INTERACTION BETWEEN INTRODUCED TROUT AND LARVAL SALAMANDERS (AMBYSTOMA-ACRODACTYLUM) IN HIGH-ELEVATION LAKES

The larval stage of the long-toed salamander (Ambystoma macrodactylum) is the top vertebrate predator in high-elevation fishless lakes in th e North Cascades National Park Service Complex, Washington (U.S.A). Al though most of these high-elevation lakes were naturally fishless, tro ut have been stocked in many of them. We sought to determine the effec ts of physiochemical factors and introduced trout on abundance and beh avior of A. macrodactylum larvae. Larval salamander densities were est imated by snorkeling. Snorkelers carefully searched through substrate materials within 2 m of the shoreline and recorded the number of larva e observed and if larvae were hidden in benthic substrates. Physicoche mical factors were measured in each lake on the same day that snorkel surveys were conducted. In fishless lakes, larval salamander densities were positively related to total Kjeldahl-N concentration and negativ ely related to lake elevation. Crustacean zooplankton, especially clad ocerans, were important food resources for larval A. macrodactylum. Cr ustacean zooplankton and cladoceran densities were positively related to total Kjeldahl-N, suggesting that increased food resources contribu ted to increased densities of larval A. macrodactylum. Differences in larval salamander densities between fish and fishless lakes were relat ed to total Kjeldahl-N concentrations and the reproduction status of t rout. Mean larval salamander densities for fishless lakes with total K jeldahl-N < 0.045 mg/L were not significantly different from mean larv al densities in lakes with reproducing trout or in lakes with nonrepro ducing trout. In fishless lakes with total Kjeldahl-N greater than or equal to 0.045 mg/L, however, mean larval densities were significantly higher than in lakes with reproducing trout where fish reached high d ensities. In fishless lakes with total Kjeldahl-N greater than or equa l to 0.095 mg/L, mean larval densities were significantly higher than in lakes with nonreproducing trout where trout fry were stocked at low densities. Reduced larval salamander densities in lakes with trout li kely resulted from trout predation. There were no significant differen ces in the percentage of larvae hidden in benthic substrates between f ishless lakes and lakes with fish. Our results imply that assessment o f the effects of fish on amphibians requires and understanding of natu ral abiotic and biotic factors and processes influencing amphibian dis tribution and abundance.