Modelling of environmental flow options for optimal Atlantic salmon,Salmo salar,embryo survival during hydropeaking

Recent findings on the causes of Atlantic salmon embryo mortality during winter in a hydropeaking river suggest that long duration drawdowns during very cold periods are the most likely cause of mortality in the ramping zone. This paper presents a framework in which thresholds for optimal embryo survival at the microscale are linked to physical habitat requirements at the mesoscale and integrated into alternative hydropower operations at the catchment scale. The connections within this framework are derived from a one-dimensional hydraulic model at the mesoscale and a hydropower simulation programme at the catchment scale. The economic costs and feasibility of several alternative options for hydropeaking operation that would comply with ecological requirements for optimal survival of embryos were evaluated. A method to assess a wide range of alternative hydropower options that considers key factors to mitigate the conflicting requirements of ecological targets, technical feasibility and economics is presented. Targeted alternative environmental flow releases to meet specific ecological objectives are often more effective than general operational rules to comply with legislation. The development of well-informed and targeted mitigation strategies is important for future environmental hydropower management