Spatial Segregation of Spawning Habitat Limits Hybridization between Sympatric Native Steelhead and Coastal Cutthroat Trout

Abstract

Native Coastal Cutthroat Trout Oncorhynchus clarkii clarkii and Coastal Steelhead O. mykiss irideus hybridize naturally in watersheds of the Pacific Northwest yet maintain species integrity. Partial reproductive isolation due to differences in spawning habitat may limit hybridization between these species, but this process is poorly understood. We used a riverscape approach to determine the spatial distribution of spawning habitats used by native Coastal Cutthroat Trout and Steelhead as evidenced by the distribution of recently emerged fry. Molecular genetic markers were used to classify individuals as pure species or hybrids, and individuals were assigned to age-classes based on length. Fish and physical habitat data were collected in a spatially continuous framework to assess the relationship between habitat and watershed features and the spatial distribution of parental species and hybrids. Sampling occurred in 35 reaches from tidewaters to headwaters in a small (20 km²) coastal watershed in Washington State. Cutthroat, Steelhead, and hybrid trout accounted for 35%, 42%, and 23% of the fish collected, respectively. Strong segregation of spawning areas between Coastal Cutthroat Trout and Steelhead was evidenced by the distribution of age-0 trout. Cutthroat Trout were located farther upstream and in smaller tributaries than Steelhead were. The best predictor of species occurrence at a site was the drainage area of the watershed that contributed to the site. This area was positively correlated with the occurrence of age-0 Steelhead and negatively with the presence of Cutthroat Trout, whereas hybrids were found in areas occupied by both parental species. A similar pattern was observed in older juveniles of both species but overlap was greater, suggesting substantial dispersal of trout after emergence. Our results offer support for spatial reproductive segregation as a factor limiting hybridization between Steelhead and Coastal Cutthroat Trout.

Received March 9, 2012; accepted September 4, 2012

ACKNOWLEDGMENTS

This work was funded by a grant from the U.S. Fish and Wildlife Service Western Washington Recovery Office (Cooperative Agreement 13410-8-J025), The Nature Conservancy (contract WAFO_193-053008), Wild Fish Conservancy, the University of Washington's H. Mason Keeler Endowment at the School of Aquatic and Fishery Sciences, and a fellowship from the Washington Fly Fishing Club. We sincerely thank the individuals who collected field data and conducted genetic laboratory work, including Isadora Jiminez-Hidalgo, Kerry Naish, Frank Staller, Ryan Klett, Schuyler Dunphy, Chau Tran, and Liane Davis. We thank John McMillan and George Pess for help in designing the study and Andrew McAninch for help with GIS analysis. We also thank Peter Kiffney, Todd Seamons, John Skalski, Clint Muhlfeld, and three anonymous reviewers for helpful comments and reviews of earlier drafts of the manuscript.