NATURAL REGENERATION OF FISH ASSEMBLAGES IN THE PILICA RIVER AFTER A REDUCTION OF POINT‐SOURCE POLLUTION

Abstract

ABSTRACTElectrofishing was conducted along the Pilica River, the biggest West tributary of the Vistula, Poland. This paper presents a comparison between the 1990s (1994–1995, 63 sites) and 2000s (2003–2005, 64 sites), that is, because the water quality started improving after 1989. In the Pilica, there is an increased risk of masking relationships between fish distribution and abiotic factors because a dam without a fish pass was constructed in the middle course of the river in 1973, perhaps resulting in fewer possibilities for potential colonizers reaching river fragments fit for colonization. The main aim of this study was to check if natural fish fauna regeneration took place in the river following a significant sewage input reduction. The fish samples were classified with a Kohonen artificial neural network and assigned to two main clusters (X and Y) and, respectively, two pairs of subclusters (X1, X2; Y1, Y2) of neurons. No species attained its significantly highest biomass and/or frequency (expressed as IndVal) in X2 (in which samples from the 1990s dominated), whereas 21 species did so in Y1 or Y2 (containing almost all of the samples from the 2000s), better testifying to the aquatic environment at the sites where the samples assigned to two latter subclusters come from. Moreover, significantly higher values for Y1 and/or Y2 when compared with X2 were recorded in the richness of the fish species, the total fish biomass and total rheophilic fish biomass. In summary, the positive reaction of ichthyofauna to the improvement of the water quality, stated after 1989, was not recorded during the sampling in 1994–1995, but with a considerable delay in 2003–2005. It should be emphasized that the improvement in fish assemblages was recorded in the Pilica both upstream and downstream from the dam reservoir. A good ecological status was recorded even for some samples from the 1990s. They were collected in the lower river course, where the fish could move freely from the Vistula. This testifies the combined role of the presence of potential colonizers and the functioning ecological corridors for fish assemblages in a river system under human impact. Copyright © 2012 John Wiley & Sons, Ltd.