Tadpoles of the tailed frog (Ascaphus truei) are common in riffles within many small, high-gradient streams of the Pacific Northwest (United States and southern Canada), where they typically graze periphyton from exposed cobbles. We conducted field observations and experiments in Clearwater Creek, southwestern Washington, to determine if tadpoles would reduce their feeding activity (i.e., emergence from crevices to graze periphyton) in the presence of non-visual cues released from each of four aquatic predators: giant salamanders (Dicamptodon spp.), cutthroat trout (Salmo clarki), brook trout (Salvelinus fontinalis), and shorthead sculpin (Cottus confusus). In absence of predators, tadpoles usually emerged from under cobbles to feed at night (2000-0100 h), and spent the remainder of the 24-h interval hidden in crevices. In the presence of giant salamanders, cutthroat trout, and brook trout that were all confined within separate, in situ enclosures immediately upstream of tadpoles, tadpole activity was reduced two-, three-, and six-fold, respectively, compared with predator-free controls. In contrast, tadpoles appeared unable to detect upstream sculpins. Subsequent consumption experiments in the laboratory showed that salamanders, sculpins, and cutthroat trout all were capable of consuming tadpoles in both structurally simple and complex habitats. We hypothesize that the inability of tadpoles to detect predaceous sculpins may explain why tailed frog tadpoles are largely absent from lower-gradient streams where sculpins are often abundant.
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