Characterization of freshwater fish assemblages from small streams in the Sava river basin

The Sava river has always been a source of good quality food for local inhabitants, but the number of fish deteriorated in the second half of the 20th century. Reasons for such losses were changes of flow characteristics of watercourses such as streams and channels which consequently lead to the altered hydrological regime, ground water levels, temperature, quantity of dissolved oxygen, concentration of nutrients in the water, and flow rate. These changes most often result in the loss of species, with the common introduction of new resistant species. The purpose of this research was to determine the composition of ichthyocenosis of small streams and canals located in eastern Croatia, to determine the dominance of species and diversity of communities between different watercourses. Additionally, ecological stress of fish communities was investigated. Species richness, Margalef’s index, Simpson’s index and Shannon Weawer index were used to investigate fish diversity and ABC index to determine ecological stress. Dominant native species in investigated streams were chub (Squalius cephalus) and gudgeon (Gobio gobio) while the most dominant alien species were Prussian carp (Carassius gibelio) and pumpkinseed (Lepomis gibbosus). Dominant native species in investigated channels were bleak (Alburnus alburnus) and European perch (Perca fluviatilis) while the most dominant alien species were Prussian carp (Carassius gibelio) and pumpkinseed (Lepomis gibbosus). The highest number of specimens was caught in the Reka stream while the highest number of species richness was found for the Gradnica stream. Margalef index was the highest for the Utinja stream, Simpson index for the Reka stream and Shannon Weawer index for the confluence of Okičnica and Gonjava. According to ABC index, all sampled waterbodies and all identified fish communities were under moderate or heavy stress. This led to conclusion that fish communities in most of streams were under anthropogenic influence. Very low number of predatory fish species (only chub is present in a higher percentage) were found and mostly small short-lived species dominated. Lower number of species has been recorded in the canals, in comparation with streams, and number of specimens of alien species predominate in sample. It is necessary to reduce pollution and human impact on small streams to eliminate stress for aquatic organisms in general, which is only model how to successfully recover native fish communities