The variability of distribution and density of pelagic fishes in the Rożnowski Dam Reservoir

Abstract

The distribution, density, species and size structures of the fish in the waters of the Rożnowski Dam Reservoir (area - 1100 ha; average depth - 12 m) were studied and their numbers and biomass were estimated. Hydroacoustic methods were applied and supplemented and verified with the results of control fishing. A SIMRAD EY-500 (split beam) 120 kHz, 7x7 deg echosounder was used. Maps of the spatial distribution and density of fish were prepared and the number of fish were determined in various areas and at various depths. The data indicate that there were significant differences in the number of fish in particular areas and at various times of day or night. At night fish density was over 6.3 times higher in the near-surface water layer in the backwater area than near the dam. The average fish density was 5.8 times lower during the day than at night; this is attributable to diurnal feeding and predator-avoidance migrations. The analyses of the nighttime acoustic data reveal that there are 13.5 million fish specimens in the pelagic zone of the Rożnowski Dam Reservoir. The total fish biomass was estimated based on the average unit weight of the fish caught (81.5 g) and was 1102.4 tons, or 1234.3 kg ha-1. This is 30-times greater per hectare than the estimates for the Solina and Klimkówka reservoirs. Two cyprinid fishes, bleak and roach, constitute 85.8% of the number of fish and 66.3% of the biomass – for a total of 818.1 kg ha-1 in the Rożnowski Dam Reservoir. Stocking only predatory fish (pike, pikeperch, wells) in this reservoir must be supported. It is recommended that net catches be intensified in the backwater area, especially those of bleak.