Each tour group is limited to 10 people, including the guide, and they can view the mass nesting, known as an arribada, for 30 minutes; previously they were allowed an hour. Ms. Rojas said her group of 15 guides had led about 30 tours this week.

More than 100,000 turtles have arrived so far, according to the union. After the chaos of the last arribada, it hustled to publicize rules meant to ensure the safety of the turtles. Visitors must not touch the turtles or use flash photography, and they must keep a safe distance from the laying females. Nelson Marin, a representative for the Environment Ministry, said in a statement that the rules would be enforced into the weekend as more tourists are expected to arrive.

Arribadas occur on two beaches in Costa Rica once or twice a month between August and November, and last four to seven days. They typically coincide with the third-quarter moon, so the next event was expected the first week of October. That this week’s arribada came early illustrates the olive ridley’s unpredictability, said Roldán Valverde, a sea turtle biologist from Southeastern Louisiana University.

The hourlong beach visit that each female turtle makes is taxing, he said. When a turtle arrives ashore, it waddles through the sand until it finds a spot to nest. Then it digs a hole with its hind flippers about 15 inches deep and lays its eggs. The egg-laying process takes 13 to 17 minutes, according to Dr. Valverde, after which the female covers its eggs in sand and darts for the ocean. An individual female can lay up to a hundred eggs in a single clutch.