Warming riverscapes annually challenge the role of thermal refuges for thermoregulating salmonids

Thermal refuge use constitutes an important thermoregulatory behavior for the persistence of cold-water-dependent salmonids, such as brown trout <i>Salmo trutta</i>, in temperate riverscapes undergoing climate change. The ability of thermal refuges to mitigate summertime physiological stress, however, is constrained by refuge availability, capacity, and individual fish behaviors, which challenges inferences made from empirical studies. We developed a spatially explicit, individual-based model (SIBM) to simulate behavioral and physiological responses of brown trout to observed water temperatures (2014 - 2022) in the Housatonic River, Connecticut, USA where fish must use thermal refuges for summer survival. We assessed thermal refuge efficacy under varying summertime thermal regimes using emergent SIBM properties including population abundance, location history, and thermal exposure. Our SIBM predicted substantial inter-summer variability in brown trout survival, with cumulative mortality rates ranging from <1% during cooler years (e.g., 2014) to as high as 86% during extreme heat years (e.g., 2016, 2020, 2022). Frequent and prolonged periods of elevated temperatures, exceeding 1,000 hours above 23.0°C in total during warmer years, led to significant energy depletion, weight loss, and mortality despite thermal refuge use. While our model assumes static thermal refuge conditions due to data limitations, our findings highlight the limits of riverscape thermal refuges as a conservation tool under intensifying climatic conditions, with implications for salmonid conservation globally. Our study suggests that adaptive management strategies should prioritize expanding thermal refuge capacity, implementing strategic discharge management during thermally stressful periods, and enhancing refuge accessibility to bolster salmonid population persistence under climate change.