The fish fauna in selected rivers of the Mazovian Lowland

In 1999–2003, investigations of the fish fauna were carried out in selected rivers of the Mazovian Lowland. Electrofishing was used to investigate species diversity and abundance at 35 sampling sites. In total, 8618 fish representing 29 species were caught, weighing in total 77.9 kg. 13 to 22 species of fish and lamprey were registered. The highest number of species was identified in the Świder River (22), dominated in abundance by roach (35.1%) and stone loach (34.9%), and in the Rządza River (20), dominated by gudgeon (33.4) and roach (24.9%). Ten-spined stickleback dominated in the Dluga River (45.5%), roach in the Klusowka river (27.0%), and gudgeon (48.4%) and sunbleak (30.0%) in the Utrata River. 9 reophilic species were identified in particular rivers, accounting for 8.6% to61.0% of total abundance. The abundance of pike, the major predator of small lowland rivers accounted for 0.1% in the Utrata River to 1.3% in the Rządza River. The highest abundance of fish in terms of numbers and the biomass per 1 km of river course were found in the Klusowka River and the Dluga River.