Cascade of Dams Amplifies Predation Risks for Rheophilic Fish Eggs

ABSTRACT Rheophilic fish populations are experiencing significant declines, primarily because of human‐induced modifications of riverine ecosystems. River damming and associated habitat changes favour generalist fish species, potentially exacerbating the challenges faced by rheophilic species. In this study, we investigated the interaction between two fish species: the rheophilic asp ( Leuciscus aspius ; Linnaeus, 1758) and the generalist common bream ( Abramis brama ; Linnaeus, 1758), with a focus on the latter's predation on asp eggs. We hypothesized that habitat modifications—specifically, reduced water flow caused by high water level retention in downstream reservoir—directly influence the accessibility of asp spawning grounds. From 2018 to 2020, we monitored asp egg abundance at a fluvial spawning ground and used passive telemetry arrays to track common bream presence. Gut content analysis was used to confirm predation of asp eggs by common bream at the site. Our findings show that high water levels in 2018 and 2019, which slowed water flow, facilitated common bream access to the spawning ground. In contrast, the significantly lower water levels in 2020 increased water flow rates by two to three times, restricting common bream access. Statistical modelling revealed that river habitat modifications intensify interspecific interactions among fish species that would otherwise have minimal interactions in unaltered environments. These subtle yet significant shifts in trophic dynamics within anthropogenically altered riverine landscapes may contribute to the ongoing decline of rheophilic fish species.