Population development of salmonids in the middle reaches of the River Ybbs with a particular focus on grayling and rainbow trout: An analysis of abiotic influences

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).