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. Below is a discussion of the most major threats to freshwater fishes in Europe, in order of severity.
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. For example, in Montenegro, the connection between the Morava drainage (which is home to many endemic and threatened species) and the Danube that is currently planned in order to increase the supply of water for hydropower generators, is likely to wipe out most of the native species of the Morava-Lake Skadar basin.
3.4.1 Water abstraction
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 (agriculture, drinking water, etc.). In the Mediterranean region of Europe, illegal water abstraction is widespread, and many countries still lack an effective enforcement of legislation that could limit the damages of excessive water abstraction to biodiversity. Commercial use of water is usually given priority over biodiversity concerns.
It is a great challenge to control the introduction of alien fish species in Europe. It only takes a few anglers to stock trout to a small spring stream, and any endemic species could be wiped out, as has happened in Croatia and elsewhere. The rapid spread of Pseudorasbora parva, a Chinese invasive species, from Romania to all over Europe in just 40 years, shows what can happen if there are no legislative barriers for alien fish dispersal. Most of the alien species in Europe are however of European native origin. For example, the pike, Esox lucius, has been introduced to Spain (where it is alien) from France (where it is native). In Spain, its introduction is responsible for the local extinction of entire species assemblages.
Freshwater resources are already very limited in many southern countries, such as Portugal, Spain, Italy, Greece and Turkey. While all these countries are home to high numbers of endemic and threatened species, their consumption of freshwater is expected to increase further in the coming years, both as a result of increasing demand and climate change (Alcamo et al. 2007, Bates et al. 2008). This is also true for the Crimean Peninsula where a highly endemic fish fauna is restricted to a few small streams, from which water is already extracted in large and unsustainable amounts.
Sadly, every impoundment is nowadays routinely stocked with alien fish species by anglers. Hence, impoundments are often the starting points of alien fish invasions into entire river basins. Impoundments offer suitable conditions for recreational fisheries, but in the Mediterranean, most angling species known by local anglers and tourists from angling magazines, are not native. Predatory species which are deemed to make good game for recreational fisheries are particularly problematic.
In those parts of Europe subject to the highest levels of water abstraction, no baseline exists for identifying geographic hotspots for the conservation of freshwater biodiversity. The awareness about the existence of threatened freshwater fish species is often missing. Concepts and approaches in place usually ignore the limited and isolated distribution patterns of fish species. Strong efforts are needed for the conservation of freshwater fishes and associated biodiversity in these regions, in order to prevent catastrophic losses in the coming years.
The poor condition of the major rivers of Italy, provide a good idea of the potential future of other large rivers in the Mediterranean. For the purpose of recreational fishing, alien predatory fishes (Silurus glanis and Sander lucioperca) have been intentionally (and often illegally) introduced by local anglers and tourists. Bait fishes, such as the central European Alburnus alburnus, Rutilus rutilus and Abrams brama, have been released and are now widespread in these river systems. Now no, or only few, native fish species remain in the main rivers. Another worst-case example is provided by the Ebro. To date, nearly all native freshwater fish species of the Ebro have been eradicated from the main channel and have been replaced by a central European fish assemblage. Whilst there has been an increased commercial benefit from recreational fisheries based on these alien species, it has
3.4.2 Alien species
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
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