Sea Turtle Nesting Beach Fidelity Revealed by Satellite Tracking

Site fidelity describes the tendency to return to or reuse familiar sites, yet some animals may use alternative sites while maintaining fidelity to primary locations, with consequences for population estimates as well as resilience to changing environments. Here we assessed and compared nesting beach fidelity of hawksbill turtles (Eretmochelys imbricata) on Frégate Island, Seychelles using data from beach foot patrols (2017-2025; n = 575 turtles) and satellite telemetry (2023; n = 10 turtles) and then examined the implications of fidelity strategies for population estimates. We also reviewed published literature on nesting beach use across sea turtle species. Satellite tracking showed that 50% of equipped turtles were study-beach faithful, laying clutches on one beach, and 50% were study-beach unfaithful, nesting across 2-4 beaches on Frégate and on nearby islands. Consequently, clutch frequency estimated through tracking was 1.9 times higher than foot patrol estimates. Estimates of adult female abundance increased as the assumed proportion of clutches laid on other beaches increased, independent of clutch frequency. Across 51 studies at 43 sites around the world, study-beach unfaithful individuals were more likely to be recorded nesting at mainland beaches (21.4% compared with 9.3% on island beaches), with mainland surveys covering greater distances (1.6-1000 km) than island surveys (1-15.9 km). Maximum inter-nest distances varied across species: green turtles (Chelonia mydas; 2250 km), leatherbacks (Dermochelys coriacea; 1849 km), loggerheads (Caretta caretta; 800 km), olive ridleys (Lepidochelys olivacea; 320 km), and hawksbills (300 km). We demonstrate mixed fidelity strategies within an island hawksbill population, suggesting that fidelity reflects natal philopatry and the proximity of alternative sites. Limited monitoring coverage at remote sites likely contributes to under-detection of nestings by study-beach unfaithful individuals. Individuals using alternative sites signal behavioural plasticity that may contribute to resilience against habitat loss. We recommend characterising beach fidelity through regional data sharing and satellite tracking to inform reliable demographic inference and identify subpopulations vulnerable to climate change.