Hybridization in Native Redband Trout in the Context of Hatchery Introductions and Habitat Modification in the Boise River, Idaho

Hybridization is one of the greatest threats to native fishes including trout, but identifying hybrids in the field is notoriously difficult.  Here we demonstrate the utility of genetic approaches for rapid assessment of hybridization in inland native rainbow (redband) trout.  We studied populations across the upper Boise River, Idaho, a system where native redband trout persist in a mosaic of conditions ranging from relatively natural to highly impacted by barriers, altered flows and intensive fish stocking.  Results suggest that complex patterns of historical introductions of exogenous trout and habitat modifications have left a spatially variable footprint on native redband trout.  Several local populations have been completely replaced by hatchery fish, whereas others exist as hybrid swarms, some are in the process of being actively invaded, and some are maintaining relatively high genetic purity.  The persistence of some populations of relatively pure redband trout is perhaps fortuitous but not guaranteed:  in the future the possibility of invasion by non-native cutthroat, hatchery rainbow trout and hybrids from nearby man-made reservoirs or other sources of naturalized hatchery trout may pose serious threats.  Opportunities to preserve the genetic integrity of native redband trout still exist, but this study emphasizes how current management options are shaped by historical legacies of fish introductions and habitat modification.