There is a cascade of hydroelectric power plants built on the Váh River. From a water-management point of view, the natural channel is used to drain extreme discharges. During most of the year, discharges are regulated by water-management structures. These discharges are not used for energy-related purposes; therefore, it is important to determine the optimal discharge that will not negatively affect the ecosystem of the stream. The minimum balance discharge (hydro-ecological discharge) was determined based on the instream flow incremental methodology (IFIM) using the riverine habi-tat simulation system (RHABSIM). Input data were obtained from direct measurements on three reference reaches in the area between the cities Piešťany and Nové Mesto nad Váhom. Hydraulic flow characteristics were derived from three measurements at different water levels. Habitat quality was represented by ichthyofauna. Data to determine the habitat suit-ability curves of fish were obtained using a diving technique to collect video footage. The modelling resulted in the quanti-fication of the effect of discharge on ichthyofauna as a bio-indicator of habitat quality, which implied the need of 20 m3∙s–1 for a minimum balance discharge in summer.

Structure of fish communities of five small water reservoirs located near the Chodelka river was investigated during the years 2002, 2008 and 2016. In the analyzed water bodies seven fish were noted, representing four families: Cyprinidae (4 species) and Esocidae, Cobitidae, Peri-cidae and Ictaluridae (1 species each). In the structure of collected fish one protected species (M. fossilis) and one alien species (C. auratus gibelio) were presented. Domination structure of ichthyofauna showed considerable differences. Among the dominant species were: R. rutilus (all five reservoir) and C. auratus. The percentage of each fish species in the biomass was slightly different in relation to the structure of domination expressed by the number of identified individuals. Regardless of the reservoir, larger share held: E. lucius, R. rutilus and C. auratus.

The paper presents the analysis of structure of fish communities in five small postexcavation reservoirs situated in Tyśmienica valley (Polesie Lubelskie). Special attention was paid to the quality of population of lake minnow Eupallasella percnurus. By the time the extensive drainage works were done in the 60s and 70s of the 20th century, Polesie Lubelskie was rich in small water reservoirs. As a result of drainage of wetlands, many of them disappeared. Among those which still exist, most of them are located in the Tyśmienica valley. The study revealed that the studied reservoir inhabited seven fish species representing two families. It was also found that structure of dominance and biomass structure is very diverse. In each reservoir the dominant species was different. The lake minnow E. percnurus was determined in three reservoires.

The upper section of the Bystrzyca River has a character of trout and grayling zone of river. The aim of the study was to determine the species composition of fish in the upper section of the Bystrzyca River and attempt to evaluate angling pressure on fish in this section of river based on analysis of records of amateur fishing. To determine the fish composition the control fishing was conducted using electric fishing gear. The assessment of angling pressure was based on the analysis of 2135 records of fishing in the mountain type rivers in 2012 year, obtained from PAA Lublin. The brown trout and grayling were a dominant fish species in the upper section of the Bystrzyca River, and the first of them was present on the whole length of the river and had the highest average density. The changes of fish community along the river course from Salmonids lead to an increase of the share of Cyprinids fish has been observed. Analysis of records of ama-teur fish catch revealed that the upper section of the Bystrzyca River is under considerable of angling pressure and the most commonly fished species was brown trout. However, the properly carried fishery management on this section of river and the establishment of „no kill” zone allows for proper maintenance of the populations of fish species valuable for anglers.

Structure of fish communities of five small water reservoirs located near the Wieprz--Krzna Canal was investigated during the years 2002, 2008 and 2014. In the analyzed water bod-ies seven fish were noted, representing four families: Cyprinidae (5 species) and Esocidae, Cobi-tidae and Ictaluridae (1 species each). In the structure of collected fish two protected species (M. fossilis and R. sericeus amarus) and one alien species (C. auratus gibelio) were presented. Domination structure of ichthyofauna showed considerable differences. Among the dominant species were: I. nebulosus (all five reservoir), C. auratus (Jabłoń Center 2, Jabłoń Church and Jabłoń Firlej-Ostrówek), and R. rutilus (Jabłoń West and Jabłoń Firlej-Ostrówek). The percentage of each fish species in the biomass was slightly different in relation to the structure of domination expressed by the number of identified individuals. Regardless of the reservoir, larger share held: E. lucius, I. nebulosus and C. auratus Moreover, one of the purpose of the study was searching for a new stands of lake minnow. However, the species was not recorded in fish communities of any of the reservoirs, despite of the ecological state of reservoirs indicating the potential habitat for lake minnow.

The brown bullhead (Ameiurus nebulosus) is an invasive species of fish fauna in Poland. This species is widespread throughout the country but its abundance and characteristics of populations present in water reservoirs are relatively little known. Aim of this study was to determine the abundance and characteristic of the population of brown bullhead in the Domaszne Lake. The control fish catches were carried out three times in 2012 and 2013 years (spring, summer, autumn). Brown bullhead had a small share in the structure of fish abundance (2.7%) and biomass (7.5%), but his presence was reported in results of all control fishing. Relative abundance of brown bullhead averaged about 7.8 CPUE and biomass about 505.5 WPUE. The fishes the greatest total length and weight had in the spring (Tl = 18.5 cm, W = 77.7 g) but the highest value of the coefficient of condition (K = 2.08) in summer were noted. Brown bullhead with a total length from 16 to 18 cm accounted for as much as 59% of the population and distribution of size structure confirm a fact that has a stable population.