Response of the reproductive fitness and population density of Gammarus fossarum Koch, 1836 (Amphipoda: Gammaridae) to damming: a case study from the dammed sub-mountain stretch of Čierny Váh River, Slovakia

Abstract The plasticity of some biological and ecological characteristics of Gammarus fossarum Koch, 1836, a species widely distributed in the rivers of continental Europe, has been the subject of many scientific studies. The effects of river damming are reflected in the disruption of the natural environmental continuum of rivers, which has been demonstrated in this and other studies. We aimed to investigate the variability of population characteristics (relative reproductive fitness, pairing success, and fecundity), and population density in six local populations distributed along Čierny Váh River, Slovakia, a sub-mountain dammed river. Our results showed that the population inhabiting the site immediately downstream of the reservoir had significantly higher density, but its individuals had lower body size, pairing success, and fecundity compared to individuals at the other sites. As a result, the lower values of these characteristics were reflected in a different distribution pattern of relative reproductive fitness among individuals at the sites. Individuals at sites downstream of the reservoir, particularly those immediately below the reservoir, had lower mean fitness values. Our study demonstrated that the disturbance in the river environment caused by the dam not only affects the structure of the aquatic community, but can also alter population-level characteristics.