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Marine population structure in an anadromous fish : life-history influences patterns of neutral molecular variation in the eulachon, Thaleichthys pacificus McLean, Jennifer Elizabeth

Abstract

Phylogeography, historical demography, conservation of biodiversity and the influence of life-history on intraspecific population structure may be investigated with molecular genetic data, in particular, mitochondrial DNA (mtDNA) and microsatellites. Due to the apparent decline in size of a number of populations, eulachon, Thaleichthys pacificus, have recently become the focus of a conservation movement in the northeast Pacific. Little is known of the marine life-history phase of this anadromous fish, and although it has been suggested that eulachon spawning in different rivers may form distinct populations, nothing is known of their population structure. Both mtDNA and microsatellite loci were employed to investigate population structure and possible management schemes. Mitochondrial DNA haplotypes, determined through RFLP analysis of four mitochondrial genes, were resolved in fish from several rivers throughout the geographic range of eulachon. My data support the idea that extant eulachon populations result from post glacial dispersal from a single Wisconsinan glacial refuge. Further, while three of the thirty-seven haplotypes recovered account for approximately 79% of the samples, many private haplotypes were observed, suggesting possible regional population structure. While a great deal of genetic variation was observed (37 haplotypes in 315 samples), an AMOVA showed that more than 97% of the total variation was detected within populations. Eulachon populations seem to be structured in a manner much more similar to marine species than anadromous species, which may be due to some unusual life-history characteristics. MtDNA results were tested against predictions made from hypotheses concerning the origin and persistence of sub-divided populations in marine species, and seem to be consistent with both the Member-Vagrant hypothesis and isolation-by-distance. Analysis of five microsatellite loci revealed that microsatellite DNA variation in eulachon is surprisingly low, yet a similar degree of population sub-division as seen with the mtDNA data was observed. Again, this level of structure was consistent with what is generally observed in marine species. Isolation-by-distance was not supported, but mtDNA data are more sensitive to population sub-division and expected to reveal patterns that nuclear DNA might not. Microsatellite data were consistent with a single Wisconsinan glacial refuge in the south. The extremely low variation observed at eulachon microsatellite loci may be due to the nature of the repeat arrays. Only 16% of the microsatellites sequenced contained perfect repeats: a small proportion compared to an average of 60% or higher in salmonids. Although eulachon numbers are declining throughout their range and especially in the southern portion of their distribution, levels of mtDNA diversity are high and microsatellite variation is consistent throughout their range. Recent demographic declines have apparently had little effect on the genetic diversity of eulachon. Despite the adequate levels of genetic diversity, population sub-division is only weakly developed in eulachon. This species represents a common marine situation of high gene flow and indeterminate population boundaries in which the definition of management units is difficult, and factors other than genetic variation must be considered in management or conservation plans.

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