– The fate of obligatory riverine fish species (rheophils), which are the objects of anglers' exploitation (chub –Leuciscus cephalus, nase –Chondrostoma nasus, barbel –Barbus barbus, gudgeon –Gobio gobio), and brown trout (Salmo trutta m. fario) and grayling (Thymallus thymallus) (in the Gwda River basin only), were investigated in large alluvial rivers (Pilica and Warta) and in the medium‐sized Gwda River basin. The Pilica (1973) and the Warta (1986–1987) were divided by large dams without fish ladders in their middle courses. The Gwda River was divided by only a few dams along its course, but its tributaries carrying pure water had numerous small dams that supplied water for fish farms. Other stresses influencing fish populations in these rivers were: pollution, overfishing, hydroelectric plants and bank revetments. Because the listed stresses occurred alternately and at various periods of time in these rivers, this enabled attributing the cause for extinction and reduction of the abundance and distribution. In the salmon Gwda River basin, a drastic decrease in spatial distribution and reduction of occurrence ranges of brown trout, grayling and barbel was evident in respect to the first study period (1980s) in the 1990s. In the large, alluvial Pilica River, nase, barbel and dace are on the edge of extinction and chub and gudgeon are vulnerable. In the Warta's tailwater, barbel is an extinct species, and chub, dace and gudgeon are vulnerable ones. In a site in the backwater, none of the above mentioned species became extinct, but their abundance and occurrence frequency decreased a bit in respect to the pre‐impoundment period. Roach‐generalist, which was used in this research as a “control” species, increased in abundance in all 3 rivers. These investigations univocally proved that the dams cause catastrophic stress for obligatory riverine species.NOTE

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In eleven sites on two small tributaries of the Parand River (North-West Parand State)| 6 .8 and 4 .0 km in length| 1263 fish specimens of 28 taxons and 14 families were collected using electrofishing . Up to twelve years ago the catchments of the two rivers were covered by tropical jungle ; this has now been replaced by pasture and arable fields . Mean diversity indices of Simpson and Shannon indices were close to 0 .6| which would indicate that human impact affected fish populations although the river beds have retained their original shape| except cleared of riparian trees . Despite their close location (about 18 km)| the two streams differed from each other in their fish faunal composition . The distinctive nature of the fish communities in the two streams was a result of: conductivity| pH| also hiding places| riparian vegetation| submerged macrophytes and depth and width of the rivers .

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Estimates were made of the ecological fish production in two small lowland rivers uninhabited by salmonids. Eight sites were examined in the Utrata River drainage basin, and five in the Zalewka Brook. Nets were placed at the upper and lower limits of each site and fish were captured using electrofishing apparatus; at the four sites estimates of fish density were verified by the use of rotenone. Twelfe fish species were found, the following being dominant: Three-spined Stickleback, Gasterosteus aculeatus L., Loach, Nemachilus barbatulus (L.), Gudgeon, Gobio gobio (L.). Ecological fish production ranged from 30.7 to 807.2 kg ha a (mean: 210.9 kg ha a). Production was highest at the sites with the lowest number of species, the maximum value occurring in the headspring section with three species.