A LONG-TERM STUDY (1985-1995) OF FISH POPULATIONS IN THE IMPOUNDED WARTA-RIVER, POLAND

Three sites, two in the tailwater and one upstream of the backwater of a large man-made reservoir in the middle course of the 808 km long lo wland Warta River (Oder catchment), were studied. The pre-impoundment study lasted one (1985), the post-impoundment one 10 yr (1986-1995). T he dam contributed to the extinction of the anadromous Vimba vimba, an d of the rheophilous Chondrostoma nasus, both cyprinids. A number of d irectly or indirectly negative changes related to the construction of the reservoir were recorded in the tailwater. These changes affected t he species number, spawning groups, diversity, density, standing crop, mean body weight, ABC index, and proportional stock density - PSD. Nu merous impacts related to engineering, water chemistry and climate wer e more negative for fish at the tailwater than at the backwater sites. The hydrology of the site upstream of the backwater continued to be d ependent on natural factors. Fish populations there suffered only from accidental impacts of the construction of the dam, such as bank revet ment or the clearance of trees and shrubs.