Can gray seals maintain heading within areas of high tidal current ? Preliminary results from numerical modeling and GPS observations

International audienceSeals are capable of navigation and orientation during long distance movements, even in absence of apparent landmarks, in open seas, and at night (e.g., Lowry et al. 1998, McConnell et al. 1999, Gjertz et al. 2000, Lesage et al. 2004). Several ideas have been put forwards about marine animals' ability to orientate and navigate at sea (Mills Flemming et al. 2006, Lohmann et al. 2008, Chapman et al. 2011). However, little work has been carried out on seals (but see Matsumura et al. 2011). A number of experiments have been conducted on captive seals in order to test their sensory systems and orientation capacities (e.g., Kowalewsky et al. 2006, Mauck et al. 2008), but such experiments are difficult to conduct on free-ranging seals. Modeling the animals' movements at sea in relation to environmental variables may elucidate the cues they use to orient and navigate. However, such free-ranging animal movements are always subject to the influence of local currents (Lohmann et al. 2008). Thus the incorporation of current data is necessary to reveal underlying navigational capabilities and strategies (Willis 2011). In this study, we model the observed sea surface tracks of two gray seals (Halichoerus grypus) that had crossed the English Channel in September 2011 (Fig. 1, 2). The seals (referred to as B23 and B24), were tracked by Fastloc GPS/GSM telemetry techniques. Their surface positions were drawn from the series of Fastloc GPS locations transmitted by GPS phone tags developed by the Sea Mammal Research Unit.2 These were glued to the animal's fur on the neck behind the head with quick-setting epoxy. The tags were configured to attempt Fastloc GPS locations every 10 min provided the seal was at the sea surface. Both seals were captured and tagged in the Molene archipelago, western Brittany, France.