Data from: Like a fish takes to water: Minimum discharge requirements to sustain rheophilic fish community dominance in bypassed river reaches

Hydropower production leads to losses of species and ecosystem types in
rivers. A major knowledge gap is represented by bypassed reaches where all
or most discharge is diverted to hydropower stations. Bypassed reaches may
have some flow, derived from tributaries and dam leakage, along with
minimum flow. In Sweden, mandates for minimum discharge are lacking for
most bypassed reaches, and when present, generally amount to only a few
percent of mean annual discharge. We asked how fish communities in
bypassed reaches differ from comparable non-regulated reference reaches,
and whether minimum discharge can help sustain fish faunas typical of
non-regulated rivers. We assembled a unique dataset of 2693 samples
collected via electrofishing by wading from 166 bypassed reaches and 247
free-flowing reaches spread across Sweden. To quantify flow-related
community changes, we compared the relative dominance of rheophilic
(preference for high flow conditions) versus limnophilic (preference for
slow-flowing to stagnant conditions) fish species, fish density, and fish
species richness between bypassed and free-flowing reaches, and
investigated the effects of minimum discharge on community structure.
Bypassed reaches had lower rheophilic dominance and fish density, but
slightly higher fish species richness than comparable free-flowing
reaches. Minimum discharge had positive effects on rheophilic dominance
and fish density but not fish species richness. For relatively low values
of minimum discharge, small changes in flow corresponded to large changes
in the predicted values of fish density (up to about 0.5 m3/s) and
rheophilic dominance (up to 2 m3/s), after which curves flattened. When
minimum discharge was expressed as a percentage of mean annual discharge,
rheophilic dominance and fish density increased almost linearly with
increasing discharge, but effects on fish density were dependent on river
size. We conclude that small increases in minimum discharge can
be particularly beneficial to the restoration of rheophilic community
structure, especially at low levels of minimum flow, even though the area
of lotic habitat has been reduced to a fraction of natural conditions.
Implemention of minimum discharge in bypassed reaches can thus be an
effective rehabilitation measure, providing ecological benefits helping to
meet environmental objectives.