Stephanie Wehkamp

• This thesis focused on the effects of coastal defence structures on the fish and crustacean community in a subtidal area in the southern North Sea. The findings represent significant effects of natural and artificial substrate characteristics on the species. Through SCUBA, fixed quadrats along line-transects were used to assess firstly the demersal fish and decapod crustacean abundances in relation to natural substratum types (rock, cobbles and large pebbles). The results indicated that significant micro-scale species-habitat associations, especially for native fish, occur. In the same study area experimental fields were established, consisting of four-footed breakwaters (tetrapods) to investigate ecological impacts on the biota. Over a two-year period the fish and decapod community was assessed again, but now in combination with the experimental fields and in distance to the artificial structures. The results demonstrate a significant decrease of fish abundances in the surrounding area caused by migration effects towards the artificial structures. Furthermore, more young-of-the-year fish were observed directly at the structures within the second year after the establishment. I suggest that the availability of adequate refuge in combination with additionally food opportunities were the main factors, which attracted native fish species in greater numbers. Concerning the macro crustaceans it was not possible to give clear statements about the impact on the assemblage in general. The species reacted differently to the alteration of their surrounding. I suggest that for some crustacean species the composition of the tetrapods are attractive as shelter. For smaller species, however, this habitat form may not be optimal because of the lack of microhabitats. I concluded that the preference for a habitat type depends mainly on ecological requirements such as substratum colouration, adequate shelter size and specific species features.