Laying on the edge: demography of green sea turtles (Chelonia mydas) nesting on Playa Norte, Tortuguero, Costa Rica

Abstract

The most important nesting site for the green turtle, Chelonia mydas, in the Atlantic basin is Costa Rica’s Tortuguero National Park (TNP). An important portion of this population nests on Playa Norte, an adjacent beach located to the north of TNP. Using 10 years (2006–2015) of mark-recapture data, we estimated demographic parameters for green turtles nesting at Playa Norte. During the 10-year study period, 1573 nesting encounters were recorded, representing 1101 distinct individuals. Open-population models estimated an abundance of 6004 (95% CI 5037–6970), representing the contingent of adult females that nested on Playa Norte at least once during the study period. This estimate includes females that died did not use Playa Norte as nesting site or became post-reproductive after the first nesting encounter as well as nesting and non-nesting adult females in a given year. This open-population abundance estimate and related demographic rates are also assumed to be robust against inter-nesting occurrences between Playa Norte and TNP, and between Playa Norte and other unmonitored and minor beaches. The mean annual survival rate was 0.85 (95% CI 0.76–0.91) and the realized annual population growth from 2006 to 2015 was 1.0179 (95% CI 0.7984–1.2978), indicating a positive trend characterized by an average 1.79% annual increase. The results presented here concur with results from studies of nesting females at TNP and highlight the importance of Playa Norte as a nesting ground for Atlantic green turtles. In addition, our study demonstrated that reliable demographic information can be derived from nesting sites at the edge of a main nesting area if proper survey designs and analytical methods are used.