Spatially diverse recovery (1986–2012) of fish fauna in the Warta River, Poland: The role of recolonizers’ availability after large-area degradation

Abstract 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.