Many fish species require various habitats within a river landscape for spawning, nursery or food, and they look for them targeted via so-called main migration routes. In this process, some diadromous species cover partly very long distances, joining thereby marine open-sea areas with the spawning areas of the low mountain range (e.g. salmon). Another significant diadromous species, which migrates as young fish into the water in order to populate its nursery areas, is eel.

Other species remain in fresh waters, but they do also use the habitats located far away from each other within a river system. Some examples for the so-called potamodromous fish species would be burbot, Vimba bream, barbel or ide.

Fifteen species were identified in the river basin district of Weser, which have originally populated the Weser and its springs and tributaries as diadromous and potamodromous species for spawning or nursery. Eleven out of these count among diadromous, whereas 4 among potamodromous species.

The appearance of the species such as three-spined stickleback (migratory form), smelt and twaite shad, which still constitute the self-reproducing populations in the river basin district of Weser, is mainly limited to lower Weser. These species are therefore excluded from strategic considerations in regard to the continuity of Weser. For the time being there are no reproductive populations of allis shad, houting and European sturgeon in the Weser catchment area. Potential repopulation areas for houting are located mainly in the lower Weser parts. The repopulation of allis shad and European sturgeon is not planned any time soon.

Atlantic salmon undergoes since many years ago the stemming measures in order to secure new populations. For further characterization of some of the named migration fish species, the following profiles were developed:

The migration of these fishes is often hampered by many hydraulic barriers, making the repopulation of migratory fishes difficult.