Fish as indicators of large river connectivity: the Danube and its tributaries

Within four-dimensional alluvial floodplain river systems, connectivity pla ys an important role for diverse fish communities. This is evident at three different temporal/spatial levels. The zoogeographic level (1) shows that connectivity at a large temporal and spatial scale is an important regulato ry mechanism in the colonization of river systems. At the population level (2), three characteristic genera of Danube fish species demonstrate from a theoretical point of view - that connectivity plays an important role in th e formation und functioning of metapopulations. The lowest level describes the significance of connectivity within various life history strategies (3) of Danube fish species. Examples presented in this paper demonstrate that fish are practical indicators of the consequences of human alterations to r iver connectivity conditions at various temporal/spatial levels. On the lif e history level, extensive and practically relevant information is now avai lable to form a sound basis for the development of ecologically orientated river restoration concepts. At the metapopulation level the theoretical sig nificance of connectivity for fish populations is evident: currently, howev er, lack of practical experience restricts its value as an indicator. Conne ctivity is relevant to ichthyogeography not only over geological time, but is also reflected in the present distribution and ongoing dispersal of fish species, a process which is additionally altered by human activity.