Population monitoring for marine turtles is often limited to assessing trends in nest counts, but individual reproductive data are required to better characterize the population dynamics underlying these nesting trends. Nest site fidelity can be low relative to the scale of tagging effort at individual nesting beaches, resulting in sparse recapture data and potentially biased reproductive parameter estimates. Genetic tagging via genotyping of maternal genomic DNA from egg samples offers an alternative approach of identifying individuals that alleviates the requirement of physically intercepting nesting females. From 2010 through 2012, we collected a single egg from approximately 20,000 loggerhead clutches detected on beaches from Georgia to Maryland, encompassing the nesting range of the Northern Recovery Unit (NRU) subpopulation. We used genotypes from 18 microsatellites to assign clutches to unique females. We quantified nest site fidelity at the subpopulation level as well as in a finer scale, spatially explicit context by assigning females to 50 km latitudinal bins based on their median nesting latitude. We estimated annual nesting female population size and clutch frequency using an open robust design framework. Finally, we compared clutch frequencies estimated from single-island physical tagging (Wassaw and Jekyll Islands, Georgia) and regional genetic tagging.
Results/Conclusions
Genetic tagging identified 5,681 unique females nesting over the study period. Mean distance between the most distant clutches laid by individual females (nest spread) was 33.4 (± 74.6) km at the NRU scale. Nesting spread at the latitudinal bin scale ranged from 12.3 (± 23.8) to 157.0 (± 213.0) km, with Georgia and South Carolina females exhibiting significantly higher nest site fidelity than North Carolina and Virginia females. The best supported models all contained time dependence in entry and persistence probabilities, with annual estimated clutch frequencies ranging from 3.9 (± 0.04) to 4.5 (± 0.09) clutches per female. Of females that were physically tagged while nesting on Wassaw or Jekyll Island, 50% also nested on another island that season. Of those, 80% did not return to their initial nesting sites over the remainder of each nesting season. Clutch frequencies estimated from physical tagging data were biased low by 16% to 38% relative to those generated from the genetic capture-recapture data. This bias indicates that “permanent” emigration at the scale of individual tagging beaches cannot be fully accounted for through standard modeling, and that regional approaches that consider the scale of nest site fidelity are critical for generating robust parameter estimates.