Assessing the effect of beach dynamics on nest site selection of green turtles (Chelonia mydas) in Playa Cabuyal, Costa Rica

Green turtles (Chelonia mydas) are listed as endangered by the International Union for Conservation of Nature (IUCN) across their entire range due to declining population numbers over the past 30 years. This study assessed the effect of beach dynamics, including beach erosion and temperature, on nest site selection across four nesting seasons, at Playa Cabuyal, Guanacaste, Costa Rica (10°67’N, 85°65’W). The study site is located in Northwest Costa Rica on the Pacific Ocean where climate is affected by El Niño Southern Oscillation (ENSO) events. The nest locations showed that green turtles at this site had a preference for nesting in Zone 3, the highest nesting area of the beach, under the canopy of trees. This zone was covered with vegetation and had open areas underneath the canopy of trees where the nests would be shaded for a portion of the day. Nesting females also showed a preference for nesting in the southern most portion of Playa Cabuyal designated as Area C, which is made up of the southern third of the beach. Factors studied to be contributing to this nest site preference were tidal inundation, distance from high tide and erosion activities. These were catalogued during the 2014-2015 nesting season to establish a baseline level of these influences by Zone (sections of the beach, 1-3, from the water) and Area (delineation of the beach from North to South into 3 distinct areas A-C). An inverse relationship was found between number of nests laid in each Zone and the number of erosion and inundation events in each zone. Zone 3 was found to have no erosion or inundation events and had the highest percentage of observed in-situ (nests that have not been relocated) nests. Zone 1 (below the high tide line) was found to have the highest percentage of erosion events and had the lowest number of nests. In addition the turtles had a significant preference for nesting in shaded areas. Shaded portions of Zone 3 had lower nest temperatures than those nests in unshaded areas. This could result in higher production of males and could also affect hatching and emergence success. Additionally, nests underneath trees had a decreased amount of surface roots so nesting females had fewer impediments when excavating a nest cavity. This could increase the chance of a successful nesting attempt and may also increase hatching and emergence success as there are fewer roots to impede hatchling development and emergence. The preference of green turtles for nesting in Zone 3 at this site is different than at other sites such as Colola, Mexico where green turtles show a preference for Zone 2. This is most likely due to the beach in Cabuyal being narrower than the beach in Colola. Turtles also showed a preference for nesting on the southern third of the beach, Area C, for three out of the four years of this study. In the 2013-2014 year there was a preference for the middle third, Area B, of the beach. In all years of the study turtles showed the least preference for the first third of the beach to the far North. These preferences may be a result of the beach being wider on the southern portion of the beach versus to the North where a rock cliff frequently causes the beach to be completely inundated. Although there was a distinct difference in levels of inundation in the three Areas of beach from North to South, the occurrence of erosion events, or seasonal changes in deposition and attrition, was similar in each Area. Site fidelity by females classified into three different categories, those with a high level of fidelity, laying all nests within 100 meters, those with low fidelity, laying all nests greater than 100 meters apart, and clustered, consisting of females with at least two nests within 100 meters of each other. There were no differences in fidelity in between zones. Turtle size was also determined if it was related to nesting strategy but no correlation was found