Food web functioning is one the main issue to understand and manage ecosystems. The Wadden Sea is an important area for fish nursery, bird migration and several marine mammals’ species. Alien species are invading the North Sea and the Wadden Sea for more than 20 years. To study the impact of these invasive species on the food web, three bays of the Wadden Sea have been chosen to be modeled with the ENA (Ecological Network Analyses). My PhD topic is mainly the updating of the Sylt-Rømø Bight food web ENA model from 2007 (Baird et al 2012) and particularly, the addition of the marine mammal compartment, not included in the model yet. To assess the diet of the seals, trophic markers as stable isotopes and fatty acids have been measured in different seal tissue (muscle, hair and vibrissae) as well as in the potential prey species from the North Sea and from the Sylt-Rømø bight.