Abstract
The Olive Ridley sea turtle (Lepidochelys olivacea) is a widely distributed marine vertebrate that is observed commonly in tropical oceans. Females nest throughout much of western Africa where they are vulnerable to poaching and fisheries by-catch. A greater understanding of the population ecology in that region is critical for effectively managing and protecting the species, including knowledge of foraging ecology, migratory routes, and population connectivity. Stable isotope analysis has become a popular method for investigating foraging locations and ecology. Studies that combine the method with satellite tracking have shown that isotopic concentrations can accurately describe and predict the different foraging grounds of sea turtles across ocean basins. This study assessed δ13C and δ15N ratios of tissue samples from female Olive Ridleys collected while they were laying eggs at Mankoadze and Anloga beaches in Ghana during the 2017-2020 nesting seasons. The purpose was to evaluate the possibility that those beaches hosted females from different breeding and/or foraging grounds, given that a recent study indicated higher than expected genetic differences from nesting Olive Ridleys at similar beaches. We found no differences in δ13C values between the two populations, but individuals from Mankoadze had higher δ15N values, which resulted in little overlap in the isotopic niche space between the populations. Comparing individuals between years at Mankoadze only, there was a difference in δ13C values between 2017 and 2018, but no differences in δ15N values across years. There was also no difference in isotope values between individuals of different sizes. Although we could not determine whether the individuals were foraging in different locations or at different trophic levels, our results suggested that individuals from the two locations may be utilizing different foraging locations and therefore may be from different population. This finding is important for management purposes, as distinct populations may face unique threats that demand different approaches to be conserved and protected appropriately. West Africa is an understudied region and Olive Ridleys are understudied compared to other sea turtle species, and this study provides groundwork for future research on both population dynamics of Olive Ridleys in the region and local isoscapes.
