New research suggests nesting turtle population is half previous estimates

New research from Swansea University has suggested that nesting turtle population figures have been overvalued and their real numbers could be nearer to half of previous estimates.

Green Turtle 2The study, by Nicole Esteban of the Biosciences Departments at the College of Science, is the cover story this week in the Proceedings of the Royal Society B.

The study focussed on female green turtles throughout their nesting season in the Chagos Archipelago in the Indian Ocean, where individual turtles often nest several times in a single season on the sandy beaches.Traditionally, the size of the turtle population is assessed by counting their tracks on the beaches and dividing them by a nominal value for the expected number of clutches per season per individual turtle.

However, in the new study the researchers used state-of-art high resolution GPS satellite tags to track female green turtles throughout their nesting season to assess when and where they nested. The research team found that the nest locations of individual turtles were often spread over several tens of kilometres of coastline.

The team assessed satellite positions and found that there was an average of 6 clutches of eggs being laid by individual turtles which was about twice as many as previously estimated. Turtle population estimates are usually calculated from the number of tracks recorded on a nesting beach, and these findings suggest that the actual number of nesting turtles may be almost 50% less than previously assumed.

Mrs Esteban who led the study said: “These results are all the more surprising because sea turtle nesting is relatively easy to observe. However our study demonstrates that the traditional approach of direct observations of nesting has led to a gross underestimate of the fertility and consequent overestimate of population levels of turtles in the Indian Ocean as well as other study sites in the Atlantic Ocean.

“Our research is important as estimating the absolute number of individuals in populations and their fecundity is central to understanding the ecosystem role of species and their population dynamics as well as allowing informed conservation management for endangered species such as sea turtles

“Our study demonstrates that high-resolution tracking can reveal the extent of coast used by individuals during a nesting period. The tendency to spread nests across multiple sites may improve the resilience of sea turtles to loss of a nesting beach.”

 Green Turtle"Our work and that of others convincingly shows the benefit of accurately assessing the clutch frequency of sea turtles by satellite tracking and we suggest there is value in extending this approach to other sea turtle rookeries around the world to improve sea turtle population estimates.”

Read the study here

Read more about sea turtle conservation research in British Indian Ocean Territory and the commemorative stamps that celebrate it here

Picture: Mrs Esteban with a green turtle