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The Warta River (795 km long) is the largest, right side tributary of the Odra (Oder) River. The study presents results from one of the best-documented long-term monitoring projects in Poland based on four terms of electrofishing: 1963–66, 1986–88, 1996–98 and 2011–12, conducted in the upper Warta. The Warta River underwent human-induced modifications typical for most European lowland rivers (damming, regulation, water pollution), of which the most destructive for fish was point-source water pollution. In the late 1980s, pollution reached its highest level and stopped increasing as the former political system collapsed and many industrial plants went bankrupt. Surprisingly, recovered fish assemblages were not recorded during the sampling in 1996–98, but in 2011–12. This is why we believe that in large degraded rivers, it takes about 10–20 years before a considerable improvement in fish fauna can be observed. Ichthyofauna recovered to a good status, but was qualitatively different compared to the good status observed in the 1960s. On the one hand, in 2011–12, high species richness and high assemblage diversity were observed, and many species, including seven rheophils, were more common than earlier. On the other hand, the populations of catadromous eel and anadromous vimba have not recovered, and these species were absent in the 2011–12 samples. Because water quality has improved, the most important factor seems to be the impact of the Jeziorsko dam reservoir which is located downstream of the study area and has no fish pass. The above patterns were recognised in this paper with a Kohonen artificial neural network, which is a tool methodically correct for analysing complex non-linear relations. Additionally, indicator species analysis allowed for the identification of significant associations of taxa with specific environmental states and was helpful in determination of ecological statuses of the river stretches. We therefore recommend the combined use of Kohonen artificial neural networks and indicator species analysis in long-term monitoring analyses. [ABSTRACT FROM AUTHOR]

Submersed plants appeared in the impounded section of the Warta River as a consequence of low discharge in summer and were colonized by epiphytic fauna dominated by Hydra sp., Simuliidae, and Chironomidae. The three-spined stickleback consumed it voraciously, and their diet composition closely reflected the available food resources. In May their alimentary tracts were filled with Cladocera drifting from the reservoir, and then by the dominant taxa associated with macrophytes, especially chironomid periphyton scrapers. A comparison of dipteran head capsule width in the food resources and in the alimentary tract proved the three-spined stickleback preference for the largest larvae (optimal foraging strategy). [ABSTRACT FROM AUTHOR]

The Warta River (795 km long) is the largest tributary of the Odra (Oder) River. This study presents results from one of the best-documented long-term monitoring projects in Poland, based on three periods of electrofishing: 1986–88 (T1), 1996–98 (T2) and 2011–12 (T3). After decades of severe point-source pollution, water quality has been improving since the early 1990s. However, the recovery of fish assemblages was recorded with a considerable delay, i.e. not in T2 but in T3. Species richness in T3 increased by ≥50% in relation to T2. The recovery process proceeded at a different pace in the upper (section X), middle (section Y) and lower (section Z) river courses. The good status recorded in X in T3 was qualitatively different from the good status observed in T1 as migratory and/or lithophilic species were less common. Section X, with relatively clean tributaries, was isolated from the rest of the Warta system by the Jeziorsko Reservoir constructed between X and Y in 1986 (without fish passage). Ichthyofauna in Y was not only in the poorest condition but was also recovering very slowly because of migration barriers and polluted tributaries, resulting in a severe shortage of sources of recolonizers. The quickest positive changes were recorded in Z because of the beneficial role played by certain less degraded tributaries and the direct connection to the recipient Odra River. Significant increases in biomass were recorded in Z for both limnophilic and rheophilic species; the former may be linked to zones of stagnant water existing between groynes and the latter to fast-flowing water near groyne heads. [ABSTRACT FROM AUTHOR]