What Is an Arribada? Costa Rica’s Mass Turtle Nesting Event Explained

Every year, on a stretch of dark volcanic sand on the Nicoya Peninsula, one of Costa Rica’s most remarkable wildlife events unfolds. Thousands, and during the biggest months hundreds of thousands, of olive ridley sea turtles come ashore at Ostional National Wildlife Refuge to nest in a synchronized event known as an arribada.

The word arribada means “arrival” in Spanish. At Ostional, it refers to a mass nesting event in which sea turtles emerge from the Pacific over several nights, dig nests close together, lay their eggs, and return to the ocean. For anyone trying to time a visit, the answer is not as simple as choosing a date. Ostional follows two overlapping calendars: the season and the moon.

Olive ridley turtles nest at Ostional throughout the year, but the largest arribadas usually happen during Costa Rica’s rainy season, especially from July through November. September and October tend to produce the biggest events, when hundreds of thousands of turtles may come ashore over three or four days.

The dry season, from December through June, can still bring arribadas, but they are usually smaller and shorter. Instead of the huge rainy-season waves, dry-season events are more likely to involve tens of thousands of turtles over two or three days.

That makes late June a transition period. Turtles may still arrive, but the peak spectacle is usually still ahead. For those planning around the 2026 season, the next new moon in Costa Rica falls on July 14, making the days before that a window to watch closely. Even then, anyone thinking of going should confirm conditions with local guides or refuge staff before making the trip.

The moon matters because arribadas are not fixed to a monthly calendar. They often begin in the darkest part of the lunar cycle, usually in the days before a new moon. The pattern repeats roughly every 28 to 30 days, but it is not perfectly predictable. That is why nobody can promise an exact arribada date, months in advance. The honest answer to “when do the turtles arrive?” is a season, narrowed to a few days around the right moon phase, then confirmed locally once turtles begin gathering offshore.

The buildup often starts in the water, where large numbers of turtles collect just beyond the surf before nesting begins. Then the first turtles come ashore. What starts slowly can become a steady stream, with females dragging themselves above the high-tide line, digging nests with their back flippers, and laying around 100 soft, white eggs before covering the nest and returning to the sea.

The process is powerful, but it is not tidy. Later turtles often dig into nests laid earlier in the event, destroying some of the eggs already in the sand. That high natural egg loss is one reason Costa Rica allows a rare legal egg-harvest program at Ostional.

Under a regulated system, members of the Ostional community may collect a limited portion of eggs during the first days of an arribada, when those eggs have the lowest chance of surviving because later nesting turtles are most likely to dig them up. In exchange, the community helps patrol the beach, remove debris, and protect the turtles and remaining nests from poachers.

For visitors, the main rule is simple: do not go alone. Access to the beach during nesting events is restricted, and travelers must enter with an accredited local guide. Flash photography is not allowed. Visitors should avoid white lights, keep a respectful distance, never touch turtles or eggs, and follow guide instructions at all times.

Ostional is not a zoo or a staged tourist attraction. It is one of the few places on Earth where olive ridley sea turtles still nest in mass numbers, and the rules are there to keep the event from being loved to death.

For the best chance of seeing a true arribada, travelers should plan for September or October, check the new moon dates, and stay flexible by a few days. Those visiting outside the peak months may still see turtles, but the largest arrivals belong to the heart of the rainy season.

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