The fish fauna of the Jordan reservoir, one of the oldest man-made lakes in central Europe

The Jordan reservoir, Czechoslovakia, was constructed in 1492 and the last full drainage and fish removal was in 1830. In this 'mature' fish community of 20 species (plus three hybrids), the roach, Rutilus rutilus (L.) (1446 fish ha^–1) and beam, Abramis brama (L.) (1074 fish ha^–1) dominated the fish stock (total of all species = 3628 fish ha^–1). Predatory species made a small contribution to the total (maximum 7.7%, obligatory predators 2%). The contribution (12%) by perch, Perca fluviatilis L., was also low, and its numbers in successive age-groups showed a steady decrease. The high variability and time-synchronization in the year-class strengths of the principal cyprinid species may indicate unstable environmental conditions or intra-community cycles. The growth rates of non-predatory species were low. There was a high fishing rate on predatory species but there was little impact by fishery management on the whole 'mature' community. Intensive eutrophication from both communal waters and agriculture probably caused the high biomass level (all species) and the low abundance of zander, Stizostedion lucioperca (L.).