For the classification of the ecological status on the basis of the fish fauna, the deviation of the reference fauna with respect to the currently existing fish fauna is investigated and assessed. Reference fauna is the fauna which appears under natural conditions in the observed waters. Definition of the reference fauna resides with the states of the Weser river catchment area. During the past years, the states have described the reference fish community in a coordination process at federal level, based on historical facts and expert knowledge on ecological requirements in regard to each individual species and every water course assessed.