The Arribada: Mass Turtle Nesting at Ostional With a Biologist

On certain nights, hundreds of thousands of olive ridley turtles come ashore on one black-sand beach, and you're there with an expert.

When the Beach Begins to Move

Playa Ostional, on Costa Rica’s Nicoya Peninsula, is a long curve of black volcanic sand that looks ordinary for most of the year. Then, on certain nights, the waterline starts to stir. One olive ridley turtle hauls herself out of the surf, then another, and another, until the beach becomes an arribada, Spanish for “arrival”: a mass nesting that can bring hundreds of thousands of turtles ashore over a few days.

Watching from above the tide line with a biologist who has studied this coast for decades, there’s an unexpected calm, and without crowds you have time to understand why it matters so much.

Safety in Numbers

The olive ridley is the world’s most numerous sea turtle, but it still faces bycatch, coastal development and climate change across the Pacific. Several times a year, most spectacularly in September and October, vast numbers of females arrive within days, sometimes half a million in one event. It’s a strategy called predator satiation: vultures, coatis and ghost crabs feast, but can eat only a fraction of the eggs.

The black sand matters too. Nest temperature decides whether hatchlings are male or female, so scientists watch these nests closely for signs of a changing climate. Most arribadas start in the last quarter of the moon and peak on the darkest nights around the new moon, so reliably that research is planned around it.

Why You Need an Expert Beside You

Access to the refuge is regulated, and rightly. On peak nights hundreds of turtles may be nesting within a few metres of each other, and it would be easy to block their path or crush nests just below the surface. With a long-time researcher, you learn to read the tracks, why red-filtered light is used, and why silence matters once a turtle enters her egg-laying trance.

When a late arrival digs up a nest laid earlier that night, your guide explains why this apparent destruction is part of a bigger pattern, and how it led to Ostional’s much-debated way of protecting its turtles.

A Harvest That Protects the Beach

Unusually, Ostional allows a legal, government-approved egg harvest. Early nests are often dug up by later waves of turtles and, left exposed, those eggs would rot and draw even more predators. Letting the community collect a regulated share of these vulnerable eggs cuts waste and predator pressure, and gives the village a real reason to guard the beach against poachers. Residents patrol the shore and work with biologists, and the turtle population has held steady, in some cycles even grown: one of marine conservation’s rare success stories.

A Night on the Sand

As the tide drops, tracks overlap across the beach. Some turtles dig steadily, scooping a flask-shaped chamber with their back flippers; others rest before the slow crawl back to sea. When a turtle lays, she goes almost still, eggs dropping in a soft rhythm. On a new-moon night there’s only starlight, warm sand, the beat of the surf and the scrape of flippers.

Your Out Of Office specialist times your visit to the moon and tides, with a briefing beforehand, transfers and permits arranged, and flexibility if the turtles are late. Costa Rica was the first country in Central America to legalise same-sex marriage, and our Costa Rica guide covers LGBTQ+ rights and practical advice.