Mark-Recapture Estimation of the Leatherback Sea Turtle (Dermochelys coriacea) Nesting Population at Matura Beach, Trinidad

by Hodge, Christina E.

Many researchers believe that Caribbean leatherback sea turtle (Dermochelys coriacea) populations are increasing. Since Trinidad currently hosts the world’s third largest leatherback nesting assemblage, accurate population estimates are needed for this area. Nature Seekers, Inc., a community-based environmental management organization, has been monitoring leatherback nesting on Matura Beach since 1990 and began its tagging project in 1999. The tagging project’s main goal was to determine the number of turtles nesting each year, but saturation tagging has not been possible given the resources available. However, as a season progresses, untagged turtle encounters constitute a smaller percentage of the overall leatherback encounters. The steady increase in percent of previously tagged turtles encountered during the season indicates that a significant portion of the population is being tagged and makes within season mark-recapture population estimation possible. This project examines the rate of recapture of tagged turtles, trends in the timing of recapture events, and the requirements for mark-recapture estimates of leatherback nesting populations. The probability of encountering an individual leatherback varies based on the time of the season and the number of days since it was last encountered. To account for this variability, each season was divided into cohorts based on a nine day nesting cycle. Closed capture, time dependent population estimates were derived for each cohort using Program Capture. Data from the 2000, 2001, and 2003 nesting seasons yielded mark-recapture population estimates averaging 62 percent greater than the minimum number of turtles encountered. This is consistent with the 40 percent beach coverage estimate provided by Nature Seekers at the initiation of this analysis. The 20 to 24 day average time between encounters of individual leatherbacks also supports the theory that Nature Seekers observes slightly less than half of the nesting events on Matura Beach since leatherbacks are known to nest every nine to ten days. Developing models to estimate sea turtle populations in situations where saturation tagging is not possible is critical for determining the status of endangered leatherback sea turtles. Future efforts should be directed toward improving nesting beach coverage, developing more accurate tagging effort indices, examination of the degree of nesting beach population closure, and the development of leatherback encounter probability models.