Posted by Rick Civelli | 09.06.2012 | Turtle Talk

Building a Nest Egg on Ostional Beaches

Sea turtles and humans are literally walking on egg shells trying to determine an optimal harvesting rate on the beaches in Ostional, Costa Rica. Recent research published in Chelonian Conservation and Biology by Valverde et al. hopes to inform these decisions, but definitive solutions may remain out of reach.

Ostional plays host to one of the most spectacular sea turtle events – the olive ridley arribada. Signaled by a common cue, thousands of females flood the Costa Rican beach and nest en masse over a one week period. This “arrival” highlights an unusual reproductive strategy unique to only two sea turtle species – the olive and Kemps ridley.

However, this influx of nesting females (observed as high as 476,550) comes with significant risk. The higher the density of nests, the lower the hatchling rate (sometimes as low as 6%). Many factors are attributed to the low nest success. In such a relatively small area, females dig up previously laid nests, killing any chance of development. Feral predators also seem to sync their arrival with that of the turtles. Insects burrow and infiltrate the nests. And with eggs in such close proximity to one another, viral and bacterial infection runs rampant.

Nest poaching is another factor, having gone on for decades, long before the scientific community even knew there was an arribada at Ostional. Hoping to combat the black market trade in eggs, officials made egg harvesting legal in 1987, but with obvious stipulations. Egg harvest only takes place during the first two days of the arribada. Collection is done  by the villagers of Ostional. The sale of the eggs brings revenue into the town, highlights the importance of the turtles, and the relatively cheap price discourages illegally harvested egg sales. Despite over 25 years of legal harvest, relatively little data had been collected to quantify the impact.

Valverde and his colleagues set out to remedy this by providing detailed figures on the both the nesting event and the collection. During their five years of research surveys they observed anywhere between 3,564 – 476,550 egg laying females climb the beaches of Ostional during an arribada. With an average clutch size around 98.9 eggs, simple math indicates that they are dealing with millions of eggs. Of the numerous nests laid in an event as few as 152 or as many as 8,138 clutches were legally harvested, an average of 21.2%.

The researchers concluded that the harvest rate seems to be above the carrying capacity on these beaches, however, since very little data has been collected the carrying capacity itself remains somewhat of a mystery. Despite the harvest of roughly a quarter of the nests, the hatchling success still remains low. Since olive ridleys become sexually mature in their early teens, researchers may be just beginning to see the impacts of this decades-long plan. And while the researchers have assembled a wealth of new data to inform the egg harvesting process, they have also raised many new questions about the program’s future.