During their life cycle, anadromous long distance migratory fish (spawning in fresh water) and the catadromous eel (spawning in sea water) migrate from the sea into fresh water or from fresh water into the sea.
Salmon spend their main growth period in the sea, but return to the rivers for spawning. They follow their sense of smell and their remembrance of the smell of the home waters. This behaviour is called “Homing”. Since this journey is extremely tiring, most salmon only do it once in their lifetime.
The eel migrates in the opposite direction. It spends most of its life cycle in the river and spawns in the Sargasso Sea, a part of the Atlantic Ocean south of the Bermuda Islands. After 3 years the eel larvae reach the European coastal waters and become glass eel which often migrate upstream the rivers in great swarms. During several years of life in the rivers they then achieve their full size. Once they are mature (females after 12-15 years) they return to the Sargasso Sea to spawn.
Formerly, the Rhine catchment used to be an important habitat for migratory fish. Quite naturally, the Rhine was free of any obstacles from the North Sea to the falls of the Rhine at Schaffhausen. From the spawning and juvenile regions in the tributaries, even in the Alps, in the Black Forest and the Vosges juvenile salmon could migrate downstream into the North Sea and the Atlantic Ocean almost without surmounting any obstacles and return to their home waters once they were ready to spawn. Thus, the life cycle of long distance migratory fish was continuous and the conservation of self-sustaining populations was granted.
Systematic river training on the Upper and High Rhine, on major tributaries such as the rivers Aare, Neckar, Main and Moselle and along several further tributaries in the entire catchment has heavily interfered with river continuity in the Rhine system. Apart from the Haringvliet sluices in the Rhine delta, the hydropower plants and weirs on the Upper Rhine represent significant obstacles for the up- and downstream migration of these fish species.
Due to the changes in the water systems, which are mostly irreversible and usage-dependant, the spawning grounds and juvenile fish habitats of migratory fish have partly been destroyed, are no longer accessible or their accessibility is considerably reduced.
The ICPR Master Plan Migratory Fish Rhine is supposed to indicate, how self-sustaining, stable migratory fish populations can again be settled in the Rhine watershed as far as the Basel area within both reasonable time and at reasonable costs. On 18 October 2007, the Conference of Rhine Ministers confirmed its intent to gradually restore river continuity in the Rhine upstream to Basel and in the waters under the salmon programme.
Based on achievements so far, the „Master Plan” defines priorities for a phased implementation of measures, lists orders of magnitude for costs and indicates further required investigations.
Master Plan Migratory Fish Rhine 2018