Early warning tool may protect river fish in heat waves

The brown trout Salmo trutta is highly heat-sensitive. Credit: Ulrich Wasem

Researchers from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) have proposed a warning tool that predicts, up to three weeks in advance, when river fish in Switzerland will be at risk from heat. As heat waves become more frequent with climate change, this early warning system gives people time to act and protect fish populations from future heat waves.

Preparing for weather extremes

When do rivers become too hot for river fish in Switzerland? To predict when river fish are at risk from heat waves, ecologists and climatologists at the Swiss Federal Institute for Forest, Snow and Landscape Research WSL joined forces with experts from across Switzerland to develop a forecasting tool. The forecasts are updated twice a week and are publicly available on drought.ch. The study is published in the journal Ecological Modelling .

With heat waves becoming more frequent and severe due to climate change, fish are particularly at risk, as their body temperature depends entirely on their environment. Unable to cool themselves, fish are ectothermic, more commonly referred to as "cold-blooded" animals. "When the water temperature rises, chemical reactions in the fish's cells change—and that's what we call heat stress," said Camille Albouy, senior researcher at the joint WSL and ETH research group Ecosystems and Landscape Evolution.

The chub Squalius cephalus has a low sensitivity to heat. Credit: Ulrich Wasem

From past data to future risk

The new tool's predictions are based on three parameters: water temperature forecasts, fish physiology and species distribution. WSL's hydrological forecasting group developed the water temperature forecasts. "Our machine-learning model , trained on data from the past 10 years, uses the latest water temperature observations and weather forecasts to predict river temperatures," said Ryan Padrón Flasher, one of the group's members.

Not all fish are equally sensitive to heat. Searching through published data, the authors gathered information on the temperature limits of 59 fish species found in Swiss rivers. This is the temperature at which their ability to swim, and therefore escape from dangerous conditions, is impaired. The authors found non-native fish species to have a temperature tolerance 1.4°C higher than native species on average. The bighead carp (Hypophthalmichthys nobilis) had the highest tolerance at 32.3°C, while the burbot (Lota lota) was the least tolerant, at 24.1°C.

Finally, because fish species are unevenly distributed across Switzerland, including information on their distribution makes the forecasts location-specific. "It's a constant compromise between the complexity and quality of the model," Albouy said. "As the idea was to develop a map of risk for the whole of Switzerland, based on the best climate predictions, we had to keep it simple and focus on what matters for management."

With the help of hindsight

To evaluate the forecasts, the scientists applied their tool to data from summer 2018, when about 3 tons (2.7 metric tons) of river fish died in Switzerland. "We wanted to see how well we could retrospectively predict the observed fish mortality," Albouy said. However, this proved more challenging than expected. "Data on fish mortality are limited," he said, "so we reached out to practitioners to see if their observations matched our predictions."

Out of the three reported cases of fish mortality in 2018, the tool predicted two correctly. Across the remaining locations, forecasts were 70% accurate, successfully identifying areas of low and moderate risk. However, at five sites the tool overestimated the level of risk, likely because it does not include finer-scale information like local population sizes or the presence of cooler refuges within rivers. "It's a promising start," Albouy said. "Now we need to see if it grows into a long-term solution for Swiss rivers."

More information

Adeline Bonaglia et al, Sub-seasonal forecasting of thermal stress for Swiss river fishes during heatwaves, Ecological Modelling (2025). DOI: 10.1016/j.ecolmodel.2025.111171

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Early warning tool may protect river fish in heat waves (2026, June 18)

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