A Genetic and Phenetic Baseline before the Recolonization of Steelhead above Howard Hanson Dam, Green River, Washington

Abstract

In 2011, steelhead Oncorhynchus mykiss (anadromous rainbow trout) will be systematically transported above Howard Hanson Dam, Green River, Washington. We are interested in providing data to help manage the stock of fish that are selected to recolonize the upper river and to track how resident rainbow trout above the dam respond genetically after 80 years of isolation. We characterized relevant gene pools in the upper Green River before fish transportation with 11 microsatellite loci to evaluate the genetic variability within and among collections. We also examined morphometric and coloration patterns as potential indicators of adaptive variation. Hatchery steelhead are clearly different from wild steelhead (F _ST = 0.037); genetic assignment tests correctly distinguished 91% of the steelhead. While there was no reduction in the amount of genetic variability in the resident rainbow trout above Howard Hanson Dam compared with that of wild steelhead collections below the dam, the two groups had low but statistically significant differences (F _ST = 0.03). The transport of juvenile and adult steelhead above the dam in the last 20 years may have affected these genetic results. Two collections of hatchery rainbow trout were highly differentiated from all other collections, and a STRUCTURE analysis indicated that there was no introgression of their genes into Green River fish. Morphologically, significant differences were seen between juvenile resident rainbow trout and steelhead. Resident trout had a stout caudal peduncle and more parr marks, whereas juvenile steelhead had a more elongate tail and fewer parr marks. Given these genetic and phenetic measures of differentiation, managers can monitor and screen the upstream passage of steelhead, and will be able evaluate the level of participation of resident rainbow trout gene pools in the recolonization event.