PACIFIC TAILED FROG (ASCAPHUS TRUEI) DISTRIBUTION IN HEADWATER STREAMS

During the low flow period (August-October) of 2000-2, we recorded the distribution of Pacific tailed frog (Ascaphus truei) among 131 non-fish bearing stream threads on basalt substrates in the Stillman Creek watershed of southwestern Washington. Older frog life stages were typically found further upstream than younger stages with adults often above the origin of surface flow, which implies adults move upstream seasonally. This pattern was supported by within-year sampling in 2002 that showed that the median position of adults in August-September was 500-m upstream of their median position in May-June. We hypothesize a cycle in which adults lay eggs downstream, move upstream as low flow approaches, and then return downstream at an unknown time. Frogs probably lay eggs in areas far enough downstream that they do not dry out during low flow, but not so far downstream as to subject hatchling larvae to scour or the possibility of passively drifting into fish-bearing waters where they may become prey. Why adult tailed frogs need upstream headwater habitats during low flow is unclear. We also found that the likelihood that tailed frog occupy non-fish bearing basins increased with basin size. No tailed frog life stages implying reproduction (eggs, larvae or metamorphs) were found in the 30 1st-order basins we sampled. We believe that length of the non-fish bearing stream thread in 1st-order basins is too short to meet minimum frog habitat requirements. This hypothesis assumes that fish-bearing reaches are poor frog habitat, but it is hard to understand why such a pattern of tailed frog occupancy among non-fish bearing basins of different sizes would occur if fish-bearing reaches were favorable habitat.