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Nicola, G. G., B. Elvira and A. Almodovar (1996). Dams and fish passage facilities in the large rivers of Spain: effects on migratory species. Archiv fuer Hydrobiologie Supplement. 113:375-379.

The construction of dams in Spain has steadily increased during this century, with a peak between 1960 and 1990. Today, there are more than 1100 operating large dams. The presence of dams without fish passes in large rivers appears to be a major contributing factor in the decline of native fish, especially of migratory species. In fact, all anadromous and catadromous fish are presently threatened and included in the Spanish Red Data Book. Species such as lamprey Petromyzon marinus, sturgeon Acipenser sturio, shads Alosa and eel Anguilla anguilla are now extinct in wide areas of central Spain, due to the blockage of fish movements caused by dams built in large rivers. As part of an ongoing national project started in 1993, we researched the existence and effectiveness of fish passage facilities at Spanish dams. The main preliminary finding is that fishways are frequently absent in large dams, while fish ladders are the most common type of fish pass used in weirs. Additional mitigating measures to reduce the negative effect of dams on fish populations are also suggested.