Evaluating Trends in Abundance of Immature Green Turtles, Chelonia mydas, in the Greater Caribbean

• Published in: Ecological applications Vol. 15; no. 1; pp. 304 - 314
• Main Authors: Bjorndal, Karen A., Bolten, Alan B., Chaloupka, Milani Y.
• Format: Journal Article
• Language: English
• Published: Ecological Society of America 01-02-2005
• Subjects: abundance; Bahamas; capture–mark–recapture; Chelonia mydas; Costa Rica; demography; Environmental conservation; Female animals; Foraging; Greater Caribbean; green turtle; Immatures; long-lived species; Marine; Marine ecology; Population estimates; Population trends; Sea turtles; Statistical variance; Tortuguero; Turtles
• ISSN: 1051-0761; 1939-5582
• DOI: 10.1890/04-0059

Many long-lived marine species exhibit life history traits that make them more vulnerable to overexploitation. Accurate population trend analysis is essential for development and assessment of management plans for these species. However, because many of these species disperse over large geographic areas, have life stages inaccessible to human surveyors, and/or undergo complex developmental migrations, data on trends in abundance are often available for only one stage of the population, usually breeding adults. The green turtle (Chelonia mydas) is one of these long-lived species for which population trends are based almost exclusively on either numbers of females that emerge to nest or numbers of nests deposited each year on geographically restricted beaches. In this study, we generated estimates of annual abundance for juvenile green turtles at two foraging grounds in the Bahamas based on long-term capture-mark-recapture (CMR) studies at Union Creek (24 years) and Conception Creek (13 years), using a two-stage approach. First, we estimated recapture probabilities from CMR data using the Cormack-Jolly-Seber models in the software program MARK; second, we estimated annual abundance of green turtles at both study sites using the recapture probabilities in a Horvitz-Thompson type estimation procedure. Green turtle abundance did not change significantly in Conception Creek, but, in Union Creek, green turtle abundance had successive phases of significant increase, significant decrease, and stability. These changes in abundance resulted from changes in immigration, not survival or emigration. The trends in abundance on the foraging grounds did not conform to the significantly increasing trend for the major nesting population at Tortuguero, Costa Rica. This disparity highlights the challenges of assessing population-wide trends of green turtles and other long-lived species. The best approach for monitoring population trends may be a combination of (1) extensive surveys to provide data for large-scale trends in relative population abundance, and (2) intensive surveys, using CMR techniques, to estimate absolute abundance and evaluate the demographic processes driving the trends.