Abstract
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.
Highlights
The European grayling Thymallus thymallus is an indicator species for the ecological integrity of an entire river region – the grayling zone, which encompasses rivers in wide mountain valleys (Huet, 1959)
In Austria, of the almost 900 river kilo meters labeled as hydropeaking-impacted, 82% are in risk of failing the objectives of the EU Water Framework Directive (BMLFUW, 2017)
We found strong evidence that the three hydrological impact types affect biomass of grayling populations (p = 0.004)
Summary
The European grayling Thymallus thymallus is an indicator species for the ecological integrity of an entire river region – the grayling zone, which encompasses rivers in wide mountain valleys (Huet, 1959). In the European Alps alone, 80% of all larger rivers with a catchment size of >500 km are affected by diverse hydromorphological impacts (Muhar et al, 2019) Of these impacts, the effects of storage hydropower plants are prevalent in hyporhithral rivers (Fig. 1). Storage hydro power plants are run according to energy demand, thereby causing artificial (sub-daily) flow fluctuations by the discontinuous release of turbined water (Greimel et al, 2016). Such water releases, called hydropeaking, have wide-ranging implications for river ecosystems. Considering that run-of-river hydropower schemes can cause hydropeaking (though usually of lower intensity) (Greimel et al, 2016), it is likely that the threat of failing to achieve ecological objec tives due to hydropeaking is even higher than initially perceived
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