FISH SPECIES COMPOSITION BEFORE AND AFTER CONSTRUCTION OF A RESERVOIRON THE GUADALETE-RIVER (SW SPAIN)

The completion of the Zahara-El Gastor dam on the Guadalete River duri ng the autumn winter of 1991 meant the final step in the spatial fragm entation of the river's headwaters, in the Grazalema Natural Park (SW Spain). Fish were sampled above and below the dam until its closing, i n the reservoir and up- and down-stream after the closing. The new res ervoir has had a strong effect on the structural composition of the ri ver community. Up to 1991, all the species captured were endemic. Stud ies prior to the dam's closing showed a predomination of the Iberian b arbel in the basin of the reservoir and downstream, while upstream the Iberian chub was dominant. From 1991, the percentages of the differen t species upstream of the dam varied (depending on the hydrological co nditions of the year), while in the reservoir and downstream the Iberi an nase became dominant, in addition, non-native species have been cap tured, in both the reservoir and the river, which had never before bee n captured in the headwaters of the Guadalete. This small-scale study newly supports the idea that even though a dam may not radically alter the thermal or hydrological regimens of the river, it can have a stro ng effect on the native ichthyofauna (in both composition and percenta ge of frequency of the different species forming the fish community).