SUPERVISOR: Stefan SCHMUTZ

PROJECT ASSIGNED TO: Stephanie POPP

Freshwater ecosystems, though covering only about 0.8% of the Earth's surface, support over 10% of known species, including 55% of all fish species (IUCN, 2016; Strayer & Dudgeon, 2010). However, they are highly vulnerable due to human interference, with nearly one-quarter of all freshwater species facing extinction (Sayer et al., 2025). Habitat loss is a major factor in species decline (Ricciardi & Rasmussen, 1999), and rivers are particularly affected by dams, pollution, and invasive species (Dudgeon et al., 2006). Large freshwater species, such as sturgeons, are especially sensitive due to their size, habitat needs, and late maturity (He et al., 2019). Since 1970, freshwater megafauna populations have declined by 88%, with large fish like sturgeons experiencing a 94% decline (He et al., 2019). Due to this development, all 26 remaining sturgeon species are now at risk of extinction (IUCN, 2022). In the Danube River Basin, sturgeon populations have drastically declined. Three of the six species are critically endangered, with the ship sturgeon (Acipenser nudiventris) considered regionally extinct and the European sturgeon (Acipenser sturio) locally extinct (Friedrich, 2018; IUCN, 2022). The sterlet (Acipenser ruthenus), the only remaining potamodromous species in the Danube River Basin, is listed as endangered and remains in larger numbers only in the Middle and Lower Danube. However, its population declined sharply after the Gabčíkovo Dam was completed in 1992 (Guti, 2008). Concerning this alarming development of sturgeon populations, destruction of habitats must be named as one of the main drivers (Friedrich et al., 2019).

Identifying, characterizing and monitoring of sturgeon habitats is crucial for understanding the interactions between sturgeon populations and their environment (Hook, 2011). It involves identifying key habitats, observing habitat use patterns, and gaining a comprehensive understanding of habitat functionality (Haxton et al., 2023). As such, this knowledge facilitates an understanding of the drivers influencing sturgeon distribution, reproductive success, and overall population viability (Reinartz, 2024). This information provides the basis for informed decision-making with regard to targeted protection and recovery measures. However, knowledge on sturgeon habitats is still scarce, with large knowledge gaps regarding their location as well as their key habitat characteristics concerning their different life-stages. 

Overall, this PhD aims to address the problems mentioned above by closing knowledge gaps concerning sturgeon habitat location and characteristics with a focus on the species present in the Danube River Basin.

In the first part of the thesis, the use of an already identified habitat will be analyzed to gain knowledge on the behavior and the habitat use of the sterlet, the only sturgeon species known to still reproduce in the Upper and Middle Danube. For this, telemetry data generated in previous projects will be processed and analyzed with the help of GIS and statistical computing environment such as R statistics. Main output is to gain knowledge on the habitat use on a temporal level as well as key parameters influencing the sturgeon behavior, in comparison with the nase (Chondrostoma nasus), a cyprinid species known for medium-distance migrations (Waidbacher & Haidvogl, 1998) in order to elucidate specific habitat requirements of sterlets.

As one of the major issues hindering the development of effective sturgeon protection plans are the massive knowledge gaps on habitat characteristics, the second aim of the doctoral project is to develop a method to assess abiotic habitat characteristics such as water depth, flow velocity and sediment composition in the Danube, as these are key parameters describing sturgeon habitats. In the course of this work, a method assessing sediment composition and water depth of known and of potential habitats by using the Side Scan Sonar (SSS) will be developed, with the aim to assess the above-mentioned abiotic parameters in an efficient and time-saving way. Additionally, it is planned to develop a Geospatial Artificial Intelligence (GeoAI) script to automate the analysis of the data generated by the SSS method. By developing an automated analyzing method, the sediment composition assessment should be made standardized and applicable on larger geographical scale, consequently supporting the efforts to close the knowledge gaps regarding habitat characteristics.

The third part of the doctoral project, which covers the largest geographical scale, focuses on assessing knowledge about sturgeon habitats in the Danube River Basin. To date, no transnational analysis or comprehensive summary of known sturgeon habitats and their recorded habitat characteristics has been conducted. Therefore, the research will focus on collecting and analyzing data on the locations of potential and confirmed sturgeon habitats as well as evaluating data consistency and quality to provide a comprehensive overview of their current status, protection levels, and threats. The foundation for this analysis is a habitat database developed in a previous project, which will be further expanded in an ongoing project (MonStur). This work is a crucial step in assessing the impact of existing and planned infrastructure developments on protected sturgeon populations.

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