Almost all large rivers in Sweden have been developed for hydropower production. Sweden has c. 2400 dams directly associated with hydropower generation, providing 35-45% (on average 67 TWH/y) of national electricity. Most hydropower exploitation was approved under the water law of 1918, with little or no consideration of ecosystem impacts. Hence, the majority of bypassed reaches at hydropower stations in Sweden have no mandated minimum discharge, and almost none have a genuine environmental allocation. Less than 10% of hydropower dams are equipped with fishways for upstream and/or downstream migration. Deteriorated hydro-morphological conditions is the primary reason why Swedish rivers fail to achieve “good ecological status” according to the EU Water Framework Directive.
We compiled a database with descriptive information about nearly all (c. 1000) bypassed reaches in Sweden, and analyzed fish community responses to mandated minimum discharges in a subset of reaches. Only c. 23% of all bypassed reaches have a mandated minimum discharge (on average equal to 3.6% of mean annual discharge), mean bypassed reach length is 1317 m (range 14 m to 68 km).
Preliminary analyses indicate that the presence of a minimum discharge have positive effects on rheophilic fish species, total fish abundance and fish species richness. A shift from lentic to rheophilic dominance occurred at on average 0.3 m3/s minimum discharge, which corresponded to 0.1-28% of mean annual discharge. Additionally, small increments in minimum discharge had large effects on the fish community up to 1.5 m3/s minimum discharge. These results indicate that rheophilic fish dominance could be restored in Swedish bypassed reaches through the allocation of a small proportion of total flow as minimum discharge, although the area of lotic habitat would remain drastically reduced under such a mitigation scenario. Work is ongoing to investigate if and how the macro-invertebrate community responds to minimum discharge.