Genetic Monitoring of Trout Movement after Culvert Remediation: Family Matters

There is an increasing need for direct evidence that fish use culverts remediated to restore fish passage.  We contrasted two individual-based genetic techniques to evaluate their utility and constraints for detecting movement of cutthroat trout over restored culverts in two different field settings.  We implemented Bayesian clustering of individual genotypes from age 1+ fish, a widely-used technique which should be able to capture current movement among previously-isolated populations.  We also investigated a novel ‘sib-split’ approach focused on young-of-year (yoy), where movement patterns are extracted from the spatial distribution of full-sibling groups inferred via pedigree reconstruction.  Family structure greatly influenced clustering results in our small headwater populations, even though field sampling was implemented to avoid siblings.  The ‘sib-split’ approach, which uses family structure to detect movement, uncovered extensive passage of yoy just weeks after emergence.  When retrospectively applied to our older age individuals, it proved essential in interpreting clustering patterns and captured passage in several families of one and two year olds.  Where family structuring may negatively affect clustering techniques or, alternatively, be prominent enough to allow application of ‘sib-split’ is difficult to predict a priori; we discuss benefits and limitations of both approaches under different ecological, spatial and management scenarios.