Fish as indicators for the assessment of the ecological integrity of large rivers

Fish communities in large rivers are characterized by a high diversity, which reflects the structural diversity and habitat richness of inshore zones and connected floodplains. The connectivity of the different habitat elements in a broad spatio-temporal context, i.e., at various scales from catchment to microhabitat - and their nestedness - define the fitness of fish species both on the individual level (e.g., growth performances) and population level (i.e., population structure, mortality, etc.). Relevant spatial scales can be the whole river course in the case of some anadromous migrators or the availability of complementary microhabitat elements, e.g., during the early ontogeny of a species. The significance of connectivity at various scales from whole river to local reach have to be evaluated based on the requirements, reaction norms, and ecological flexibility of individual species. Integrity has to be evaluated in several respects: (1) with regard to population genetics over extensive biogeographic areas and in long time scales; (2) with regard to supplementary habitats in the course of the life cycle of individual species with ontogenetic habitat shifts and specific requirements during the reproductive phase; (3) with regard to longitudinal and lateral transport and exchange processes determining local habitat conditions and the food supply for fish. Due to these dependencies the status and condition of the fish fauna is a critical sensor of integrity at different scales and thus a good monitoring tool especially with regard to river engineering. The structure of the fish population - in particular the relative abundance of specifically adapted floodplain species - provides information on the overall conditions. For a more detailed evaluation the local distribution pattern of the fish fauna, population structure, seasonality, and growth performances of the early life history stages can indicate the quality and extent of connectedness at a finer scale and can be used to define management and engineering criteria.