Multistate mark–recapture modeling reveals Brown Trout movements during thermal refuge use

ABSTRACT Objective Salmonids rely on movements within and around thermal refuges for summer survival. Although thermal refuges can mitigate thermal stress, there is still limited understanding of how individual movement patterns respond to environmental variability during refuge use. We assessed Brown Trout Salmo trutta movements within thermal refuges and between refuges and adjacent main-stem habitats and emigration from refuges in the Housatonic River, Connecticut, USA, during the summers of 2022 and 2024, using passive radio telemetry. Methods We defined spatial zones within thermal refuges and the nearby main stem and applied a Bayesian multistate Cormack–Jolly–Seber model to estimate movement, survival, emigration, and detection probabilities among zones. Our model incorporated the influence of river discharge, water temperature, and terrestrial predator visitation on movement patterns. Results In 2022, Brown Trout exhibited high spatial zone fidelity (0.398–0.755) and diel movement patterns, preferring central thermal refuge zones when temperatures exceeded 19.19°C. In 2024, Brown Trout exhibited reduced spatial zone fidelity (0.188–0.300) and increased interzone movement with no diel patterns as main-stem discharges approached 23.5 m3/s following a precipitation-driven flow pulse. Brown Trout exhibited high emigration probabilities during both years. Conclusions Our findings demonstrate that Brown Trout movements responded to dynamic riverscape conditions, with discharge and temperature serving as context-dependent drivers of spatial behavior. These results underscore that thermal refuge conservation requires managing refuges as spatially heterogeneous, hydrologically dynamic habitats requiring connectivity to adjacent main-stem habitats rather than isolated coldwater patches.