Rapid Colonization of Upstream Habitats by Oncorhynchus Mykiss Following Culvert Modification.

Habitat fragmentation threatens the persistence of fish populations, and road crossings (e.g., culverts) are particularly problematic because they are globally ubiquitous and can block access to critical habitats. Barrier removal or modifications that allow upstream access can result in expanded distributions, increased abundances, and gene flow among historically isolated populations. Actions that promote connectivity, gene flow, and the potential expression of multiple life history forms (e.g., anadromy in Pacific salmon Oncorhynchus spp.) are thought to buffer against extinction and are increasingly important given the high rates of imperilment for many fish species. We collected genetic samples from juvenile O. mykiss (anadromous steelhead and resident Rainbow Trout) prior to and following the modification of a culvert that served as a historical barrier to upstream movement and tested for changes in genetic composition through time. Prior to culvert modification, levels of genetic diversity were low and genetic differentiation with reference collections of anadromous steelhead was high. After treatment, we observed increased levels of genetic diversity, larger estimates of effective population size, and decreased genetic differentiation with steelhead, indicating an influx of individuals from a downstream population. Using an inexpensive genetic‐based monitoring approach, we show that culvert modification can result in the rapid reestablishment of connectivity between historically isolated populations. [ABSTRACT FROM AUTHOR]