Are Fish Assemblages Establishing at Restored Sites Predictable?

It is commonly assumed that the species pools in the surroundings of restored reaches are important for the outcome of river restoration projects; however, their impact has rarely been quantified. Using electric fishing data from 18 river restoration projects and their surroundings, we analyzed how species occurrence patterns in the surroundings affect the species assemblages that establish at restored river reaches. 98.3% of the species that established at a restored site had the closest known population within a radius of 5km around the restored site. The number of species establishing at the restored sites depended on the number of species present within this range. Even though restorations shifted the fish assemblages marginally towards their natural reference state, the assemblages at the restored sites were still far from reference conditions since the species pools in the vicinity of the restored reaches were greatly impoverished. The assemblage structure of fishes in the 5km species pool explained 45% of the deviance in a generalized linear model on the species establishment after a restoration. Species presence at unrestored conditions as well as the species occurrence rates and population densities in the species pool were the most important variables in the model. This study demonstrates that knowledge on the species pool can be used to estimate the prospect of planned river restoration projects to restore natural species assemblages.