Background and aims
Freshwater fish populations are declining globally under the combined effects of river regulation, altered flow regimes, habitat degradation, water-quality decline, invasive species and climate change. Environmental flows are widely used to reduce the ecological effects of flow alteration, yet their effectiveness for fish conservation remains difficult to evaluate. We examined how evidence for fish responses to environmental flows varies across assessment methods, implementation contexts, flow components, ecological processes, non-flow stressors, geography and species traits.
Methods
We conducted a systematic review of published studies assessing fish responses to environmental flows. Evidence was examined across methodological approach, implementation status, flow-regime component, ecological process, response variable, non-flow stressor, geographical region and species traits.
Results
Our synthesis show concentration of evidence in habitat-hydraulic and ecohydrological assessments, with modelled evaluations more common than studies of implemented environmental flows. Reported responses focused largely on habitat availability, connectivity, movement and spawning, whereas population-level outcomes were less frequently assessed. Flow magnitude and timing received the greatest attention. Water quality and barriers were incorporated more often than sediment, invasive species and climate-related pressures. Evidence was also unevenly distributed across regions, species and ecological traits.
Conclusion
Environmental flows can create conditions favourable to fish, but evidence that these conditions lead to population recovery remains limited. Stronger inference will require more evaluations of implemented flows, longer monitoring, clearer population-level response metrics and explicit consideration of non-flow constraints.