| Introduced trout have often been implicated in the decline of high mountain amphibian populations, but few studies have attempted to understand whether fish stocking also influences the distribution and abundance of amphibians throughout entire mountain basins, including in the remaining fishless lakes. I examined this relationship using the relative abundance of long-toed salamanders (Ambystoma macrodactylum) and Columbia spotted frogs ( Rana luteiventris) in fish-containing and fishless lentic sites in basins with varying levels of historic fish stocking. All lentic waters were surveyed for fish and amphibians in 11 high-elevation basins in the Frank Church—River of No Return Wilderness, Idaho, between 1995 and 1998. I found introduced trout (Oncorhynchus clarki, O. mykiss, O. m. aguabonita ) in 43 of the 101 sites, representing 90% of the total surface area of lentic water bodies. At the scale of individual water bodies, after accounting for differences in habitat characteristics between fish-containing and fishless sites, the abundance of amphibians at all life stages was significantly lower in lakes with fish. Trout predation is likely responsible for this pattern. At the basin scale, densities of overwintering life stages of amphibians were lower in the fishless sites of basins where more habitat was occupied by trout. Using mark-recapture and radiotelemetry on R. luteiventris, I found that many frogs traveled from breeding sites to summer habitats and then to overwintering sites each year. Male frogs usually moved within 100 m of breeding sites, whereas females made annual migrations of more than 2 km (roundtrip) between breeding sites and summer habitats, traveling up to 500 m across dry, uplands. R. luteiventris that bred in shallow, fishless wetlands often migrated to deep, fish-containing lakes to overwinter. Trout predation on overwintering R. luteiventris may explain why basin fish stocking appeared to negatively affect frog populations in fishless water bodies. These results suggest that many of the remaining fishless habitats are too shallow to provide suitable breeding or overwintering sites for these amphibians and that current trout distributions may eventually result in the extirpation of amphibian populations from entire basins, including water bodies that remain in a fishless condition. |
