Can fish habitat restoration for rheophilic species in highly modified rivers be sustainable in the long run?

As a result of the ongoing loss of freshwater biodiversity, restoration of riverine habitats is high on the agenda. However, it remains controversial if commonly used instream fish habitat restoration techniques have sustainable effects in highly modified waterbodies. This study compared the effects of introducing four different instream structures (bank rip-rap, benched bank rip-rap, successively grown riparian wood and introduced dead wood, nine replicates each) on the fish community distribution in the river Günz in Germany. To assess the sustainability of the restoration measures, different time points (seven and two years after the restoration) and seasons were considered. Out of all measures, the introduction of dead wood had strongest effects on fish aggregation (biomass and density) as well as on species richness and diversity. Even seven years after restoration, no alterations in the density and demography of target species in conservation was detectable. These results suggest that instream habitat restoration measures are unlikely to fully mitigate deficiencies in highly modified rivers. Consequently, the investment of resources for aquatic restoration may have greater effects in systems that are closer to an optimal state, unless greater effort into restoration beyond the main channel is made.