Image taken on Nov 7, 2015 shows Lora sea turtles leaving the sea to lay their eggs in nests at Ostional Beach, 295km to northwest of San Jose, capital of Costa Rica. [Photo/Xinhua]

SAN JOSE - Over a quarter of a million Olive Ridley sea turtles have lumbered ashore by Monday morning to nest at Ostional beach on Costa Rica's north Pacific coast.

Around 68,000 turtles laid eggs on Friday night, another 107,000 nested on Saturday, and 80,000 more on Sunday night, said Mauricio Mendez, a marine biologist and deputy director of the Tempisque Conservation Area.

It was the thirteenth mass nesting arrivals this year and Saturday's number of arrivals was probably the largest for a single day in recent years, he said.

The Ostional Wildlife Refuge is one of the two most important breeding sites in the world for the endangered species. The turtles, which lay their eggs over three to five days related to the moon's cycle, began arriving for November's nesting period on Friday.

The mass turtle arrivals, known in Spanish as "arribadas," occur from July through December, during Costa Rica's rainy season, when access to Ostional beach becomes more difficult due to flooding from the Nosara river.

But September, October and November are the peak nesting months, the expert said.

Due to the huge number of turtles arriving, and the amount of eggs they lay each day, subsequent groups of turtles often end up destroying nests dug up earlier.

As a result, Ostional is the only beach in the world where harvesting and selling sea turtle eggs is legal. Starting in 1987, authorities have allowed vendors to collect eggs during the first three nights of the arribadas.

In return, the villagers protect the turtles by keeping the beaches clear of debris, and running day and night patrols against poachers.