Environmental factors associated with reproductive barrier breakdown in sympatric trout populations on Vancouver Island

The incidence of hybridization between coastal cutthroat (Oncorhynchus clarki clarki) and rainbow trout (Oncorhynchus mykiss) varies widely among populations. The breakdown of reproductive isolation is of concern to managers, and raises the question: how have the two species retained their genetic and morphological divergence? Using a combination of mitochondrial DNA and nuclear DNA markers coupled with watershed attribute and disturbance data, we determined the distribution and frequency of trout hybridization on Vancouver Island, BC and the environmental factors associated with the hybridization. We found 284 hybrids (among 1004 fish) in 29 of 36 sampled populations. High variation in levels of hybridization was observed among populations, and no single environmental factor was found to dominate in determining hybridization levels. However, logging activity, urban infrastructure development, and stocking of hatchery rainbow trout played significant roles in determining hybridization levels, and populations in small watersheds are more at risk of reproductive barrier breakdown. This study illustrates that cutthroat–rainbow trout reproductive barrier breakdown is widespread on Vancouver Island and that anthropogenic disturbance plays a role in the process. As similar environmental disturbance is common in much of coastal trout habitat, large-scale hybridization may be occurring elsewhere and thus may represent a critical management issue for Pacific trout species.