The beach of Ostional is the scenery for a rarely-seen biological wonder. Around the beginning of the last quarter of the moon cycle, hundreds, and sometimes hundreds of thousands sea turtles come to one specific mile of beach at Ostional to dig their eggs into the black, volcanic sand. Only olive-ridleys and their close relatives, Kemp’s Sea Turtles, the latter an Atlantic species synchronize their nesting in mass emergences or “arribadas”, the Spanish word for arrivals.
Some days or weeks before the mass nesting, the “flotilla”, an increasing number of turtles, congregates close offshore. After some days, prompted by some secret signal, the “arribada” will begin. At first, a few hundred turtles will come out on the beach, followed by a steady stream of animals for the next three days.
Turtles nest at Ostional year round, but peak time is during rainy season. From August through December arribadas occur regularly once, sometimes even twice a month and the number of nesting females are in the range of hundreds of thousands as opposed to tens of thousands for the dry season months. The largest “arribada” thus far recorded in Ostional, took place in November 1995 when a calculated 500 000 females came ashore. The turtles generally ride in on the high tide at night but during an arribada they start arriving around 4 p.m. and keep coming until 7 a.m. the next morning.