Few Impacts of Introduced Cutthroat Trout (Oncorhynchus clarki) on Aquatic Stages of Boreal Toads (Anaxyrus boreas boreas)

Abstract

Introduced salmonids can impact aquatic ecosystems through direct predation and indirect effects. We explored the effects of introduced Cutthroat Trout (Onchoryncus clarki) on Boreal Toad (Anaxyrus boreas boreas) survival and habitat use during two aquatic life stages, the embryo and tadpole, at Boreal Toad breeding sites with and without Cutthroat Trout. We found no difference in embryo survival and higher tadpole survival at the site with Cutthroat Trout. Cutthroat Trout are unlikely to use the shallow areas where Boreal Toad eggs are deposited; however, during the tadpole stage, Cutthroat Trout and tadpoles overlap broadly in near-shore aquatic habitats. Frequency of tadpole habitat use is lower in Cutthroat Trout–used areas, but we observed no behavioral or temporal avoidance of Cutthroat Trout by tadpoles. Our results suggest that Cutthroat Trout do not have a negative effect on Boreal Toad embryo or tadpole survival in wild settings and that Cutthroat Trout presence does not preclude tadpoles from using habitats.