Frontiers in Marine Science (Nov 2015)
Spatial and temporal (1963-2012) variability of ichthyofauna in the large lowland Warta River, Poland
Abstract
The Warta River is a tributary of the Odra (Oder) River. It is 795.2 km long. In 1986 the large Jeziorsko dam reservoir was constructed in the 306th km of its course. In the late 1980s, the river pollution assumed its highest level and stopped increasing as the former political system collapsed and many industrial plants went bankrupt. Unified fish electrocatches have been performed along the Warta River since the 1960s. During the last sampling, in 2011-2012, fish fauna in the middle course of the river was in the poorest condition due to the destabilizing upstream impact of the Jeziorsko dam reservoir, large amounts of wastewater input to the river, and the lack of unpolluted tributaries that could serve as sources of recolonizers. The weakest human pressure was reported for the upper and lower courses, which resulted in higher numbers of species significantly preferring them, and the higher species richness. Species richness significantly increased in comparison with the previous sampling occasions (in 1963-66, 1986-88, and 1996-98). Significant increases in the stability of occurrence, abundance or biomass were recorded for many species including burbot, chub, dace, ide, gudgeon, bleak, bitterling, perch and spined loach. Significant declines in the above mentioned population parameters were rare and related mainly to European eel (a migratory species). The previously recorded strong negative trend (declines in rheophils, increase in the dominance of roach and perch) has been reversed. However, regenerated fish assemblages were not recorded in 1996-98 (i.e. several years after the beginning of the improvement in water quality) but in 2011-2012 (i.e. about one decade later). We have noticed a similar delay in ichthyofauna recovery also in the Pilica River (Vistula system). This is why we believe that about 15 years are necessary to observe a considerable improvement in fish fauna in larger degraded rivers.
Keywords
