Genetic Evaluation of Cutthroat Trout Movement Through Remediated Culverts

Thousands of culverts across the western U.S. present passage barriers to inland trout, and culvert remediation is an increasingly common tool for management.  Determining that fish are actually moving through restoration sites, however, is costly and there is little direct evidence of fish passage to confirm the effectiveness of restoration.  We contrasted a suite of genetic techniques to capture movement of cutthroat trout from different age classes over three remediated culverts in Idaho and Montana; genetic data prior to culvert removal and samples from unrestored neighboring sites allowed for before-after and ‘control’ comparisons in Montana.  Heterozygosity was unchanged in the control and treatment sites in MT over the study period.  Effective size decreased slightly while allelic richness was unchanged in the control population, but allelic richness increased significantly in both treatment sites 3 years after passage restoration.  Individual assignment tests captured movement of age 1+ fish across restored culverts and sibship analyses, based on pedigree reconstruction of young-of-year fish and power-validated with simulated pedigrees, demonstrated movement across culverts even just shortly after emergence.  Our results provide guidance on the efficacy of different genetic techniques to detect movement over short time-frames and small spatial scales.