Stable isotope (δ13C and δ15N) data indicate minor changes in the trophic niches of native whitefish and Arctic charr despite partly overlapping niches with the introduced smelt. Survey fishing data collected before (2007) and after (2016) the introduction indicates that smelt has become the numerically dominant fish species both in benthic and pelagic habitats, with concurrent reductions in the relative abundance of native European whitefish (Coregonus lavaretus) and Arctic charr (Salvelinus alpinus) populations. Here, we describe a case from a large lake in southern Norway, Storsjøen, where the illegal introduction of a small, predominantly planktivorous fish species, European smelt (Osmerus eperlanus), has led to changes in the native fish community and lake food web. Invasive and introduced fishes can affect recipient ecosystems and native species via altered competitive and predatory interactions, potentially leading to top-down and bottom-up cascading impacts. In most cases we have evaluated, the main focus has normally been on ecological mitigations of downstream impacts from hydropower operation, while upstream impacts, e.g., with lack of ecological flow, seem to be the case in many bypassed rivers. We also exemplify, however, that several recent revisions of hydropower licenses lack key measures to meet the expectations for best practise, in particular related to ecological continuum. Several Norwegian hydropower schemes have included many emerging best-mitigation measures. Updated knowledge about environmental effects and documentation of the ecological mitigation effects from measures are essential. In order to reach these objectives, best available mitigation measures for all rivers and lakes need to be implemented and regularly updated and revised in the river basin management plans, in line with EEA-relevant frameworks and regulations. Relevant knowledge about ecological conditions and possible cumulative impacts from multiple stressors, adaptive management, and pressure-pays principles are essential components in ecosystem-based management. A sequence of key management principles are incorporated in the Norwegian Nature Diversity Act and the Norwegian version of the EU Water Frame-work Directive as complementary regulations to ensure sustainable use of rivers and lakes modified e.g., for hydropower regulations.
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