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The mudminnow ( Umbra krameri ) has its westernmost distribution area in Austria near Vienna (Wanzenböck 1995, for historical and current distribution maps see the Power Point presentation in the Suppl. material 1). The species is considered the most threatened fish species in Austria and was believed to be extinct in the second half of the 20th century (Böhm 1978, Hacker 1983, Herzig-Straschil 1994). In 1992 it was rediscovered in the Fadenbach, a backwater system within the Danube Floodplain National Park and shortly thereafter in a wetland south of Vienna (Jesuitenbach) (Wanzenböck and Spindler 1995, Spindler and Wanzenböck 1995). Over the past four decades, several conservation measures have been implemented in the National Park in order to protect the mudminnow, including studies on habitat preferences (Sehr and Keckeis 2017), monitoring programs (Jung et al. 2018, Jung et al. 2024), habitat restoration projects (Spindler 2006) and captive breeding efforts. Furthermore, a reintroduction attempt was carried out in the Hanság wetland, a drainage-ditch system on the Austrian-Hungarian border. Captive bred mudminnows, whose parental generation originated from the Fadenbach population, were stocked there (Benesch 2004, Benesch 2008). The Fadenbach-population was the only one that had been regularly monitored over the past decades starting immediately after its discovery. During the 1990s, mudminnow densities were relatively high, but showed large fluctuations most likely due to high winter mortality (Fig. 1). In 2013 even higher densities were recorded and the population appeared to have extended its range within the Fadenbach system (Keckeis and Sehr 2014), most likely due to the creation of so called “survival ponds” – deeper sections within the backwater system. Between 2018 and 2023 severe droughts led to a dramatic decline. Currently, the population remains at a very low level and the species has disappeared from the upper reaches of the Fadenbach system. The recent droughts and persistently low water levels are part of a long-term trend: in the area, mean groundwater levels have dropped by approximately 1.5 metres since 1940 (Fig. 2) mainly due to river regulation (diking), agricultural irrigation, incision of the Danube river bed and a decline in precipitation. Therefore, a new conservation initiative was launched by the Danube Floodplain National Park with the project “PonDiversity” which focuses on the European pond turtle ( Emys orbicularis ), water pineapple ( Stratiotes aloides ) and mudminnow. The project includes comprehensive surveys of the mudminnow habitats within the National Park using electrofishing, an assessment of existing captive stocks established in the 1990s and genetic analysis of both, wild and captive populations. In a second phase, habitat restoration measures and improved captive breeding programmes will be implemented based on the outcome of population genetic studies. The reintroduced population in the Hanság area survived for around 15 years - at least until 2018 - but was no longer detected in 2023 (Jung et al. 2024). Its current status must be regarded as uncertain, as no extensive surveys have been carried out so far. In contrast to the Fadenbach- and Hanság-populations, the Jesuitenbach-population has recently shown a significant increase with highest densities recorded in 2024 (Jung et al. 2025). A possible explanation for this development is the decline of roach and pike populations during the recent dry periods, which may have led to reduced competition and predation pressure without deteriorating habitat conditions for the mudminnow. The population increase provided an opportunity to establish an ex situ population of Jesuitenbach mudminnows at Zoo Schönbrunn.