Salmonids elicit an acute behavioral response to heterothermal environments.

Brown trout parr actively avoid cold water (≤8°C) through rapid behavioral responses, but do not avoid warmer water with temperature contrasts up to 8°C.

• Why it matters: Understanding how fish respond to sudden and strong thermal disturbances is crucial for predicting their resilience to anthropogenic water releases and climate change impacts.
• What they did: The study used a novel laboratory approach with image-based tracking to expose acclimated brown trout parr to rapidly forming cold- and warm-water interfaces ranging from 4 to 20°C, and quantified their behavioral responses.
• The result: Findings demonstrate that brown trout parr swiftly employ thermotaxis and thermokinesis to avoid cold water, enabling them to cope with rapid thermal changes in their environment.