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The biogeography and conservation of rainbow trout, Oncorhynchus mykiss McCusker, Megan R.

Abstract

Northwestern North America has been repeatedly glaciated over the past million years, with the most recent glaciation occurring between 60,000 and 10,000 years ago. Where species survived glaciation and what dispersal routes they used during recolonization most likely had a profound effect on their intraspecific genetic variation. In this study, molecular techniques were used to investigate biogeographical, taxonomic and conservation issues in the rainbow trout, Oncorhynchus mykiss. Allozyme data were consolidated from the literature to assess relationships of rainbow trout throughout its range, and a divergence between coastal and inland populations from California to Kamchatka in eastern Siberia was supported. For greater detail, a mitochondrial DNA analysis of restriction fragment length polymorphism was done, focusing mainly on British Columbia and the northern distribution of the species. Two phylogenetically distinct mitochondrial lineages were found with an average of 1.03% sequence divergence, with overlapping distributions that included both coastal and inland populations. Diversity and distributional analyses strongly suggested that both a coastal and an inland refuge were used during the last glaciation with extensive post-glacial gene flow and secondary contact. The Queen Charlotte Islands and the Columbia River were most strongly supported as refuge locations. An additional refuge may have existed in Beringia, but post-glacial dispersal from Beringia was most likely quite limited. Sequencing analysis of mitochondrial haplotypes revealed higher diversity in California than in the northern part of the species range, indicating a more ancient presence of rainbow trout in California relative to the north. The phylogeographic divergence in British Columbia among coastal and inland groups predates adaptive variation in the species as indicated by two life history characters in this analysis. Genetic variation resulting from historical isolation, therefore, warrants high conservation priority. However, due to the degree of secondary contact between these groups post-glacially, subspecies designations of coastal and redband (inland) trout were not supported.

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