Seals in the Wadden Sea are easy to study and count when they are exposed at haulout sites. However, our knowledge of seal activity at sea is limited. Consequently, in spring and autumn of 2002 and 2003, we equipped 19 seals with a logger/PTT-combined system to record at 15-second intervals, swim speed and direction, dive depths, water temperature and water turbidity. After a predefined time, the devices were automatically released to be washed ashore, where they could be located by satellite signal or found by beach walkers. The stored data showed that the seals did not forage in the Wadden Sea, but travelled to specific
in the North Sea where they foraged on benthic prey
, usually for several days, before returning straight back to their sandbank. Animals almost always dived to the seabed during both commuting and foraging. However, the dive profile was more irregular in the Wadden Sea compared to the deeper North Sea where the diving pattern was very regular, particularly with respect to depth and duration. Seals in the Wadden Sea not only rested on land at their haulout spots but also
in the water, sinking down to the seafloor where they lay motionless for around 7 minutes, surfacing only for a short period to breathe before sinking back again. In deeper water (over 10 metres deep), the seals rested for up to 50 minutes at the surface, apparently drifting and showing no diving activity.