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

Upstream movements and migrations of benthic macroinvertebrates are an impo rtant component of running water dynamics. Many lotic organisms perform ups tream movements at some stage in their life-cycle. We investigated whether a boulder ramp and a concrete bypass fishway on two streams in Baden-Wurtte mberg, Germany, are successfully used by benthic invertebrates to reach ups tream areas. We compared macroinvertebrate distribution at sites upstream a nd downstream of the fishways, rated the fishways according to their constr uctional features and conducted migration experiments with marked Gammarida e in the passages. The number of taxa and densities of macroinvertebrates d id not significantly differ between upstream and downstream sites in both r ivers. Constructional features and the results of the mark-recapture experi ments indicated that the boulder ramp allows upstream migrations in princip al, whereas the bypass is more difficult for macroinvertebrates to ascend. Both fishways immediately exit into an impoundment, which is a potential ba rrier 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 conclud es that in constructing fishways suitable for upstream movements of lotic i nvertebrates, more attention should be paid to their specific requirements.