Refuge for more details: 2682-0937 or 2682-0400.

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