Compensatory Effects Drive Human Harvest-Natural Predation Dynamics of Sea Turtle Nests in Costa Rica

abstract: Human consumption of sea turtle eggs, meat, and other products is considered to be a major threat to sea turtle populations worldwide. Declining populations are often attributed to anthropogenic pressures despite evident additional pressure from natural processes; however, depredation by natural or feral species such as raccoons, crabs, and dogs are often unknown. A popular tool for the conservation of marine turtle eggs is hatcheries, but their protection is limited by factors such as carrying capacity and timing or location of nests. The Rescue Center for Endangered Marine Species (CREMA) runs four sea turtle conservation projects on the nesting beaches of the Southern Nicoya Peninsula in Costa Rica, where the predominant nesting activity is from Olive Ridley turtles (Lepidochelys olivacea), followed by sporadic Green (Chelonia mydas), Leatherback (Dermochelys coriacea), and Hawksbill turtles (Eretmochelys imbricata). Two of these nesting projects are based in Costa de Oro and San Miguel, which are adjacent beaches divided by an estuary. Both projects use a hatchery where nests are usually relocated; however, when the hatchery is not available, nests are either relocated or left in situ at the beach. The aim of this study is to: 1) compare human harvest and depredation rates of nests relocated and left in situ. We reviewed data collected from 2012 to 2018 of nests relocated to a hatchery and left in situ at both nesting sites, and these data represent nesting conditions prior to relocation to a hatchery. We found that the nesting beach at Costa de Oro exhibits high rates of human harvest, which has decreased since the conservation project was established, while San Miguel exhibits comparatively low egg harvest but much higher depredation. Egg harvest in Costa de Oro decreases from approximately 50% of all nests in 2013 to 15% in 2018 while depredation on both beaches fluctuates year to year. Our results demonstrate that different pressures impact nesting beach success in the Southern Nicoya Peninsula along with natural threats, possibly due to contrasting community values and human populations. San Miguel has been protecting nests for over 20 years, whereas the Costa de Oro project only began in 2012. It also important to consider that depredation on the San Miguel nesting beach may be increased by human pressure such as in the case of domesticated animals, especially when the human population in San Miguel is consistently higher than in Costa de Oro. Persistence of depredation and human egg harvest alongside conservation efforts exhibit the prevalence of these pressures and suggest increased pressure if measures such as nightly patrols and hatchery protection were not utilized. We suggest a continuation of hatchery and patrol based conservation efforts as well as community outreach to attempt to merge cultural values with sea turtle conservation