Nature-based fish habitat enrichment of non-damming beaver structures positively affects fish species richness and density

The return of beavers to the strongly structurally altered Central European stream systems results in a variety of conflicts, potentials and opportunities. Among monetary compensation issues for landowners, target species conflicts with fish conservation remain unresolved. This work investigated the impact of beaver structures of the Eurasian beaver ( Castor fiber L.) on the fish community of a Bavarian stream system to quantify potential ecosystem services provided by the ecosystem engineering activities of beaver. In addition to beaver structures, artificial structures, constructed from bank clearcuttings, were introduced for comparison. Electrofishing and abiotic measurements were carried out to quantify the effects of the morphological and functional beaver structures on the fish community. In addition, structural characteristics, such as building material and volume, were characterized. Individual fish densities and species diversity were significantly higher in the beaver structures compared to both the reference reaches and the artificial structures. Species such as European chub ( Squalius cephalus L.), common dace ( Leuciscus leuciscus L.), European spirlin ( Alburnoides bipunctatus Bloch 1782), and common nase ( Chondrostoma nasus L.) benefited most from the beaver structures, particularly smaller size classes <15 cm. Artificial structures had a lower number of species and individuals. They not only differed from the beaver structures in their general fish communities, but particularly in the presence of target species of conservation and fish sizes. In addition to the already well-documented effects of beaver dams, our findings contribute important knowledge to the ecosystem engineering capabilities of the Eurasian beaver. Since the beneficial fish habitat effects of the beaver structures was mostly related to construction material, positioning, total and pore volume as well as flow velocity, these identified properties can also be used to guide future efforts of nature-based structural enrichment of stream habitats. • Beavers are ecosystem engineers creating valuable deadwood habitats. • Fish diversity in beaver- and man-made dead wood structures was assessed. • Fish diversity and abundance were highest in beaver-made structures. • Artificial structures did not fully mimic all functions of beaver-made ones. • Material composition, positioning, pore volume and flow determine effect size.