Photo left: the bullhead or miller`s thumb is an established member of the Kall fish fauna (Photo left: Dr. B. Stemmer)

The species richness of the fish fauna and other aquatic organisms depends to a great extent on the availability of a variety of subhabitats in the stream. For instance tributaries and other water bodies as spawning grounds or habitats for young fish must be accessible, while movement between stream bed and bank or areas prone to flooding, or between the stream water body and the intersticial zone (the spaces within the gravelly substrate), are of great importance for the aquatic fauna.

Photo right: Culverts with a sharp drop (Tiefenbach II) represent insurmountable obstacles

Existing impassable obstacles like drops or channelling lead to the isolation of subhabitats, e.g. for the brown trout or the bullhead. In addition, the Kall, along with the central section of the Rur, are regarded as potentially the most significant spawning grounds for the salmon in the entire Rur hydrological system. While the accessibility to migratory fish of the River Maas in the Netherlands from the North Sea up to its confluence with the Rur has been greatly improved in recent years by the installation of modern fish ladders, there remain many hindrances in the Rur and Kall for salmon on their way to spawning grounds.