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The rapid river flow variations due to hydropower production during peak demand periods, known as hydropeaking, causes several ecological impacts. In this study, we assessed the potential of an overhead cover and velocity-refuge structure in an indoor flume as an indirect hydropeaking mitigation measure for the cyprinid species Iberian barbel (Luciobarbus bocagei). We designed a lab-scale LUNKERS-type structure using two different materials which were used separately: Wood (opaque) and acrylic (transparent), tested under hydropeaking and base-flow events. Physiological (glucose and lactate) and behavioral (structure use) responses were quantified. The structure use (inside and in the vicinity) and the individual and schooling behavior was assessed. Although there was no evidence that the hydropeaking event triggered a physiological response, the wood structure use was significantly higher than the acrylic one, where the metrics of use increased in the hydropeaking event. Differences between individual and group behavior were only higher for the entrances in the wood structure. The higher frequency of wood structure use under hydropeaking conditions suggests that the visual stimulus conferred by this shaded refuge enables fish to easily find it. The results suggest that the use of overhead and velocity-refuge structures may act as an effective hydropeaking flow-refuge mitigation measure.

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