Abstract: Fish assemblages in reservoirs and lakes are mainly assessed by multiple sampling gear. The challenge exists in how to combine all the data from the different types of gear to develop a fish-based index. In this paper, we describe a novel approach to this challenge in reservoirs in Flanders. The developed approach can also be used for natural lakes in the same eco-region and for any combination of fishing methods. In a first step, we defined a reference list of fish species occurring in man-made Flemish reservoirs. To compile this reference list, we adapted the reference for Dutch lakes with recent data from freshwater reservoirs in Flanders. This reference list contains guild-specific information needed to define metrics. To pre-classify the reservoirs, a habitat status for each reservoir was set using abiotic parameters (pressures). Fish gear-dependent metrics were selected according to their response to these pressures. Threshold values for metrics were determined based on the species reference list and occasionally on the calculated metric values. The ecological quality ratios derived from the index calculation were validated with an independent set of data. The developed index proved to successfully assess the ecological status of the reservoirs in Flanders.