The multitude and ecological specificities of the local fish species count among the remarkable characteristics of a catchment area. In the last 25 years, altogether 116 fish and roundmouth species (river lampreys) have been detected in the water courses of the Weser river catchment area, including the areas of the lower and outer Weser. Over 50 % of these species belong to the marine fish species. Based on historical evidence it could be proven that 37 fresh water species originate from rivers and streams. The remaining 12 species appeared not until 1900, often isolated, and are therefore considered as neozoa. In order to perform characterization of the fish fauna in the Weser river catchment area, a documentation of fish and roundmouth species (cyclostomata) in the river catchment area of Weser was compiled.

Many people find the long distance migrating fishes especially fascinating, although they require both marine habitats of oceans and also streams and rivers with their clear fresh water in order to complete their life cycle. Already in 1996, the fundamentals for the repopulation were published in a brochure. There is current evidence for 11 migrating fish species. Solely as related to Atlantic sturgeon there is evidence on historical presence only. Other species combine the necessary habitats to spawn, aestivate or hibernate and migrate many kilometers within a river (potamodromous migrating fishes).

Due to their endangered populations and specific dissemination, many fish species are subject to special protection on the basis of European Fauna-Flora-Habitats Directive. Long term conservation and development of their populations in the notified FFH-areas must consider spawning and nursery habitats, as well as the continuity of the migration ways. They require therefore a comprehensive engagement on a federal level, supported by the European Water Framework Directive.