In search of the “missing majority” of nesting loggerhead turtles: improved inter-seasonal recapture rates through subpopulation-scale genetic tagging

Abstract

Capture–mark–recapture (CMR) studies on marine turtle nesting beaches provide data on reproductive periodicity that inform population trends and models. Annual survival is estimated from observations of remigration, the return of females in subsequent nesting seasons. However, a significant proportion of tagged females are never encountered remigrating in many studies, presumably due to weak nest site fidelity (NSF). We employed a genetic CMR approach based on subpopulation-scale clutch sampling to conduct a 5-year evaluation of inter-seasonal recapture rates and NSF for Northern Recovery Unit loggerhead turtles (Caretta caretta). Of 1770 females genetically tagged from Georgia through Maryland in 2010, 1156 (65%) remigrated between 2011 and 2015. Inter-seasonal NSF, measured as shifts in median latitude nesting locations between years, was highly variable among individuals but strong overall (mean: 15.08 (± 44.61) km, median: 1.84 km). Among three focal beaches with nocturnal tagging projects, 69 of 173 females (40%) remigrated onsite whereas 115 (66%) were detected overall. Regional genetic sampling therefore yielded significantly higher inter-seasonal recapture rates, which may improve precision in future survival analyses. However, despite sampling ~ 1000 km with high annual detection probabilities (p* ≥ 0.94), 35% of 2010-females were not detected remigrating. Several non-exclusive hypotheses to explain these remaining “missing” remigrants should be considered: longer remigration intervals, imperfect detection within the study area, emigration to Florida, anthropogenic mortality, and natural mortality or senescence. This genetic tagging approach can be applied over large spatial scales where nesting densities permit, better characterizing inter-seasonal dispersal.