Twelve grey seals (Halichoerus grypus) were captured in the Baie de Somme, Picardie (France) between May and September 2012 and fitted with Fastloc GPS/GSM tags (Sea Mammal Research Unit - SMRU, UK). The project was funded by the Région Poitou-Charentes. The study was conducted by Cecile Vincent at the University of La Rochelle (France) in collaboration with SMRU, the Parc naturel marin des estuaires picards et de la mer d’Opale, Picardie Nature, PELAGIS (CNRS/La Rochelle University) 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.