34 ATLAS OF BIODIVERSITY RISK CHAPTER 6 The native freshwater fish fauna in the Iberian Peninsula is characterized by high diversification at species level, with the largest number of endemic species in Europe, due to the geologi- cal history of each basin (Doadrio 2001). After habitat destruction, the introduction of exotic species is the second major threat to this region. Over the last few decades, exotic species have flourished in the Iberian Peninsula, mainly released for angling purposes or biological control, but also as a result of accidental introductions from aquaculture facilities. The bleak Alburnus alburnus (Linnaeus, 1758) is Distribution of Alien Bleak Alburnus alburnus (Linnaeus, 1758) in the Northeastern Iberian Mediterranean Watersheds: Past and Present ALBERTO MACEDA-VEIGA, ADOLFO DE SOSTOA, EDGAR SOLORIO-ORNELAS, MARIO MONROY, DOLORS VINYOLES, NUNO CAIOLA, FREDERIC CASALS, EMILI GARCIA-BERTHOU & ANTONI MUNNÉ  Figure 2. ........................................ .......................... .......................... .......................... Figure 1. ........................................ .......................... .......................... .......................... .......................... considered a forage species by anglers who have repeatedly introduced it in various reservoirs in the Iberian Peninsula, to improve the populations of exotic fish predators such as the northern pike Esox lucius (Linnaeus, 1758), the largemouth bass Micropterus salmoides (Lacepède, 1802), the zander Sander lucioperca (Linnaeus, 1758) and the wells catfish Silurus glanis (Linnaeus, 1758). The bleak is also frequently used as a bait fish (Elvira 1995). The bleak is a small cyprinid which feeds mainly on zooplankton. Its native distribution area ranges from the east- ern slopes of the Pyrenees to the Urals (Doadrio 2001) (Figure 1). Although its impact on native fish fauna has not been studied in depth, there is concern that it may outcompete native fish due to its high reproductive output. Another potential source of danger is hybridization with native fish. Hybridization has already been report- ed with cyprinid species of genera Squalius, Blicca, Rutilus and Abramis. Besides its impact to native fish fauna, it also affects the trophic dynamic of reservoirs, which are the main source of water for human populations. It feeds on cladocerans and other small invertebrates which play an important