SUPERVISOR: Stefan SCHMUTZ

PROJECT ASSIGNED TO: Jakob NEUBURG

Introduction

The Danube River Basin is the most international river basin worldwide. Its catchment expands over the territories of 19 countries with a size of ~ 801 000 m2. At the same time, the Danube is the second longest river of Europe, with a length of 2 826 km (Sommerwerk et al., 2010). Starting in the 19th century, the Danube was changed extensively through human interventions. Channelization for flood protection, navigation and agricultural expansion caused an increasing disconnection of the river and its floodplains and tributaries (Habersack et al., 2016). The construction of hydropower plants (HPP) along the Danube during the 20th century led to additional habitat loss for migrating species (Friedrich et al., 2019). As a consequence, populations of rheophilic fishes decreased (Peňáz, 1996) and were fragmented into sub-populations (Ludwig et al., 2009). Impacted rivers developed towards low species richness and low fish densities and biomasses (Carrel & Rivier, 1996; Schiemer et al., 2003). Along the Austrian Danube, a chain of ten hydropower plants left only two remaining free-flowing sections in the Wachau and along the National Park Donau-Auen (Habersack et al., 2016). The residual stretch consists out of large impoundments.

In order to prevent further deterioration and ensure achievement of an acceptable ecological condition of all water bodies in the EU, the Water Framework Directive (WFD, 2000) was implemented in the year 2000. Consequently, restoration measures like the reconnection of oxbows or tributaries were realized. Monitoring data of implemented restoration measures suggested an immediate acceptance of fish assemblages and occurring positive effects are considered to expand to adjacent river sections (Friedrich et al., 2018b).

Nevertheless, a multitude of human interferences caused a massive decline of fish populations. One group that was hit particularly severe are the sturgeons (Acipenseridae Bonaparte, 1831), which are currently at historic low levels (Lenhardt et al., 2006). In the Danube, five species used to enter the Austrian part during their spawning migrations (Hensel & Holčík, 1997). Nowadays, migrations end at the first dam at the Iron Gate (Hensel & Holčík, 1997) and the sterlet (Acipenser ruthenus Linnaeus, 1758) is the only remaining species. The sterlet is classified as “critically endangered” by the Austrian Red List because the population is considered to be lower than 1 000 spawners (Wolfram & Mikschi, 2007). Moreover, only one self-reproducing population is known in Upper Austria (Ratschan et al., 2017). The last evidence of reproduction in another section of the Austrian Danube dates back to 1986 (Friedrich et al., 2014) but the respective spawning site north of Vienna was lost for the downstream population through the construction of the HPP Freudenau (Friedrich, 2018a). In addition, population estimates are based on expert judgement and a regular monitoring is established in only two small areas, below the HPP Jochenstein and the HPP Freudenau, respectively.

Among other species, sturgeons use the deepest parts of rivers (Bemis et al., 1997) and sampling those areas is methodologically challenging and often requires a combination of several methods (Radinger et al., 2019). As a result, historic perceptions of patterns of fish abundances in large rivers have been shaped mainly by observations from more easily sampled shallow aquatic areas (Dettmers et al., 2001a). However, several species were observed to use the main channel of large rivers as a habitat during all seasons and occur in considerable numbers (Dettmers et al., 2001a; Wolter & Bischoff, 2001) and many of those species are of special conservation concern (Galat & Zweimüller, 2001; Sandu et al., 2013; Szalóky et al., 2014). Also in the Danube, several species are predominant in samples targeting deeper areas of the main channel (Loisl et al., 2013; Szalóky et al., 2014). Nevertheless, the large-scale organization of fish assemblages in the Danube is poorly known and often based on shoreline sampling (Szalóky et al., 2014). Hence, without adequate sampling of the main channel, the importance of this aquatic area to the fish community is overlooked (Dettmers et al., 2001a), since it is considered as both, an important habitat and a link between different habitat types in large floodplain rivers (Dettmers et al., 2001b; Galat & Zweimüller, 2001).

This PhD thesis aims to assess the status of bottom-dwelling fish populations in the Austrian Danube by quantitatively assessing the bottom fish fauna using several methods suitable for sampling the main channel. Moreover, the usage of different habitat qualities (free-flowing section, head of the impoundment and impoundment) by the bottom fish fauna in a chain of hydropower plants will be assessed during all seasons. Due to its conservation value and the lack of monitoring-based populations estimates, a special focus lies on the sterlet in the free-flowing section along the National Park Donau-Auen. The enhancement of the knowledge about the bottom fish fauna, which involves several species of high conservation value, and the detailed investigation of the main channel of the fragmented Austrian Danube, serve as input for the development of sustainable ecological measures for future management of the Austrian Danube.

References

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