Flow restoration and the impacts of multiple stressors on fish communities in regulated rivers.
1. River regulation for hydropower is undertaken worldwide, causing profound alterations to hydrological regimes and running water habitats. Regulated catchments are often subjected to additional stressors, arising inter alia from agriculture, forestry and industry, which are likely to interact with impacts of river regulation on fish and other biota. Such interactions are poorly understood, hindering planning of effective mitigation and restoration. 2. We investigated fish responses to increased discharge (as a restoration measure) in regulated rivers in Sweden. We compiled electrofishing data from river channels downstream of hydropower dams, each of which either has or lacks a mandated minimum discharge corresponding to c. 5% of pre-regulation discharge. We further analysed interactions between flow restoration and co-occurring local and regional stressors. 3. River channels without a mandated minimum discharge were characterized by a low diversity of fish species with traits favouring persistence under unpredictable environmental conditions, including omnivory, short life cycles and small size. Additional stressors further reduced diversity and increased dominance by broad-niched, opportunistic species. 4. Both the presence and magnitude of a mandated minimum discharge were positively related to fish diversity and density, and the relative density of three economically and recreationally valuable species. However, the size of these relationships frequently varied with the presence of additional stressors. 5. Increasing levels of hydrological degradation and reduced connectivity at the catchment scale reduced positive flow-ecology relationships and hindered the restoration of fish communities towards reference conditions. However, application of a mandated minimum discharge also assisted in mitigating impacts of some co-occurring stressors, especially reduced riparian integrity. 6. Synthesis and applications. Additional stressors can strongly influence the outcomes of flow restoration for fish community diversity and composition. Our approach combining fish species and trait data from multiple flow restoration projects with information on additional stressors yielded valuable insights into factors affecting flow restoration success, useful for (a) identifying the systems most likely to benefit from mandated minimum flows, (b) modelling influences of multiple stressors on flow-ecology relationships, (c) prioritizing additional measures to manage co-occurring stressors and enhance outcomes from flow restoration.