Publications

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

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

T. W. Buehrens, J. Glasgow, C. O. Ostberg, T. P. Quinn

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.

To link to this article:  http://dx.doi.org/10.1080/00028487.2012.728165