Response of European grayling, Thymallus thymallus, to multiple stressors in hydropeaking rivers.

Rivers of the large Alpine valleys constitute iconic ecosystems that are highly threatened by multiple anthropogenic stressors. This stressor mix, however, makes it difficult to develop and refine conservation and restoration strategies. It is, therefore, urgent to acquire more detailed knowledge on the consequences and interactions of prevalent stressors on fish populations, in particular, on indicator species such as the European grayling Thymallus thymallus. Here, we conducted a multi-river, multi-stressor investigation to analyze the population status of grayling. Using explorative decision-tree approaches, we disentangled the main and interaction effects of four prevalent stressor groups: flow modification (i.e., hydropeaking), channelization, fragmentation, and water quality alteration. Moreover, using a modified variant of the bootstrapping method, pooled bootstrapping, we determined the optimal number of characteristics that adequately describe fish population status. In our dataset, hydropeaking had the strongest single effect on grayling populations. Grayling biomass at hydrological control sites was around eight times higher than at sites affected by hydropeaking. The primary parameters for predicting population status were downramping rate and peak amplitude, with critical ranges of 0.2-0.4 cm min-1 and 10-25 cm. In hydropeaking rivers, river morphology and connectivity were the preceding subordinated parameters. Repeating the procedure with pooled bootstrapping datasets strengthened the hypothesis that the identified parameters are most relevant in predicting grayling population status. Hence, hydropeaking mitigation based on ecological thresholds is key to protect and restore already threatened grayling populations. In hydropeaking rivers, high river network connectivity and heterogenous habitat features can dampen the adverse effects of pulsed-flow releases by offering shelter and habitats for all life cycle stages of fish. The presented approach of explorative tree analysis followed by post-hoc tests of identified effects, as well as the pooled bootstrapping method, offers a simple framework for researchers and managers to analyze multi-factorial datasets and draw solid management conclusions.

Advancing towards functional environmental flows for temperate floodplain rivers.

Abstraction, diversion, and storage of flow alter rivers worldwide. In this context, minimum flow regulations are applied to mitigate adverse impacts and to protect affected river reaches from environmental deterioration. Mostly, however, only selected instream criteria are considered, neglecting the floodplain as an indispensable part of the fluvial ecosystem. Based on essential functions and processes of unimpaired temperate floodplain rivers, we identify fundamental principles to which we must adhere to determine truly ecologically-relevant environmental flows. Literature reveals that the natural flow regime and its seasonal components are primary drivers for functions and processes of abiotic and biotic elements such as morphology, water quality, floodplain, groundwater, riparian vegetation, fish, macroinvertebrates, and amphibians, thus preserving the integrity of floodplain river ecosystems. Based on the relationship between key flow regime elements and associated environmental components within as well as adjacent to the river, we formulate a process-oriented functional floodplain flow (ff-flow) approach which offers a holistic conceptual framework for environmental flow assessment in temperate floodplain river systems. The ff-flow approach underlines the importance of emulating the natural flow regime with its seasonal variability, flow magnitude, frequency, event duration, and rise and fall of the hydrograph. We conclude that the ecological principles presented in the ff-flow approach ensure the protection of floodplain rivers impacted by flow regulation by establishing ecologically relevant environmental flows and guiding flow restoration measures.

Effects of river bank heterogeneity and time of day on drift and stranding of juvenile European grayling (Thymallus thymallus L.) caused by hydropeaking.

High-head storage hydropower is deemed to be the ideal renewable energy source in Alpine regions to meet the increasing demand for daily peak electrical energy. However, this mode of operation - called hydropeaking - can imply severe hydrological and hydromorphological consequences for river ecosystems, affecting fish populations by e.g. drift and stranding of young life stages. Several fish-stranding experiments using physical models have been performed in the past, but until now very little is known about influences of time of day or gravel bank heterogeneity. We performed experiments during late summer 2013 with juvenile European grayling (Thymallus thymallus) (mean length: 53mm) in a nature-like experimental channel enabling hydropeaking simulations. In the first experiments (n=21) we observed relative drift and stranding rates for a single hydropeaking event focusing on the effect of time of day on a homogenous gravel bank. The second test series (n=15) focused on two dewatering potholes installed as potential traps. Additional experiments (n=6) were done with a reduced downramping rate to gain information about potential mitigation effects on stranding risk. During daytime and decreasing water level, we observed low drift rates of 15% and stranding rates below 5% in dewatering potholes and on homogenous gravel banks. However, in the presence of dewatering potholes, nighttime drift rates were about three times and stranding rates about ten times higher than on the homogenous gravel bank. A lowered downramping rate reduced drift to about a quarter and almost eliminated nocturnal stranding risk. These results might be used to effectively regulate water releases from high-head storage hydropower plants in a more suitable way for sensitive life stages of fish. Reducing the downramping rate or shifting peaks to daytime can reduce negative effects of hydropeaking in consideration of the morphological character of affected rivers.