Hydrobiologia 422/423: 279–289, 2000. M. Jungwirth, S. Muhar & S. Schmutz (eds), Assessing the Ecological Integrity of Running Waters. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 279 A multi-level concept for fish-based, river-type-specific assessment of ecological integrity S. Schmutz, M. Kaufmann, B. Vogel, M. Jungwirth & S. Muhar Dept. Hydrobiology, Fisheries and Aquaculture, Univ. of Agricultural Sciences Vienna, Max Emanuel-Strasse 17, A-1180 Vienna E-mail: schmutz@mail.boku.ac.at Key words: water quality, assessment criteria, biological hierarchy, reference conditions, MuLFA, Austria, Europe Abstract We propose a multi-level concept for fish-based assessment (MuLFA) of the ecological integrity of running waters. This concept is designed for large-scale monitoring programmes such as required for the proposed Water Frame- work Directive of the EU. Out of five different biological organisation levels (fauna, community, guild, population and individual), we propose seven criteria: River-type-specific species, species with self-sustaining populations, fish region, number of guilds, guild composition, population size and population age structure. The principle of the MuLFA is based on assessing the deviation from undisturbed reference conditions. Reference conditions have to be compiled for every distinct river type using historical fish and abiotic data, present river-type-specific reference sites and reference models. The final assessment procedure is done by comparing the assessment reach with the reference conditions using a 5-tiered normative scheme and assigning that reach to the level of highest coincidence. The benefit of the MuLFA is its potential for consistent sensitivity to low- and high-dose human alterations, and due to its general character, its adaptability to all river types. Introduction Associating the health of fish stocks with water quality conditions has a comparatively long tradition. Non- etheless, standardised fish-based methods to assess the ecological integrity (EI) of running waters have been available only since the 1980s. Various single-species- oriented methods were subsequently developed for salmonids, among them the HQI (Habitat Quality Index, Binns & Eiserman, 1979), the HSI (Habitat Suitability Model, Raleigh et al., 1984; Wesche et al., 1987) and other models (Bowlby & Roff, 1986; Scarnecchia & Bergersen, 1987; Fausch et al., 1988; Jowett, 1992; Barnard & Wyatt, 1995) based on fish and habitat relationships, and the community oriented IBI (Index of Biotic Integrity, Karr, 1981) for non- salmonid rivers. The transferability of these methods to other catchments or ecoregions without consider- able adaptations is limited (Angermeier & Karr, 1986; Miller et al., 1988). In Europe, the development of new and the adaptation of existing fish-based methods is still in the early stage (Oberdorff & Hughes, 1992; Oberdorff & Porcher, 1994; Hughes & Oberdorff, 1998). However, these methods are rapidly gaining importance, not least because the new EU water policy specifies fishes as one of the four biotic elements which are decisive for assessing EI (Water Framework Directive, WFD). The principle behind the new EU approach is based on assessing the deviation of the present status from the undisturbed river-type-specific conditions. The assessment procedure is carried out by assigning the present status to five predefined EI-levels ran- ging from ‘high’ to ‘bad’ quality. While the general outline of the new method is already embodied in the Directive, detailed information on the definitive assessment approach is still entirely lacking. The pur- pose of this paper is to develop a new concept for a fish-related EI-assessment method based on existing methods and embedded in a theoretical framework. The new method should enable a large-scale, nation- wide EI-assessment in accordance with the proposed WFD by simultaneously considering the uniqueness of different river types. Special emphasis is placed