Managing nesting hawksbill turtles (Eretmochelys imbricata) on a small tropical island.

Abstract

Although human impact has caused the hawksbill turtle (Eretmochelys imbricata) to be on the brink of extinction, conservation efforts are on the increase and management strategies are being re-defined to manage the species towards rapid population increases. Numerous conservation measures have been applied and tested, with varying levels of success, to minimize threats and increase hatchling recruitment success rates. Conservation management is imperative for the survival of this species and has been shown to be easiest and most feasible at the nesting and hatching stages. On Cousine Island where this study took place, a turtle monitoring and management program was initiated in 1994 to protect nesting females and incubating eggs at the nesting and hatching stages of the lifecycle. Controversial conservation measures such as the handling of eggs, relocating clutches and application of crab proof barriers, have been applied on an ad hoc basis to minimize the high levels of mortality from natural erosion and crab predation. The merit of these practices, such as the translocation of egg clutches and the implementation of these specific barrier methods has never to our knowledge been scientifically tested using controlled experimentation. The first aim of this study was to find the most effective crab barrier method for increasing hatchling recruitment success and minimising predation rates with limited negative consequences. The second aim was to decipher crab density trends in order to offer a proxy for the number of crabs a nest might be exposed to within an area and develop beach profiles along the entire nesting beach to provide an expression of beach morphology to minimise nest loses from erosion. Finally, we analysed 10 years of nesting ecology and hatching success data with the aim of providing nesting and hatching trends to inform management decisions on Cousine Island into the future. Results across this study indicated that management measures can have both positive and negative effects on the nesting ecology and hatchling recruitment success in particular. We found that hawksbill clutches incubating without crab barriers are susceptible to losing on average a third or more eggs to crab predation than those with a crab barrier in place. Nests which were protected with netting had significantly higher hatchling recruitment success rates and nests protected with fencing had significantly lower predation rates than control nests. The use of netting, however, had a substantial cooling effect which can potentially distort natural sex ratios. Results across the entire study indicated that spatial distribution and environmental variations have an effect on embryonic development, hatchling fitness, sex determination, hatching success and the risk of predation. The hatchling recruitment success on Cousine Island was also found to be affected by nest losses each season from seasonal beach erosion. Beach dynamics were also found to be cyclical and we discovered that the nesting beach is prone to higher levels of erosion than accretion which created significant changes to beach width across the season. This study showed that when focusing turtle conservation measures at the nesting site, hatchling recruitment success can be increased by minimising mortality at the egg and hatchling stage. The aim in the long-term is to assist with the analyses of local and global population dynamics, deciphering threats and minimising the threat of extinction.