Do dams also stop frogs? Assessing population connectivity of coastal tailed frogs (Ascaphus truei) in the North Cascades National Park Service Complex

Abstract

We investigated the effects of three hydroelectric dams and their associated lakes on the population structure and connectivity of the coastal tailed frog, Ascaphus truei, in the North Cascades National Park Service Complex. Three dams were erected on the Skagit River in northern-central Washington state between 1924 and 1953 and subsequently changed the natural shape and movement of the Skagit River and its tributaries. We collected 183 frogs and tadpoles from 13 tributaries and generated a dataset of >2500 loci (unlinked SNPs) using double digestion restriction site-associated DNA sequencing (ddRADseq). An analysis of molecular variance (AMOVA) identified $$\sim$$ 99% of the genetic variation within groups, and the remaining variation among groups separated by dams, or the Skagit River. All populations exhibited low F $$_{ST}$$ values with a maximum of 0.03474. A “de novo” discriminant analysis of principal components revealed two populations with no geographic structure. However, testing groups that were partitioned a priori by the dams revealed distinctiveness of groups down-river of the lowest dam (Gorge Dam). Coalescent-based analyses of recent migration suggest that up to 17.3% of each population is composed of migrants from other populations, and an estimation of effective migration rates revealed high levels of migration heterogeneity and population connectivity throughout the study area. Our results suggest that although the populations down-river from Gorge Dam are distinguishable using SNPs, A. truei population connectivity is high throughout the North Cascades National Park Service Complex.