European fish-based assessment reveals high diversity of systems for determining ecological status of lakes

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,

Automated summary

Triggered by the adoption of the Water Framework Directive, various fish-based systems were developed in Europe to assess the ecological status of lakes. A total of 24 systems were developed by 21 European countries, with the most common approach being the use of fish communities in comparably undisturbed natural lakes as reference conditions. Different countries used a variety of fish sampling methods and combined multiple metrics for index development.