DOWNSTREAM MIGRATION: PROBLEMS AND FACILITIES

Abstract

1. OVERVIEW Downstream fish passage technology is much less advanced than it is for upstream fish passage facilities. This is simply due to the fact that efforts to re-establish free movement for migrating fish began with the construction of upstream fish passage facilities, and that downstream migration problems have only been recognised and addressed more recently. It is also because it is much more difficult and complex to develop effective facilities for downstream migration. This situation is not restricted to France, since as yet, no country has found satisfactory solutions for downstream migration problems, especially where large installations are involved (EPRI, 1994). 2. SPECIES AND STAGES INVOLVED IN DOWNSTREAM MIGRATION Downstream migration, i.e. migration descending a river towards the sea or a lake, concerns fish at different stages of development, depending on the species. The diadromous and potadromous species which are legally recognised and listed as protected migratory species in France, and the particular development stages involved in downstream migration, are as follows:-Atlantic salmon (Salmo salar) and sea trout (Salmo trutta trutta): juveniles (smolts) migrating to the sea to reach their feeding grounds, and adults (kelts) after spawning. With respect to kelts it should be noted that the provision of downstream passage facilities for adult sea trout is much more important than for salmon because multiple spawning is much more frequent in trout than in salmon.-Allis shad (Alosa alosa): mainly juveniles (since most adults die after spawning).-Twaite shad (Alosa fallax): both juveniles and adults.-Eels (Anguilla anguilla): adults only, during migration to their marine spawning grounds.-Sea lamprey (Petromyzon marinus) and River lamprey (Lampetra fluviatilis): juveniles of both, and also adults in the case of river lamprey.-Brown trout (Salmo trutta fario) and Grayling (Thymallus thymallus): juveniles migrating to downstream feeding grounds, and adults after spawning.-Pike (Esox lucius): both adults and juveniles.