Flow regime is a fundamental driver in fluvial ecosystems, shaping habitat structure and biodiversity, and sustaining ecological integrity. Fish respond to flow fluctuations but whether aspects of flow variability represent organism-level stressors is poorly understood. To find existing evidence of fluvial fish organism-level responses to flow variability (natural or anthropogenic), and whether it resulted in quantifiable stress (i.e. departure from homeostasis), we reviewed literature on the physiological responses of fish exposed to flow. Among 58 articles that we identified to be relevant to the research question, 40 reported whole-animal responses (tertiary responses to stress), 37 reported blood and tissue level changes (secondary responses to stress) and 18 reported neuroendocrine changes (primary responses to stress), exclusively or combined. Whole-animal responses (e.g. growth or disease resistance) were more commonly assessed due to their broader use in population and community dynamics studies. Due to their long-term character it was difficult to isolate flow variability as the only stressor and to understand the underlying mechanisms that culminated in a stress response. Our review indicates that flow variability can be a stressor for fish but it remains unclear if events such as floods or hydropeaking are inherently stressful per se. More experimentation is needed to find out if flow variability presents a stressor to fluvial fish, what thresholds trigger a stress response and to better understand the relative role of the different flow components. This knowledge can be used to define robust stress biomarkers (particularly for field studies) and propose adequate flow thresholds.

Hydropeaking is the rapid change in the water flow downstream of a hydropower plant, driven by changes in daily electricity demand. These fluctuations may produce negative effects in freshwater fish. To minimize these impacts, previous studies have proposed habitat enhancement structures as potential mitigation measures for salmonids. However, the recommendation of these mitigation measures for cyprinids remains scarce and their effects unknown. In this study, the effects of potential habitat mitigation structures under simulated hydropeaking and base-flow conditions are examined for Iberian barbel (Luciobarbus bocagei) in an indoor flume. Solid triangular pyramids and v-shaped structures were evaluated as potential flow-refuging areas and compared with a configuration without structures. A novel, interdisciplinary approach is applied to investigate individual and group responses to rapidly changing flows, by assessing physiological (glucose and lactate), movement behaviour (structure use, sprints and drifts) and the pressure distribution using a fish-inspired artificial lateral line flow sensor. The major findings of this study are four-fold: 1) Under hydropeaking conditions, the v-shaped structures triggered a lactate response and stimulated individual structure use, whereas solid structures did not elicit physiological adjustments and favoured individual and group structure use. Overall, both solid structures and their absence stimulated sprints and drifts. 2) The hydrodynamic conditions created in hydropeaking did not always reflect increased physiological responses or swimming activity. 3) Each event-structure combination resulted in unique hydrodynamic conditions which were reflected in the different fish responses. 4) The most relevant flow variable measured was the pressure asymmetry, which is caused by the vortex size and shedding frequency of the structures. Considering the non-uniform nature of hydropeaking events, and the observation that the fish responded differently to specific flow event-structure combinations, a diverse set of instream structures should be considered for habitat-based hydropeaking mitigation measures for Iberian barbel.

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.