Abstract
Nest count surveys provide an important means for tracking trends in marine turtle populations. In 2015 the IUCN Red List global status review changed the loggerhead turtle (Carretta caretta) from Endangered to Vulnerable status, and, with a revised regional management unit (RMU) assessment system in place, concluded that the Northwest Indian Ocean loggerhead turtle RMU was Critically Endangered (CR). We analyzed annual time series nest count survey data between 1978 and 2016 to evaluate population trend and characterize the existing spatial and temporal attributes of a globally significant rookery on Masirah Island, Oman. A two-point trend assessment using a subset of historical data (1985-1996) combined with recent data (2008-2016) on a representative, 9-km section of index beaches revealed a change from 584 tracks-1 km-1 season-1 (SD = 109.5) to 122.43 tracks-1 km-1 season-1 (SD = 37.3), representing a statistically significant 79% decline. Comparative data were tested for spatial and temporal conformity, and the influence of other parameters on the trend result were considered, such as nesting success ratios and spatial shifts in density. Our analyses confirm that the decline occurred within a single loggerhead generation. Mean multiyear nest counts from recent surveys provides an estimate of 55,202 nests year-1 (SD=16,814, CI 0.95 53,326- 57,078) across 83 km of nesting beach. Considering recent clutch frequency estimates the mean number of nesting loggerheads per year is estimated between 10,223 (CI 0.95 9,875- 10,570) and 11,500 (CI 0.95 11,109- 11,891). The Northwest Indian Ocean RMU is thought to represent 35% of the global nesting population. Recognizing a pressing need to address the decline in this population we identify potential threats on nesting beaches and at sea which point towards priority actions for conservation of this CR RMU.
Highlights
Animals with long generation times pose a challenge for population trend assessments
Annual nest counts remain the most helpful index of adult female sea turtle abundance due to inaccessibility of other life stages, these counts are influenced by short-term environmental factors other than female abundance (Richards et al, 2011)
72% of effort was applied to index beaches (SD = 10)
Summary
Animals with long generation times pose a challenge for population trend assessments. Annual nest counts remain the most helpful index of adult female sea turtle abundance due to inaccessibility of other life stages, these counts are influenced by short-term environmental factors other than female abundance (Richards et al, 2011). Because of this annual sampling error in the total female population, sea turtle population assessments strive to represent time series approaching a generation or more (Snover and Heppell, 2009)
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