Triggered by the adoption of the Water Framework Directive, a variety of fish-based systems were developed throughout Europe to assess the ecological status of lakes. This paper provides a comprehensive overview of all existing systems and summarizes sampling methods, fish community traits (metrics) and the relevant anthropogenic pressures assessed by them. Twenty-one European countries developed fish-based assessment systems. Three countries each developed two distinct systems to approach different ecoregions, either to use different data, or to assess different lake types leading to a total number of 24 systems. The most common approach for the setting of reference conditions, used in seventeen systems, was the utilisation of fish communities in comparably undisturbed natural lakes as reference. Eleven used expert judgment, nine historical data and eight modelled relationships. Fourteen systems combined at least two approaches. The most common fish sampling method was a standardized fishing procedure with multimesh-gillnets. Many countries applied combinations of fishing methods, e.g. non-standard gillnets, fyke nets and electrofishing. Altogether 177 metrics were used for index development and each system combined 2-13 metrics. The most common ones were total standardized catches of number and biomass, relative abundance of Perca fluviatilis, Rutilus rutilus, and Abramis brama, feeding preferences, sensitive species, and non-natives. The pressure-response-relationships for these metrics were supported with both correlations established during system development and scientific publications. However, the metrics and their combinations were highly diverse and no metric was applied universally. Our analysis reveals that most fish-based assessment systems address multiple pressures (eutrophication, hydromorphological alterations, fishery pressure and occurrence of non-natives), whilst few are pressure-specific, tackling only eutrophication or acidification. We argue that the value of fish-based systems for lakes lies in their capacity to capture the effect of many different pressures and their interactions which is lacking for most assessment systems based on other biota.

Balaton Limnological Research Institute Klebelsberg K u 3 H 8237 Tihany Hungary

Biology Centre of the Czech Academy of Sciences Na Sádkách 7 37005 České Budějovice Czech Republic

CNR Water Research Institute L go Tonolli 50 28922 Verbania Pallanza Italy

Environment Agency Research Analysis and Evaluation Air Land and Water Team Worcester Road Kidderminster DY11 RA United Kingdom

European Commission Joint Research Centre Via Enrico Fermi 274 21027 Ispra VA Italy

Institute for Inland Fisheries Im Königswald 2 D 14469 Potsdam Germany

Leibniz Institute of Freshwater Ecology and Inland Fisheries Müggelseedamm 310 D 12587 Berlin Germany

Natural Resources Institute Finland P O Box 2 FI 00791 Helsinki Finland

Research Institute for Nature and Forest Dwersbos 28 B 1630 Linkebeek Belgium

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