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ABSTRACT1. During 2001 and 2002, 12 tributaries of the River Duero basin, north-west Spain, were sampled. Pearl mussels Margaritifera margaritifera (L.) were located in two of them. Isolated individuals were found in the River Tera, a tributary of the River Esla; and only a few colonies with more than 20 mussels were found in the River Negro.2. All of the specimens found were adults (mean length 95 mm), with juveniles being absent in the river bed. The River Negro plots showed a low median mussel density (0.11 m À2 ), and that of greatest size consisted of 164 ageing individuals.3. Mussels were mainly located in permanent rivers and in shallow stretches with little flow of water, with heterogeneous substrates and heavily shaded by the riparian woods. The whole basin has well-conserved rivers with cold oligotrophic water and poor mineralization due to the insolubility of the rocks.4. This relic population has a severely restricted distribution because of a series of large reservoirs in a hydropower complex of the rivers Duero and Esla (caused by the construction of the Ricobayo Dam in 1947) and their negative impact on the rheophilous fish. The mussels of the River Tera have been isolated from the rest of the Duero basin since 1969 (Cernadilla Dam), whereas in the River Negro they have been isolated since 1995 (Agavanzal Dam).

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animals, has focused the interest of water-resources and water-quality managers on these organisms (Sivonen & Jones, 1999).There have been several hypothesis presented concerning the relationship between environmental conditions and cyanobacteria abundance