Upstream movements of benthic macroinvertebrates in two different types of fishways in southwestern Germany

Upstream movements and migrations of benthic macroinvertebrates are an important component of running water dynamics. Many lotic organisms perform upstream movements at some stage in their life-cycle. We investigated whether a boulder ramp and a concrete bypass fishway on two streams in Baden-Wurttemberg, Germany, are successfully used by benthic invertebrates to reach upstream areas. We compared macroinvertebrate distribution at sites upstream and downstream of the fishways, rated the fishways according to their constructional features and conducted migration experiments with marked Gammaridae in the passages. The number of taxa and densities of macroinvertebrates did not significantly differ between upstream and downstream sites in both rivers. Constructional features and the results of the mark-recapture experiments indicated that the boulder ramp allows upstream migrations in principal, whereas the bypass is more difficult for macroinvertebrates to ascend. Both fishways immediately exit into an impoundment, which is a potential barrier for migrating invertebrates. High average current velocities, lack of lenitic banks and sheltered interstices, as well as problems caused by the operation of a power plant are factors explaining why the Enz River bypass is only marginally functional for invertebrates. The investigation concludes that in constructing fishways suitable for upstream movements of lotic invertebrates, more attention should be paid to their specific requirements.