Although pollution is one of the most widespread threats, impacting the highest number of species, it is not the most serious threat. Other significant threats such as water abstraction and the introduction of alien species are causing much more rapid population declines for some freshwater species.
The single most important threat to European freshwater fishes is the abstraction of water; from underground, or from the streams and rivers themselves. Water is abstracted for many reasons, for example, agriculture or drinking water.
Many European fishes are highly susceptible to the impact of introduced alien species. These may be predators or competitors, especially under "insular" conditions of Mediterranean catchments, where rivers and streams may be naturally devoid of predatory fish, or where naturally species poor communities exist with little inter-specific competition. Local populations are easily wiped out from such catchments if ecologically more competitive species, from species-rich central and eastern European fish communities, invade their habitats.
Most freshwater fishes are very sensitive to alterations of their natural habitat. In addition, many require long distance migrations to fulfil their life cycle.
There are few rivers in Europe that have not been impacted by dams for hydropower or irrigation purposes. The first dam upriver of an estuary is now usually the end point for the migration of anadromous species, especially in Eastern Europe. Dams are also a major threat for catadromous species, such as the European Eel, which forage in freshwater and spawn at sea. In most cases, dams block the migration route of fishes.
Pollution was the major factor threatening freshwater fish species in Europe during the late 19th and in the 20th centuries, and is still a major driver of population decline and habitat loss for freshwater fishes in Europe. Pollution is caused by a number of sources, including domestic waste, industrial and agricultural effluent, river transportation, and sedimentation.