Wasted effort or promising approach – Does it make sense to build an engineered spawning ground for rheophilic fish in reservoir cascades?

Anthropogenic alterations such as the construction of dams and reservoirs led to a loss of quantity, quality and connectivity of habitats for riverine fish species. The resulting decline of freshwater fishes has prompted many restoration efforts addressing bottlenecks in their life cycles such as improving spawning habitat quality. Whilst there is a wealth of studies addressing the benefits and challenges of spawning ground restoration for rheophilic fishes in lotic ecosystems, there is a paucity of information on the usefulness of such measures in almost lentic systems. In this study, an engineered spawning ground with areas of different grain sizes, built as a mitigation measure for a hydropower plant construction in a near-stagnant reservoir cascade, was assessed including physical and chemical characterisation, spawning habitat use and recruitment of target species for conservation. During the six-week investigation period, about 4000 larvae and 18,000 eggs of mainly ubiquitous species such as roach (Rutilus rutilus, L.) and European perch (Perca fluviatilis, L.) were detected by drift netting and sampling of the gravel bed. The engineered spawning ground was successfully used for recruitment by the rheophilic gravel spawning cyprinids asp (Leuciscus aspius, L.) and ide (Leuciscus idus, L.). For all species, the highest density of eggs and larvae was found in the area with the highest current velocity directly at the turbine outlet. The findings of this study illustrate the potential of engineered spawning grounds for fish conservation even in heavily modified water bodies such as reservoir cascades. However, evidence of successful reproduction of rheophilic species on the spawning ground alone does not allow any predictions on further population development, since crucial prerequisites in the further ontogeny may pose additional bottlenecks.