Wpływ małej zapory na zmiany składu ichtiofauny w cieku Sitna w ciągu 9 lat (otulina Drawieńskiego Parku Narodowego)

Many authors point out the negative impact of hydraulic engineering constructions, not only on the structure of fish fauna, but also other aquatic organisms. This applies particularly to transverse bulkheads dividing the flowing waters into two different parts in terms of hydrological, biological and environmental components. Most studies relate to large rivers, where with no doubt it was found that the dams change physicochemical conditions of water. Research in this study was conducted in Sitna stream located in the buffer zone of Drawieński National Park (DNP), a part of the central Drava basin. This fiveorder, right tributary of middle Drawa river, flows into Adamowo Lake. Drawa river flowing out from Adamowo Lake is the northern border of the DNP (Fig. 1). In 2005, on kilometers section of stream water swelling dam was built by the State Forests plantation forest. During the year the water swells to a height of about 50 to 100 cm. The study of fish fauna of Sitna were conducted in August 2000, 2006 and 2009. In order to determine the species composition of fish fauna and the length of individual fish, the fish were obtained using the battery unit IUP 12. Catches were conducted throughout the length of watercourse.  The results of the research show that a small dam influences very negatively on changes in the composition of fish fauna in small stream. Results also show that a small dam on a small watercourse, significantly affects the fish fauna. After the building of river dam the number of cyprinids significantly increased. While the number of typical river species, particularly brown trout and gudgeon essentially decreased. Besides, in the watercourse the rapid increase of pike was observed. This species found in the watercourse good spawning conditions. In last year of the study, 2009 in stream section over the dam the river species were absent. Currently, the section suitable for salmonids is the section below the dam. It seems that also in this section, the hydrological conditions will be worse and river fish will be absent in the whole watercourse. It is expected that a progressive changes will lead in consequence to the total extinction of valuable species of fish and to irreversible changes both biological and abiotic.