16 grey seals (Halichoerus grypus) were captured in the Molene archipelago, Brittany (western France) between April 1999 and August 2003 and fitted with SRDLs (Satellite Relay Data Loggers constructed at the Sea Mammal Research Unit - SMRU, UK). The project was funded by DIREN Bretagne and FEOGA (EU). The study was conducted by Cecile Vincent and Vincent Ridoux at Oceanopolis (Brest, France) and the University of La Rochelle (France) in collaboration with SMRU and the Office national de la chasse de la faune sauvage (ONCFS). Seals were caught under licenses delivered by the French ministry of the environment.