Seasonal Variation of Harbor Seal's Diet from the Wadden Sea in Relation to Prey Availability

The Wadden Sea has an important role for marine mammals in terms of resting, nursing
and foraging. Harbor seal is the most abundant marine mammal species in this area. The
use of the food resources of the Wadden Sea by seals is not clear, and previous studies
showed that this species can travel kilometers away from their haul-outs to forage in the
North Sea. In this study, we analyzed the stable isotopes of vibrissae from 23 dead harbor
seals found on the island of Sylt to investigate their diet. The predator´s carbon and nitrogen
isotope compositions were compared to the compositions of different potential prey items
from the Sylt-Rømø Bight and from the North Sea in order to study seasonal pattern in the
diet and in the foraging location. In parallel, seasonal variation of abundance and biomass
of the potential prey items from the Sylt-Rømø Bight were studied and compare to their contribution
to the seal´s diet. The results revealed a change in the seal´s diet from pelagic
sources in spring to a benthic based diet in summer, and an increasing use of the North Sea
resources in fall and winter in accordance with the seasonal variation of the availability of
prey in the Sylt-Rømø Bight.