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 dis-tinct populations, nothing is known of their population structure. Molecular genetic data were used to investigate population structure and possible management schemes. Mito-chondrial DNA genotypes, determined through restriction fragment length polymorphisms (RFLP) analysis, were resolved in fish from several rivers throughout the geographical range of eulachon. Our data support the idea that extant eulachon populations result from postglacial dispersal from a single Wisconsinan glacial refuge. Further, while three of the 37 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 > 97% of the total variation was detected within populations. As yet, it is unclear whether genetically distinct populations of eulachon exist, or if these fish may be treated as one or a few large populations. Results were tested against predictions made from hypotheses concerning the origin and persistence of subdivided populations in marine species, and seem to be more consistent with the Member–Vagrant hypothesis than isolation by distance. Eulachon present an interesting situation that illustrates the difficulties involved in defining management units in organisms with high levels of gene flow.
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