Characterization of sections of the Sava River based on fish community structure

Sampling was undertaken, with the same fishing gear and along the Sava River, from its source to its confluence, in September 2014 and September 2015. In total, 44 fish species were identified, of which 37 were native species and 7 were alien. Fish samples revealed independence in terms of both species composition and their abundance under different hydrological conditions. During flooding and high water levels in 2014, pelagic fish species were sampled in greater proportion than at lower water levels in 2015 when benthic fish species were more abundant. The flood wave in 2014 was accompanied by catch of common carp, Cyprinus carpio, a typical lower rhithron fish species in the upper course, and of tench, Tinca tinca, a typical potamon fish species of backwaters, in the main channel of the lower Sava River. One specimen of bighead goby, Ponticola kessleri, which is common in the potamon fish community, was caught during the 2015 sampling close to the boundary between the upper and middle sections of the Sava. This is the first record of Ponto-Caspian gobies in the inland waters of Slovenia. Its finding far upstream indicates a strong effect of an as yet unidentified stress along the Sava River up to the spot where the bighead goby was sampled. Finally, these results indicate that pelagic fish species are more resistant to the stressful effect of flooding than benthic species, and that the structure of fish communities is influenced/affected by flooding as a short-term stressor. The progressively increasing number of alien fish species downstream in the Sava River point to the effects of long-term human-induced stressors in the area.