The construction of the Římov Reservoir on the Malše River in 1978 significantly influenced the fish assemblage within the river, above the reservoir. Fish species that had successfully colonised the reservoir after impoundment expanded to the inflowing river. The changes in the riverine fish assemblage up to the first impassable weir (4 km upstream) reflected the development of the reservoir assemblage after an impoundment boom of pike (Esox lucius), then the perch (Perca fluviatilis) dominated phase and later the final cyprinid-dominated phase. The changes in the fish fauna over the period 1978-2002 were significant (χ2, P < 0.001). During the cyprinid-dominated phase, roach (Rutilus rutilus) became the most abundant species both in the reservoir and in the river, outnumbering the pristine river species. We conclude that the cyprinid-dominated assemblage in the reservoir can be more aggressive in interfering with the river assemblage than the percid-dominated one.

The construction of the Římov Reservoir on the Malše River in 1978 significantly influenced the fish assemblage within the river, above the reservoir. Fish species that had successfully colonised the reservoir after impoundment expanded to the inflowing river. The changes in the riverine fish assemblage up to the first impassable weir (4 km upstream) reflected the development of the reservoir assemblage after an impoundment boom of pike (<i>Esox lucius</i>), then the perch (<i>Perca fluviatilis</i>) dominated phase and later the final cyprinid-dominated phase. The changes in the fish fauna over the period 1978–2002 were significant (&chi;², <i>P</i> < 0.001). During the cyprinid-dominated phase, roach (<i>Rutilus rutilus</i>) became the most abundant species both in the reservoir and in the river, outnumbering the pristine river species. We conclude that the cyprinid-dominated assemblage in the reservoir can be more aggressive in interfering with the river assemblage than the percid-dominated one.