Urbanisation is driving freshwater fish declines in rivers in Europe and the USA

Urbanisation is a key driver of freshwater fish declines in rivers across Europe and the USA, according to a new study. An international team of scientists assessed data from over 30,000 sites across the two continents to examine how fish populations respond to different human land-uses in river catchments.

Freshwater fish are threatened by multiple human pressures across the world. However, the ways in which fish are affected by pressures – such as pollution and habitat loss – can vary in different areas. As such, there is a need to better understand how fish populations respond to different stressors caused by human activity on a continental scale, in order to support effective conservation strategies.

The authors of the new study – published in Science of the Total Environment – used large-scale datasets to assess how the frequency and severity of different human stressors affects river fish populations. Their results paint a clear picture of the impacts of intensive human activity on river ecosystems.

“Overall, urbanisation and human population density most frequently lead to a significant decrease in fish populations,” says lead author Rafaela Schinegger, Assistant Professor for Nature Conservation Planning at BOKU in Austria. “In Europe, urban land use was also identified as the most severe stressor, whilst in the USA, we more often identified agricultural land use as the most severe one.”

Urbanisation and intensive agriculture can cause a wide range of stressors on river ecosystems. Urban land development around rivers often leads to habitat loss, alterations to water flows, barriers to fish movement and increased risk of pollution and flooding. Intensive agriculture can generate a similar range of threats, often with a heightened risk of fertiliser pollution, increased demands for water abstraction, and clearing of riparian ecosystems.

The characteristics of different fish species impacted how they responded to stressors. As might be expected, species which are particularly sensitive to changes in water quality and habitat loss were strongly affected by urbanisation and agricultural stressors.

“Our study showed that fish species intolerant to habitat degradation in general, including water quality impairments and hydro-morphological changes such as channel alteration, barriers and spawning habitat loss are the most responsive and sensitive traits over a wide range of ecoregions and across continents,” says lead author Maria Magdalena Üblacker from IGB Berlin in Germany.

Species which rely on gravel and rocky areas of river bottom to spawn – so-called ‘lithophilic’ species – were similarly sensitive to stressors in both European and USA rivers. This is likely due to the loss of suitable spawning grounds due to human development.

In the USA, two more characteristics in fish species made them particularly susceptible to stressors. Migratory fish and species which live in fast-flowing water – so-called ‘rheophilic’ species – were also strongly affected by human activity. These species are likely to be vulnerable to river flow barriers and habitat alterations resulting from human development.

The research team used the huge dataset on European and USA rivers to identify ‘threshold’ values at which human stressors had a significant effect on fish populations. “The identified thresholds can provide guidance for prevention, conservation, and restoration of riverine fish habitats,” says Maria Magdalena Üblacker. “Our study also provides evidence about the urge to analyze interconnections between aquatic and terrestrial ecosystems and their implications for future nature conservation planning,” continues Rafaela Schinegger.

“Our work shows the value of international collaborations among institutions, as well as the supportive and productive relationships among our author team,” says lead author Dana Infante, a Professor in the Department of Fisheries and Wildlife at Michigan State University. “Our results for Europe and the USA clearly indicate that a landscape scale approach and the reduction of stressors in urbanised areas have to be a focus for management.”

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Üblacker MM, Infante DM, Cooper AR, Daniel WM, Schmutz S, Schinegger R, (2023), “Cross-continental evaluation of landscape-scale drivers and their impacts to fluvial fishes: Understanding frequency and severity to improve fish conservation in Europe and the United States,” Science of The Total Environment, https://doi.org/10.1016/j.scitotenv.2023.165101.