Arribadas
Posted by: 
alvarojm
N 09° 59.457 W 085° 41.956
16P E 642570 N 1104692
The beach of Ostional is the scenery for a rarely-seen biological wonder. Around the third quarter of the moon, hundreds, and sometimes hundreds of thousand sea turtles come to one specific mile of beach at Ostional to dig their eggs into the black, volcanic sand.
Waymark Code: WM7J3E
Location: Costa Rica
Date Posted: 10/30/2009
Views: 18
The Ostional Wildlife Refuge in Costa Rica was created in 1984 to protect one of the world's most important nesting sites of the olive ridley sea turtle.
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 to seven 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.
Used to a life in the ocean, the turtles painfully drag their heavy bodies over the beach until they get over the high tide line. There, flicking clouds of sand, they dig a nest with their flippers to deposit about 105 white, ping pong-shaped eggs.
Over the course of a five-day arribada nesting turtles will leave up to 10 million eggs on the beach of Ostional.
Amazingly, Ostional is the only beach in the world where egg poaching is legal. Scientists found out that most of the eggs deposed in the first nights of an arribada are destroyed by subsequent turtles who dig their nests.
Therefore, since 1987, the government of Costa Rica allows on an annual, temporary suspension of the international ban on turtle-egg taking that the community of Ostional may harvest the doomed eggs on the first two dawns of an arribada. In return, the community must protect the turtles, clean debris from the beaches and patrol day and night for poachers.
The baby turtles hatch within 45-54 days depending on incubation temperatures, which will also determine if they will become male or female. They face varying degrees of success in each of the clutches that are laid in large groups to increase their success of surviving.
In general the baby turtles hatch at night, but it may also happen that you are sitting in the afternoon on the beach and suddenly, next to you the sand becomes live and small heads pop up.
As soon as the hatchlings have struggled out of the sand, the race to the ocean begins. With eyes barely opened, the mini turtles smell the breeze and instantly know the right direction.
Women and children from the community of Ostional accompany the hatchlings as they clamber toward the sea, protecting them from dogs and vultures.
If you also want to help the small turtles you shouldn't carry them all the way to the beach as they need the run to develop their lungs. You can carry them a piece of way over the hottest stretch of sand and let them run the rest just trying to provide them some shade and keep the vultures away.
Visiting Ostional:
The Ostional Wildlife Reserve extends 15 km along the shoreline, including the beaches of Ostional, Nosara and Guiones. It is only a narrow, 200 m wide strand of beach, but extends inland along the estuaries of the rivers and mangrove swamps, protecting large colonies of birds. Offshore the reserve incorporates another 3 miles stretch of maritime zone.
Vultures in Ostional preying on turtle eggs
Without the occurrence of an arribada you can walk alone to the beach which is littered with the white shreds of broken turtle eggs.
During the day crowds of black vultures sit in wait for the small turtles or, impatiently with their beaks, pick and scratch in the sand to get to the tasty turtle eggs.
During nesting season (Aug - Nov) solitary turtles arrive almost every night at Ostional and sometimes the rare and endangered giant leatherback turtles and green sea turtles come to nest as well.
During an arribada you must check in with the ranger booth at the southern end of Ostional where you pay the entry fee of $ 6 and another $ 7 for the guide without whom you may not go to the beach. No flashlights or flash photography is permitted.
Species Link: [Web Link]
How often turtles cross: 
Months most seen crossing: Jul-Dec
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Visit Instructions:
Describe what happened. Example
"Mother Blanding had made her nest between the goldenrod and bluestem outside my window late last June.
By chance we were sitting on the porch when we noticed little blandings in the same location we had seen the mother take 2 months earlier."