Evaluation of biological integrity of a small urban stream system by investigating longitudinal variability of the fish assemblage.

We investigated the integrity of fish assemblages of a small anthropogenically impacted urban stream system in south-western Germany. Heavy point and non-point pollution physiologically stresses the aquatic fauna while migration barriers and river morphological alterations negatively influence habitat availability. Investigations conducted in 1998 and 1999 revealed that the fish assemblage was altered and several site-specific fish species were missing while non-site-specific species were relatively common. Highest species diversity was recorded in a reach downstream of the main migration barrier, while upstream only a few species were present. In general, a shift to ubiquitous and limnophilic species as well as to less pollution sensitive species was observed. Adult fish dominated the population structure throughout the stream course. Recruitment was confined mainly to the less polluted headwater. In the present study we introduce an assessment system based on the similarity of the potential natural and the actually established fish assemblage as demanded by the Water Framework Directive of the European Union for the evaluation of the ecological status of a fish assemblage in a stream [EC of the European Parliament and of the Council of Europe establishing a framework for community action in the field of water policy. Brussels, 30 June, 2000. 1997/0067 (COD)]. Both, stream morphological quality and pollution data sampled within the two study years confirm the validity of the suggested assessment method.