University of California Water Resources Center

Wilderness water resources often provide wildlife habitat and associated recreational opportunities, such as angling or birdwatching. Introduced trout in mountain lakes could affect terrestrial wildlife by changing ecosystem subsidy, which is the flow of nutrients and organisms from aquatic to terrestrial habitats. Trout prey upon larval amphibians and aquatic insects, and the adult stages of aquatic insects and amphibians are prey for bats, birds, snakes, and other terrestrial insectivores. The indirect effects of introduced fish on terrestrial wildlife have rarely been considered, and there have been no prior experiments testing effects of fish stocking on the Cascades frog (federal and California species of special concern). We conducted a four-year replicated whole-lake experiment to assess whether changes in fish abundance could aid frog recovery and whether trout predation of larval amphibians and aquatic invertebrates indirectly affects the density of terrestrial predators. Results show that non-native trout suppress the numbers of the declining Cascades frog and other amphibians, as well as large-bodied aquatic insects such as dragonflies and damselflies. After trout removals, these groups show a marked increase in abundance. We found differences in the species and abundance of garter snakes feeding at lakes with and without trout present. Over 100 species of birds and more than six species of bats use the lake basins during the snow-free season and we are currently assessing the data for patterns associated with trout abundances.