Camera traps reveal that terrestrial predators are pervasive at riverscape cold-water thermal refuges

Perceived predation risks by terrestrial predators are major ecological forces in aquatic systems, particularly for aggregating fish. Riverscape thermal refuges are discrete, localized cold-water patches where fish temporarily aggregate to buffer against heat events. Predation pressures by terrestrial predators at thermal refuges may decrease the thermoregulatory benefits of refuge use, but quantifying such effects can be challenging and controversial when sampling can impose additional stress on fish. We passively monitored terrestrial predator visitation patterns and predation at four thermal refuges in the Housatonic River, Connecticut, USA, between May 18th and September 29th, 2022, with camera traps, a common wildlife monitoring method. Specifically, we (1) assessed diel visitation patterns by different categories of terrestrial predators at thermal refuges and determined if patterns varied among predator categories or with prevailing environmental conditions, and (2) estimated the probability of predation by hour of the day combined across all predator categories, quantifying general predation pressures at refuges. We detected at least one terrestrial predator at a thermal refuge each day, and mean hourly visitation rates (count/h) were highly variable across predator categories and sampling dates. The most supported generalized additive mixed model indicated that terrestrial predator visitation rates (count/h/day) varied with mean daily river discharge and water temperature differential, and relationships differed across categories of terrestrial predators. We observed 22 separate predation attempts on thermoregulating salmonids and predicted that the probability of predation by any terrestrial predator increased from 0.002 to 0.017 throughout a 24 h day (p = .004). Camera traps provided novel evidence that terrestrial predators are pervasive at riverine thermal refuges, which is relevant for refuge conservation and management globally. We recommend the implementation of a coordinated monitoring network across riverine thermal refuges using camera traps, further enriching our ecological understanding of cumulative predator effects in refuges across complex riverscapes.