In Europe, water policy is currently subjected to considerable changes in line with the recent European Water Framework Directive (WFD - 2000/60/EC), which requires the assessment of their hydro-morphological, chemical and biological characteristics for the restoration and maintenance of "healthy" aquatic ecosystems. Compared to previous policies, the WFD gives strong priority to biological quality goals by introducing measurements of aquatic biota necessary to identify the structural and functional integrity of ecosystems. Furthermore, the WFD introduces river basin management throughout Europe, which could have major impacts on the conservation and restoration of aquatic systems.

The ecological quality of an aquatic ecosystem is based on the spatiotemporal integration of the different physical, chemical and biological components. In order to improve the protection, the long term use and the adequate monitoring of the aquatic ecosystems, it is required to identify and take into account some global metrics, complementary to those used in traditional water quality assessment. Among the potential bio-indicators, fish exhibit several characteristics (presence at different levels of the trophic chain, long life, aptitude to integrate environmental alterations occurring at different levels of the aquatic ecosystem, sensitivity to habitat degradation), providing original and complementary information to those already available from other organisms.