Defining hybridization thresholds for native species conservation in the genomic era

Human-induced hybridization among genetically distinct groups of fish is a widespread and complex problem in fisheries management. A particularly challenging facet of human-induced hybridization is deciding which fish should be prioritized for conservation action or investment, and which should not. The increasing availability of genomic data in fisheries management demands that explicit hybridization frameworks and associated hybridization thresholds be developed, as increasing resolution will inevitably demonstrate that small amounts of nonnative ancestry are present in populations or species that were previously thought to be nonhybridized. A key question then becomes, how do we make rational decisions regarding resource prioritization for populations or species with, for example, 10, 1, 0.1 or even 0.01% nonnative ancestry? We use extensive data from Westslope Cutthroat Trout Onchorhynchus lewisi to describe how objective, data-based decision frameworks can be developed to help managers conserve genetic variation, while minimizing nonnative ancestry and the risk of outbreeding depression. While the conservation implications of hybridization are nuanced and context-dependent, the approach described herein is general and can be extended to other species.