In all reservoirs the fish fauna is recruited from their riverine antecedents. The nature of the riverine environment, created by its unstable hydrological regime, favours the evolution of generalist species. Of these species, only a few are preadapted to lacustrine conditions. When riverine fish communities are trapped in a reservoir, most of the fish stay close to the shore, the mouth of tributaries and in shallows. The pelagic and deep waters are poorly utilized and the yield of fish is below the natural productivity of these water bodies, especially in the case of deep and large reservoirs. This occurs unless the dammed river contains fish species from families which are preadapted for lacustrine conditions. A positive correlation between the percentage of pelagic fish and the total yield of fish in reservoirs indicates that the pelagial represents a vacant habitat, only seldom invaded by riverine species. This situation probably applies to the deep water of reservoirs too, although sufficient data to demonstrate this are not available.

At least 134 exotic and/or translocated fish species belonging to 34 families which were introduced to 29 of 33 European countries are documented. These include 35 species of Cyprinidae (introduced to 28 countries), 17 Salmonidae (28), 11 Coregonidae (12), 10 Cichlidae (8), six Centrarchidae (23), five Acipenseridae (8), four Ictaluridae (18), four Poeciliidae (11), four Percidae (9), four Mugilidae (1), three Catostomidae (6), three Gasterosteidae (5), two Anguillidae (3), two Atherinidae (2), two Clariidae (4), two Gobiidae (2), two Umbridae (6), two Pleuronectidae (1), two Thymallidae (3) and one species each of Anabantidae (1), Channidae (2), Clupeidae (1), Cobitidae 91), Eleotridae (1), Engraulidae (1), Esocidae (3), Mullidae (1), Osmeridae (1), Percichthyidae (1), Petromyzontidae (1), Polyodontidae (2), Siluridae (5), and Syngnathidae (1). True exotic species amount to 74 species/forms belonging to 21 families, of which 11 families are exotic for Europe. The remaining 60 species (21 families), are native in Europe and were translocated among various watersheds and/or countries. Most species were intentionally released since 1945 to enhance sport, commercial and subsistence fisheries, for fish farming and aquaculture, and for purposes such as mosquito, macrophytes and algal bloom control. Poor success was registered in most cases, along with adverse or unexpected effects on native fishes and their habitat.

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The discrepancy between high primary production and poor fish yields in some tropical reservoirs, especially in Souht East Asia, is apparently due to the fact that not all available trophic levels are utilized . Data analysis has shown that this is due to the riverline origin of fish communities which are not well adapted to lacustrine habitats in such lakes and reservoirs . These fishes are able to inhabit only a relatively narrow belt of the shallow inshore area . It has been shown that fish are rare in the deep water and pelagial area of geologically young lakes and reservoirs . Although most riverine fish are also able to inhabit the lacustrine habitat, only a few are preadapted to take advantage of deep or open water environments . In the tropics, Cichlidae and freshwater Clupeidae thrive in lakes and reservoirs .

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Slovak Federative RepublicThe world's biggest salmonid, the huchen, Huchu hucho (L.), is now threatened with extinction, especially the nominate form-the Danubian huchen-which is confined to the basin of the River Danube. About 50 years ago the huchen occupied almost 12 000 km of river in Europe but it is now common in only about 33% of its former distribution, is rare in 28% and has disappeared from 39%. The continuous area formerly inhabited by the huchen is now disrupted into isolated sections. This large predatory fish has been pushed back to the submountain zone of rivers, and its density has decreased considerably. This is thecombined result of increased regulation of streams, damming of rivers, industrial and agricultural pollution and other factors causing the destruction of the huchen's habitat. Protective measures have been applied, involving legal restrictions, restocking, the establishment of reserves and introductions into rivers not previously inhabited by the huchen. These measures have failed due to continuing pollution of rivers and the low numbers of fingerlings available for restocking.