A number of fish species require passage along the length of rivers during at least short periods of their life-cycle.  In places the migration of fish is an annual event and dams and other instream structures constitute barriers to their movement.  In some cases the long-term sustainability of fish populations depend on this migration, and local economies can be reliant on sustainable fish populations as a source of income.

A number of fish species migrate from freshwater to saltwater and back again at various stages of their life cycle.  Salmon and eels are examples of anadromous fish, in which the adults migrate upstream to spawn and the young descend downstream.  Catadromous fish are species such as eels, which do the reverse – adults migrate downstream to spawn and the young migrate upstream.  Other freshwater fish move within river systems such as up tributary streams to spawn.  Depending on their location, dams can present barriers to these species for migration in both upstream and downstream directions. As well as creating direct physical barriers, flow and water quality characteristics of the natural river regime which may act as migratory cues.

Where species are of commercial value, considerable dollars have been spent by the hydropower industry in facilitating the passage of these fish both upstream and downstream of dams, although not always with success.