Estimation of the Net Nesting Effort of Olive Ridley Arribada Sea Turtles Based on Nest Densities at Ostional Beach, Costa Rica
Ostional Beach, Costa Rica, supports a large mass nesting (arribada) aggregation of Olive Ridley Sea Turtles (Lepidochelys olivacea). A large number of egg clutches is lost to egg harvest and to nest destruction by nesting females after every arribada. Accordingly, the purpose of our study was to generate estimates of the net number of clutches left incubating from clutch densities with the use of a quadrat methodology, and to compare these data with nesting population estimates resulting from the strip transect in time methodology that is currently applied. After the conclusion of each arribada, we randomly performed 50 excavations in 1-m2 quadrats to count the number of clutches present. We extrapolated quadrat density data to the entire nesting area of the beach to estimate the total number of clutches remaining following each arribada and egg harvest. The mean total clutch density was 4.09 ± 0.18 SE nests/m2. Our results showed that quadrat and transect estimate differences ranged from 0.04 to 52.6%, with quadrat estimates typically being lower. Our results demonstrated that, in the rainy season, a large number of clutches (47.4–99.9%) was left incubating in the beach after every arribada and egg harvest. By omitting clutches that were harvested or destroyed throughout the arribada, we evaluated the reproductive potential and estimated the magnitude of clutch loss on Ostional Beach, both of which play important roles in the management of the egg harvest as a sustainable conservation strategy. Playa Ostional, Costa Rica presenta el anidamiento masivo o arribada de la tortuga marina Lora (Lepidochelys olivacea). Un número elevado de huevos se pierde durante cada arribada debido a la cosecha de huevos y a la destrucción de nidos por parte de tortugas anidadoras. Así pues, el objetivo de este estudio fue el de generar estimados del número neto de nidos que permanecen incubándose a partir de la densidad de nidos utilizando una metodología de cuadrantes y la comparación de estos datos con los estimados poblacionales obtenidos con la metodología de transectos fijos sobre el tiempo que se está utilizando actualmente. Después de la conclusión de cada arribada, realizamos al azar 50 excavaciones en cuadrantes de 1m2 para contar el número de nidos presentes. Nosotros extrapolamos los datos de densidad de estos cuadrantes a la playa de anidación entera para estimar el número de nidos que quedan en la playa luego de cada arribada y de cada cosecha de huevos. La densidad promedio total de nidos fue de 4.09 ± 0.18 SE nidos por m2. Nuestros resultados mostraron que las diferencias entre los estimados transectos y de cuadrantes variaron de entre 0.04% a 52.6%, siendo los estimados de cuadrantes típicamente menores. Nuestros resultados demostraron que en la época lluviosa un gran número de nidos (entre el 47,4% y el 99,9%) permanecieron incubándose en la playa luego de cada arribada y cosecha de huevos. Al omitir los nidos que son destruidos por las tortugas durante cada arribada o cosechados, este nuevo procedimiento nos permitió hacer una evaluación del potencial reproductivo y de la magnitud de la destrucción de nidos en playa Ostional, ambos siendo factores que juegan un papel importante en el manejo de la cosecha de huevos como una estrategia de conservación sustentable.Abstract
Resumen
Diagram of the (A) transect and (B) quadrat methodology as implemented in this study.
Power analysis varying the effort of sampling from 50 excavations (current data) to 1 for each arribada event. Error bars represent a 95% confidence interval (2.5–97.5% quantiles). The y axis is the mean number of clutches per excavated plot.
Ninety-five percent confidence intervals (standardized by the confidence interval for 50 excavations) as a function of the sampling effort (number of excavations). The two plain lines represent the 95% confidence interval of the linear log-log fit of the data. The dashed line shows the average confidence interval for 50 excavations.
Mean clutch density ± SE at Ostional Beach, Costa Rica of new and old clutches for the rainy season (May through November) and dry season (December through March). Seasonal means with letters of different case are significantly different according to the Bonferroni-adjusted LSD a posteriori test (F3,546 = 57.44; P > 0.001) for old vs. new clutches. Seasonal totals were not significantly different.
Comparison of estimated number of clutches (±CI95%) based on the transect and quadrat methodologies for each arribada at Ostional Beach, Costa Rica. Rainy season arribadas occurred September through November of 2010; dry-season arribadas occurred January through April of 2011. Nonoverlapping confidence intervals indicate that means are significantly different (indicated by asterisks). Transect estimate data for September through December 2010 are published in Valverde et al. (2012).
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