Decline of native freshwater fishes in a Mediterranean watershed on the Iberian Peninsula: A quantitative assessment

We examined changes in the distribution of 9 native and 18 introduced fresh water fishes in the south-eastern Pyrenees watershed, Iberian Peninsula, us ing data from 1996, 1984-1988 and historical information. This region suffe rs many modifications to its freshwater ecosystems that are linked to human activity in the Mediterranean regions. Fish communities, stream physical h abitat and environmental degradation were assessed at 168 sites from 11 bas ins in 1996. Seven native species (78%) showed decline from previous data, one of which became extirpated in the first half of the 20th century. On th e other hand, introduced species are expanding. As a consequence, intact na tive communities are increasingly rare, declining from presence in 22% of r iver courses in 1984-1988 to 15% in 1996. The most typical community type i s a mixture of native and introduced species occupying 30% of river courses . Stream degradation seems to be the main cause of this process because fis h communities differed between degraded streams and streams suffering less impact. A principal component analysis showed that water pollution and modi fications to the habitat were the two anthropogenic factors that accounted for most changes in the fish community integrity. Habitat alteration, prima rily through construction of dams and water diversions, has fragmented habi tats and isolated native fish communities in headwater streams. Current pro tection measures do not offer effective conservation of threatened species and communities. A global conservation and restoration programme from an ec osystem-based approach is essential to reverse the trend affecting native f reshwater fishes in this Mediterranean region.