The Czarny Dunajec River, Polish Carpathians, has been considerably modified by channelization and gravel mining-induced channel incision and it varies in morphology from a single-thread, incised or regulated channel to an unmanaged, multi-thread channel. For twelve cross-sections with between 1 and 4 flow threads, the abundance and diversity of fish fauna were determined by electrofishing and compared with an assessment of hydromorphological river quality and the variation in flow depth, velocity and bed material size. Hydromorphological quality of the river varied between high-status conditions (Class 1) in unmanaged, multi-thread cross-sections with varying proportions of islands and Class 4 in channelized cross-sections. The increased number of low-flow channels in a cross-section was associated with a larger aggregated width of low-flow channels and greater variation in flow depth, velocity and bed material size. Single-thread cross-sections hosted only 2 fish species and 13 individuals on average, whereas 3-4 species and 82 individuals on average were recorded in cross-sections with four low-flow channels. Regression analysis indicated that both the number of fish species and individuals increased linearly with increasing variation in depth within a cross-section and exponentially with improving hydromorphological river quality, while they were unrelated to flow width, suggesting that it is the increase in variability of habitat conditions rather than simple habitat enlargement, that supported the increased abundance and diversity of fish fauna in the multi-thread cross-sections. This study shows that the simplification of flow pattern and the resultant degradation of hydromorphological river quality, caused by human impacts, is reflected in remarkable impoverishment of fish communities and that recovery of these communities will require an increase in morphological complexity of the river. Figure 3. Scores given by four evaluators for the hydromorphological river quality in the investigated cross-sections of the Czarny Dunajec and the average of the four estimates. The number of flow threads in each cross-section is also indicated Figure 5. Scatter plots of physical characteristics of the Czarny Dunajec versus the number of flow threads in the investigated cross-sections.The significance of the difference of the parameter means between the cross-sections with one and four low-flow channels were determined by Mann-Whitney test. The dashed lines link the parameter means between the two groups of cross-sections

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A cline in microsatellite diversity appeared among populations of Barbus carpathicus in four adjacent river basins on the northern slopes of the Carpathian Ridge, with western populations more variable than eastern ones (HS equals 0·45, 0·32, 0·23 and 0·13, respectively). Overall genetic variation was low, likely reflecting population bottlenecks during postglacial expansions through the Danube–Vistula watershed and eastward into Carpathian tributaries of the Vistula River. Genetic similarities between populations in the westernmost tributary of the Dniester (Strwiąż River) and those in the adjacent Vistula tributary (San River) suggest that at least the western part of the Dniester basin was colonized from the Vistula River system.

Common bream Abramis brama, roach Rutilus rutilus and their hybrids were collected in the Dobczyce Reservoir in southern Poland in 2006-2013 to study whether it is better for a hybrid individual to resemble and compete with one of its parents, or to minimize competition by having a distinctive phenotype. All hybrids were F crosses and originated predominantly (93·2%) from matings between female A. brama and male R. rutilus parents. In morphometric analyses, a newly defined coefficient, L = 2·5 (body mass) (L × body depth) , which enables forms with similar length-depth proportions but different length-mass relationships to be distinguished was used. Morphometric and meristic characteristics of the hybrids were intermediate in comparison with the parental species, with small but significant deviation towards R. rutilus in longitudinal body dimensions (trunk and tail length) and towards A. brama in body cross-sectional shape (body depth and L coefficient). This may result in a more R. rutilus like propulsion in hybrids and a more A. brama like ability to manoeuvre.

Planning and implementation of effective restoration projects require appropriate assessment of a river's hydromorphological status. Two European standards on hydromorphological assessment of rivers and hydromorphological assessment methods used in Poland are reviewed in the context of their applicability for river restoration purposes. River Hydromorphological Quality assessment method is presented with a case study of the Biała River, Polish Carpathians, where this assessment was used as basis for a restoration project aimed to establish an erodible river corridor. The results of the assessment revealed significant differences in hydromorphological quality between unmanaged and channelized river crosssections, indicating channel regulation as a major cause of the hydromorphological degradation of the Biała and confirming the choice of the erodible river corridor as an appropriate method of its restoration. The assessment indicated hydromorphological features of the river that were severely modified within the channelized reaches and which are likely to improve the most with the removal of bank protection and allowing free channel migration.

We studied the effect of input of allochthonous organic matter on rotifers in a medium-sized (9.8 km 2 ) reservoir correlating rotifer abundance to values of d 13 C in a sestonic fraction of !10 mm. Total density of rotifers during the study period was significantly correlated with tributary inflow during the time preceding the sampling date, with a delay of about 10-13 days. Concomitant with elevated tributary inflow, the d 13 C value in the sestonic fraction of plankton samples decreased approaching a level typical of terrestrial material. Thus, the response in rotifer production might be due to the input of energy in the form of dissolved and/or particulate allochthonous organic matter that becomes available to the rotifers through the microbial food web.

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