Arribada nesting of olive ridley sea turtles (Lepidochelys olivacea) at La Escobilla, Mexico

Graduation date: 2011Olive ridley sea turtles (Lepidochelys olivacea) present an interesting case study of the conservation of wildlife species that aggregate in high densities and have served as resources for human consumption. Ridleys exhibit synchronized mass-nesting behavior, \ud during events called "arribadas," where thousands of females lay eggs together in a small area over the span of a few consecutive nights. Some of the largest arribadas occur at La Escobilla, Mexico, with over a million nests estimated per season. These \ud aggregations have made olive ridleys vulnerable to negative impacts from harvest and density-dependent influences on nest success. There are many interconnected \ud determinants of nest success, including intraspecific competition for space, temperature, and predation. Nest destruction by conspecifics is an apparent potential impact of high density nesting, as later-arriving turtles often dig up previously laid \ud nests. Nest destruction is the main scientific argument for "sustainable" egg harvest, since local communities could utilize eggs otherwise destroyed as an economic resource. In 2009 at La Escobilla, I explored 1) the historical context of harvest and \ud community members' current perceptions of turtles through 12 semi-structured interviews with key informants; 2) nest destruction rates during arribadas through a field study to quantify nesting behavior. My first objective was to understand the shifting human-turtle relationship and how it informs current community dynamics \ud and future conservation and research at arribada beaches. Historical research and interviews indicated that many local residents are familiar with turtle behavior and agree on the importance of conservation efforts for turtles and the local community. Nevertheless, residents struggle for economic stability within what has largely been an externally-imposed protectionist framework. Future efforts should integrate long-term employment with local involvement in research, conservation, and non-consumptive use. The second objective of my project was to quantify nesting activity and investigate the relationship between nest densities and nest destruction. Nesting behavior of 1293 turtles was observed in 26 sample plots during two consecutive \ud arribadas. Cumulative nest densities estimated over two arribadas ranged from around 1 to 8 nests/m². The odds of a turtle digging up eggs increase 21% for every additional nest in a 1m² area surrounding a nesting turtle. No hatchlings emerged from the arribadas I studied, likely due to unfavorably warm temperatures in 2009 and widespread beetle predation. Estimation of hatchling production at the beach level is necessary for accurate projection of the population's status; the empirical findings and \ud methodologies considered in this project can be used in such future models. My field study illustrates the complexity of predicting hatchling production because of temporal and spatial variation, as indicated by cumulative effects of multiple arribadas on incubating nests