Research

Fish populations in Austrian rivers have declined sharply in recent decades due to hydromorphological degradation, energy-related development and climate change. Meanwhile, the impact of fish-eating predators on these already stressed communities is a hotly debated topic. The project aims to quantify the additional predation pressure while taking ecological conditions into account. To this end, historical and current fish stock data on abundance, biomass and population structure, collected since the return of predators, are analysed and examined by water body type and degree of stress, in order to separate the effects of predation from those of habitat and climate factors. In parallel, cross-state data on the occurrence and population trends of predators are compiled and used for relative trend analyses. Using distribution and feeding ecology data, we estimate the consumed fish biomass and spatiotemporal predation pressure, linking them to fish stock trends. The analysis is conducted across scales, from the national level down to the level of selected case study rivers. The results will provide a scientific basis for managing fish-eating predators in the context of protecting and sustainably using Austrian river fish stocks.

Instrumental water level and discharge measurements in Lower Austria for medium-sized and smaller rivers only date back a little over 100 years. Historical flood statistics are therefore limited in time, which seems insufficient, especially in times of climate change with predicted more frequent and larger extreme floods. For individual rivers, historical floods have been documented and analyzed on the basis of local chronicles and flood marks (see, for example, the work of Heinz Wiesbauer). However, it can be assumed that many documents on historical floods prior to instrumental measurements have not yet been collected and consistently evaluated. The aim of the project is to collect and process information on historical floods in Lower Austria prior to the start of instrumental hydrological recordings. The surveys focus on flood marks, but written sources and historical literature will also be collected and evaluated.

The grayling (Thymallus thymallus) and rainbow trout (Onchorhynchus mykiss), which are key species for the fishing industry, show strong population fluctuations and sometimes unsatisfactorily low stocks in large parts of the Ybbs. In particular, a decline in the grayling population in the middle reaches of the river documented in 2024 and the noticeable decline in rainbow trout raise questions about the causes responsible. The proposed project aims to explain population demographic developments using abiotic descriptors – specifically flow and water temperature. The focus here is on the critical phase of incubation and emergence, a period that has been shown to be of great relevance for the further population development of salmonids (Unfer et al., 2011, Pinter et al., 2025). An additional focus is on the development of cohorts (year classes), i.e. the survival of individual age stages from year to year. The data available for the Ybbs is ideal for these analyses, as in addition to continuous hydrological data, there is a complete seven-year data series (2019–2025) of fish stock data from a district upstream of Amstetten – a unique data basis in the hyporhithral zone that enables well-founded and detailed analyses of reproductive success and cohort development. As part of the interpretation, influencing factors such as water morphology and potential stressors such as predation pressure and interspecies competition should not be ignored. By considering seamless population development, a new basis for future fisheries management steps and considerations is created. This redefines the prerequisites for sustainable and forward-looking management and lays the foundation for further considerations on current problems in fisheries management (status of the huchen population, consequences of climate change, etc.). Such research results are of great importance for this key species in the grayling region, especially in view of the unstable stocks of grayling, which is classified as vulnerable (Wolfram & Mikschi, 2007).