Ostional, Costa Rica
During January 1999 I worked as a volunteer with the Douglas Robinson Marine Turtle Research Centre at the Refugio Nacional de Fauna Silvestre de Ostional (Ostional National Wildlife Refuge) in Costa Rica. The centre researches the population of Olive Ridley turtles (Lepidochelys olivacea.) that nest on the beach at Ostional. The Olive Ridley is known locally as Tortuga Lora.
The Olive Ridley Turtle
The Olive Ridley is the smallest of the seven species of marine turtles. A mature turtle has an average carapace length of about 60cms and weighs about 40kgs. While little is known about their life expectancy, it is known that females mature at around 25 years, indicating that the turtles are very long lived.Occasional individual Olive Ridleys will come at night to the 1km beach at Ostional to nest throughout the year. However, the species is unique amongst marine turtles in that the vast majority of turtles nest at the same time of the month.
At around the time of last quarter of the moon (half-moon, waning) the females will come to nest in large aggregations known as arribadas (arrival). During the peak nesting season, around August, up to half a million turtles will nest on the 1km beach over a ten day period. During the off-peak season, January to May, the arribadas are smaller and shorter, with "only" a few thousand arriving over a few days.
This mass nesting behaviour is all the more surprising considering the range of the Olive Ridley. The population that nests at Ostional feeds on shrimp off of the western American continental shelf, up to 60kms offshore, between the north of Chile up to Baja California. Why such a wide ranging creature is so fussy about where it nests, and why the entire population should choose a single 1km stretch of beach is a mystery.
Each female will come and nest between three and six times a year. She will walk up the beach to above the high tide line, create a depression with her body, and then use her hind flippers to dig a hole in the sand. She will lay about 100 ping-pong ball sized eggs, then fill in the hole and scatter the sand around in order to hide the nest. She will then return to the sea, this being the only time she will spend on land. The whole nesting process typically takes 60 to 90 minutes and this is the females' only contributing to the success of her offspring.
The eggs take approximately 40 days to hatch and the hatchlings then take another three to seven days to dig themselves out. Their final emergence appears to be temperature dependent, most of them emerging at night. As with all reptiles the ratio of males to females hatched depends upon the ambient temperature, more females being produced when it is warmer.
The hatchlings will walk down the beach and swim off into what is known as "the lost year", for nobody has yet determined where turtles go in the first year of their life. While the fertility of the eggs usually exceeds 80%, it is estimated that only 1% will survive to maturity.
While the Olive Ridley is not considered to be endangered, little is known of their global population. This is due to the lack of research world wide, exacerbated by the difficulties of tracking pelagic species and the fact that the turtles have a life expectancy far greater than that of any research programmes.
What is known is that the number of arribada sites has greatly decreased over the last 20 years. In 1979 large arribadas were regular occurrences at 17 different locations globally, including beaches in Mexico, Costa Rica, the Guianas, India, Sri Lanka, Malaysia, the Arabian peninsula, and the Eritrean coast. They were then known to range along the Pacific American coast, from west Africa to north-east America in the Atlantic, and from Japan to Africa in the Indian ocean.
Now there are only three major arribada sites world wide, two on the Pacific side of Costa Rica (Ostional and Nancite) and one in India. There are still some other beaches where a few hundred turtles will nest over a year -- and some beaches in Mexico are experiencing increasing numbers -- but most of the large arribada sites are no longer used.
The problems faced by the Olive Ridley are, of course, both natural and man-made.
The risks start with the nesting sites. The turtles are scared by artificial lights, so as stretches of coastline become more populated and acquire electricity, the already fussy turtles find fewer and fewer suitable nesting sites.
Once laid, the eggs face being dug up by dogs, raccoons, crabs, and humans.
The tiny hatchlings, of course, are likely to become food for a variety of species when at sea, however they may not make it that far. On their way from the nest to the water they are likely to fall prey to a variety of predators, including large flocks of vultures (the only case I've seen of vultures actually killing something).
A recently discovered problem in adult turtles is Fibropapilloma. This is a cauliflower like cancerous growth, connected with a strain of herpes virus, that appears on the flippers, eyes, and throat. The turtle does not die as a direct cause of the cancer. Rather, the fibropapilloma results in the turtle not being able to see food so well, not being able to catch it so well when it does see it, and finally not being able to swallow the food. The turtle dies of starvation. Fibropapilloma is a world wide problem affecting all species of marine turtles as well as whales and dolphins. It is estimated that 10% of the Ostional Olive Ridley population are affected. 90% of the Hawaiian Green turtle are affected.
Removing a long line hook from a washed up dead turtle which had eaten the hook and got tangled in the line.
Adult turtles may be attacked by sharks, but it is more likely that they will be killed by human fishing activity.
Shrimp trawlers fish in the feeding areas of the turtles. While turtle excluder devices (TEDs) exist for trawler nets, few fishing fleets in the developing world fit them. The turtles get caught in the nets and, being dragged around underwater for a long period, finally drown.
indiscriminate technique of long line fishing also kills turtles, either as a
result of the turtle eating the hook, or by getting caught on the hook and
tangled in the line.
Long line fishing is a commercial technique that deploys a line several kilometres long, supported by floats, from which hooks are hung every few metres.The fishing boat runs along the length of the line, taking fish off of the line brought onto the bow and redeploying the line off of the stern. Long lines are a totally indiscriminate way of fishing.
Turtle eggs, in Costa Rica and in many other parts of the world, are considered to be an aphrodisiac (for men!). International trade in all turtle products, including eggs, is banned by the Convention on International Trade in Endangered Species (CITES).
In fact, the beach at Ostional is the only place worldwide where the collection of turtle eggs is legal. This scheme was introduced by the founder of the refuge and biologist of the Douglas Robinson Marine Turtle Research Centre, Anny Chaves, as a conservation measure.
As the turtles come to nest in such large numbers on such a small beach, it is inevitable that some of the later turtles will inadvertently dig up and destroy the nests of the earlier turtles. In fact, about 30% of the eggs laid during any arribada will be destroyed by later nesting turtles. The community of Ostional (pop. ~400) is allowed to dig up eggs during the first couple of days of each arribada, resulting in about 10% of the eggs being collected. These are the eggs that would most likely have been destroyed anyway.
The eggs are then distributed and sold throughout Costa Rica at a government regulated low price -- half the price of a chicken egg. This has had a dramatic effect on the previously strong black market. With turtle eggs easily and cheaply available in the markets, it is no longer worthwhile for people to sneak onto beaches in the middle of the night to dig up a few eggs. This has had a positive effect on all five marine turtle species that nest in Costa Rica.
My work with the research centre
The voluntary work at the Douglas Robinson Marine Turtle Research Centre during my stay consisted of:
- population size studies by:
- counting tracks early in the morning outside of the arribada period
- statistical counting by quadrats, every 2 hours at night during the arribada,
- counting of individuals when it was light (I counted 805 turtles on the beach one morning)
- examining turtles for tags and for fibropapilloma and other obvious health problems
- protecting hatchlings from predators and helping them reach the water
- noting washed up dead turtles killed by trawler nets and long line fishing
- beach patrols and education of over zealous tourists using lights and flashguns
In addition I had a little excitement in San Jose where Leslie du Toit and Anny Chaves of the research centre, and myself, found some market stalls selling tortoise shell (taken from the Hawksbill turtle) and black coral products, both illegal under Costa Rican law and by international treaty. We feigned interest and returned the next day accompanied by officials from the Ministry of the Environment and Energy, who confiscated all the goods.
Ostional Wildlife Gallery
Storks, Spoonbills & Vulture