With the entry into force of the EC WFD in 2000, Member States have committed themselves to achieve the good ecological and chemical status until the end of 2015.

Fish fauna represents thereby an important biological component in assessing of the ecological status of the surface water. Apart from the water quality, both spawning and nursery water must be safeguarded, as well as an appropriate interconnection between various habitats.

The assessment of the ecological status of water courses on the basis of fish fauna is performed uniformly in Germany, by means of the fish based assessment system (FBAS, orig. abbrev.: fiBS).
Comparison of the reference fauna, i.e. the fauna which can be expected under the natural environmental conditions with the actual fauna in the natural water course represents the starting point of the assessment. The difference between the often present main species, water-type-specific species and rare, but regularly present secondary species is made thereby.

First categorization of the surface water ecological status on the basis of the fish fauna is presented within the River Basin Management Plan 2009 for the Weser river catchment area. The conclusion is hereby clear, that the predominant number of water bodies show moderate or worse ecological status as regards fish fauna.