Natural Regeneration of Fish Assemblages in the Pilica River After a Reduction of Point‐source Pollution

Abstract

Electrofishing 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 (X 1 , X 2 ; Y 1 , Y 2 ) of neurons. No species attained its significantly highest biomass and/or frequency (expressed as IndVal) in X 2 (in which samples from the 1990s dominated), whereas 21 species did so in Y 1 or Y 2 (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 Y 1 and/or Y 2 when compared with X 2 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][2004][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.

Disturbed fish fauna zonation as an indicator of large-scale human impact: A case study (2011-2012) of the large, lowland Warta River, Poland

“…Section S3, which was the most degraded Warta section in previous decades (Kruk, ; Przybylski, ), not only continues to be polluted by its tributaries but it cannot be supplied with recolonisers coming from them either (compare with Kruk & Penczak, ). Also, section S2, which is rich in rheophils (Figure , Table ), cannot effectively serve as sources of recolonisers for S3 due to the 20‐km stretch of stagnant water in the Jeziorsko dam reservoir (Ciepłucha et al., ).…”

Application of a Kohonen's self‐organizing map for evaluation of long‐term changes in forest vegetation

“…IndVals complement the species planes produced by the SOM software, presenting the associations of a given species with SOM regions (Tszydel et al. ; Kruk & Penczak ; Penczak et al. ).…”

The Role of Streams of Different Sizes in Recovery of Fish Fauna in the Pilica River Catchment

“…While making the present comparison, we also presumed (as in the Kruk and Penczak, , study) that water quality improvement might be one of the factors of fish assemblage changes, but we also suspected that the improvement might act indirectly, through other factors, particularly stream size differences. Hence, the aims are to (1) determine if the rate of ichthyofauna recovery is the same in Pilica River tributaries as that recorded previously for the Pilica, and (2) determine the role of streams of the same river system for ichthyofauna recovery with respect to their size.…”