The Wadden Sea is constantly being formed by local dynamics and large scale climatic oscillations. These abiotic characteristics define the species composition and abundance in the habitat, Wadden Sea. As the Wadden Sea represents a very dynamic and open system, species might be easily introduced which may affect native species and in the course of time the whole food web. To investigate the impact of changing abiotic characteristics and invasive species on the ecosystem Wadden Sea, this thesis was conducted. To uncover possible ongoing changes fish were chosen as the organismic subject group of the study. Fish are able to avoid unfavorable conditions by migration to other areas, acting as an indicator for ongoing changes.