The Sylt-Rømø bight is situated in the northern Wadden Sea, between the islands of Sylt and Rømø, on the western German coast. This bight is rather well studied and its food web has been modeled, taking into account most of the trophic compartments (Baird et al. 2012). But top-predators are not yet included in the model due the difficulties to determine their diet based in a wide range of possible prey types. The aim of this study is to determine if harbor seals (Phoca vitulina), the main present mammal species in the area, take significant part in the ecosystem. The first step was to determine the seals diet using stable Isotope analysis performed on harbor seals muscle and vibrissae. In a second time, the seal compartment was included in the existing Ecological Network Analysis model to study the impact of harbor seals on the Sylt-Rømø bight food web.