As marine turtles traverse entire ocean basins, a number of techniques have been developed to gather information on their movements at sea. These range from basic methods, such as mark and recapture using flipper tags, to more technically advanced, expensive approaches, such as satellite tracking. Here we report the first published use of geolocation by light, using global location sensing (GLS) units, to estimate the locations of marine turtles in the Mediterranean. We attached units to 9 green Chelonia mydas and 3 loggerhead Caretta caretta turtles which had nested in Cyprus in previous years. One individual of each species was also fitted with a satellite transmitter for comparison. GLS units were retrieved on subsequent visits made by instrumented turtles during the same nesting season. In total, 28 GLS units were recovered from 12 individuals. The mean great-circle distance between the mean daily position calculated from satellite tracking and the corresponding mean light-based geolocation over the same time period was 50.3 km for green turtles and 57.6 km for loggerhead turtles. We suggest that GLS technology offers potential utility for identifying large-scale movements, migratory pathways, and over-wintering or foraging sites of adult female marine turtles. \u
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