Oceans & Turtles · Ostional, Nicoya, Guanacaste

Ostional Arribada: Thousands of Olive Ridleys Nest in September

Thousands of olive ridley sea turtles began coming ashore at Ostional National Wildlife Refuge in Guanacaste before dawn on Sept. 4, 2026, in what local guides called one of the biggest arribadas of the year. A second, unexpectedly long arribada started Sept. 16 and was still going nine days later, when refuge officials also reported five arrests for alleged egg theft and nest destruction.

By Pura-Verde.com Editors · Published October 30, 2026
Original report: The Tico Times: Thousands of Olive Ridley Turtles Come Ashore in One of the Year's Largest Arribadas (2026-09-04)

Before sunrise on Friday, Sept. 4, 2026, the beach at Ostional on Guanacaste's Nicoya Peninsula started filling with olive ridley sea turtles. By daylight, thousands were digging nests side by side in one of the mass nesting events known in Costa Rica as an arribada, Spanish for "arrival." Local guides and residents said it was among the largest of the year so far.

What happened at Ostional in September

The Tico Times reported the arribada the same day, noting that turtles were expected to keep arriving through Friday night and into Saturday. Teletica's coverage that afternoon also called it the biggest of the year and said local guides expected more turtles overnight.

No official count was released with those first reports. Turtle and nest totals are normally compiled by wildlife authorities and the local association after monitoring ends, so any figure you see quoted for this specific night should be treated with care.

The month didn't stop there. A second arribada began on Sept. 16 and turned out larger and longer than expected, Teletica reported on Sept. 24, when it had already lasted nine days with three or four more nights forecast.

Why September and October bring the biggest arribadas

Ostional is one of only a handful of beaches in the world where olive ridleys nest in these synchronized waves. Arribadas here tend to cluster around the last quarter moon, which fell on Sept. 4 this year, and can start a couple of days before or after it.

The rainy months from September through November usually bring the largest events. In a big year, tens of thousands of turtles can come ashore in a single night, and an arribada can stretch on for many nights. Dry-season events, roughly January to July, tend to be smaller.

There may be a wild card this season. In a Sept. 6 report, Teletica cited SINAC experts warning that El Niño conditions could alter how many turtles arrive, since temperature plays a key role in nesting. It's too early to say what that means for the season as a whole.

A refuge built around a community

The Ostional National Wildlife Refuge was created in 1983 and is managed by SINAC's Tempisque Conservation Area. It covers about 468 hectares of land and 8,000 hectares of sea along roughly 16 kilometers of coast, taking in the beaches of Ostional, Nosara, Pelada and Guiones.

What makes Ostional unusual is its legal, regulated egg harvest. When an arribada is large enough, the community development association, ADIO, can declare it an "arribada aprovechable" and its members collect a share of eggs during the first days of nesting. The logic is that eggs laid early are likely to be dug up and destroyed by later waves of turtles. The model has run for more than three decades, and Ostional is one of very few places on Earth where a turtle egg harvest is allowed.

Outside that system, taking eggs is a crime. During the mid-September arribada, refuge administrator Carlos Pizarro told Teletica that 5 people had been taken before prosecutors in Santa Cruz for alleged egg extraction and nest destruction. Officials also freed an adult turtle caught in illegal fishing gear and asked boat operators to slow down near the beach. Pizarro urged anyone buying eggs to buy only from ADIO's legal supply.

How to see an arribada responsibly

Watching an arribada is unforgettable, and it's also easy to do harm without meaning to. During a declared arribada, the beach is closed to anyone not with a certified local guide. Here are the main rules, plus a little common-sense courtesy:

• Go with a certified Ostional guide. Group size is capped at 9 visitors. Don't try to visit the nesting beach on your own, especially at night.
• No flash and no white light. Light can confuse and frighten nesting females, which may return to the sea without laying. Follow your guide's lead on any lighting.
• Watch your feet. Nests can be crushed underfoot, and hatchlings from earlier arribadas may be emerging at the same time.
• Keep your distance. Never touch, ride or block a turtle, and don't stand in front of one heading to the water.
• Book ahead. Guided slots fill fast during big events, so contact the local guide association in advance.

Hatchlings usually emerge about 45 to 50 days after nesting, so nests from early September would have been hatching into late October. For more on why lights matter so much on nesting beaches, read marine biologist Daniel Arauz's warning about beachfront lighting. You can also find other community sea turtle projects on our map, see our guide to sea turtle conservation in Costa Rica, or read about leatherbacks arriving early on the Caribbean coast.

Frequently asked questions

When is the best time to see an arribada at Ostional?

The biggest arribadas usually happen from September through November and tend to start around the last quarter moon. Smaller events also occur in the dry season. Local guides can tell you when one is underway.

Can you visit Ostional beach during an arribada without a guide?

No. Once an arribada is declared, the beach is closed to anyone not accompanied by a certified local guide, and groups are limited to 9 people.

Is it legal to buy turtle eggs in Costa Rica?

Only eggs from the regulated harvest run by the Asociación de Desarrollo Integral de Ostional are legal. Taking or selling eggs from any other source is a crime.

How many turtles come to Ostional in an arribada?

It varies widely. Dry-season events may bring fewer than 20,000 turtles over several nights, while the largest rainy-season arribadas can bring tens of thousands in a single night. Official counts for September 2026 weren't published in the initial reports.

Sources and original reporting

1. The Tico Times: Thousands of Olive Ridley Turtles Come Ashore in One of the Year's Largest Arribadas (2026-09-04)
2. Teletica: Video: así fue la arribada más grande del año en Ostional (2026-09-04)
3. Teletica: Por qué el fenómeno de El Niño afectará el arribo de tortugas lora (2026-09-06)
4. Teletica: Gigantesco arribo de tortugas lora ya suma nueve días en Ostional (2026-09-24)

We wrote this story from the published reports and official sources above. We didn't interview the people named unless the story says so.