Using Data from Satellite Telemetry and Nesting Beach Monitoring to Improve Estimates of Marine Turtle Clutch Frequency

Population abundance data is often used to define species’ conservation status. Abundance of marine turtles is typically determined using information from nesting beach monitoring such as clutch frequency and remigration interval of nesting females. However, studies have shown that clutch frequency determined solely from nesting beach monitoring data can be underestimated. To obtain reliable estimates of clutch frequency for hawksbill turtles in northeastern Brazil (-6.273356 S., -35.036271 W), the region with the highest nesting density in the South Atlantic, data from beach monitoring and satellite telemetry were combined from 2014 to 2019. Beach monitoring data indicated the date of first nesting event, whilst state space modelling of satellite telemetry data indicated the departure date of turtles, allowing calculations of residence length at breeding site and therefore estimation of clutch frequency based on internesting intervals. Nesting females were estimated to nest up to six times within the nesting season and had an average clutch frequency of 4.7 nests per female. This estimate is almost twice larger than previous estimates based only on beach monitoring. The new estimates of clutch frequency will allow for more reliable population abundance estimates for this critically endangered population. Further, to guide future estimates of marine turtle clutch frequency methods to estimate clutch frequency were compared and their advantages and biases were discussed. Our approach and findings highlight the need for reconsideration of how clutch frequency is commonly determined for marine turtle populations and the use of beach monitoring data and satellite telemetry for estimations of clutch frequency.