Thermal Heterogeneity and Habitat Complexity as Drivers of Brown Trout Resilience in Boreal River Restoration

Restoration strategies in boreal rivers should aim to enhance ecosystem resilience to climate change. This requires better understanding of fish habitat preferences, protection of thermal refuges, and mitigation of habitat loss.

In TRIWA LIFE-project, we examined the role of ground water in boreal rivers for brown trout (Salmo trutta). The streams surveyed varied in ecological status, ranging from near natural to restored and channelized conditions. To assess trout habitat, use and availability, we employed innovative methods including thermal infrared imaging, machine learning, alongside electrofishing surveys.

Preliminary results indicate that trout were more abundant in stream sections characterized by higher sinuosity and the presence of groundwater. Machine-learning models further revealed that trout occurrence was strongly associated with thermal conditions and habitat structure, with temperature metrics, coarse gravel substrate, water depth, and flow velocity emerging as the most influential predictors. Model performance was high (AUC > 0.9), indicating robust discrimination of suitable habitats and highlighting the combined importance of thermal heterogeneity and geomorphological complexity for trout habitat use.