Marine population structure in an anadromous fish: life-history influencespatterns of mitochondrial DNA variation in the eulachon, Thaleichthys pacificus

Due to the apparent decline in size of a number of populations, eulachon, T haleichthys pacificus, have recently become the focus of a conservation mov ement in the northeast Pacific. Little is known of the marine life-history phase of this anadromous fish, and although it has been suggested that eula chon spawning in different rivers may form distinct populations, nothing is known of their population structure. Molecular genetic data were used to i nvestigate population structure and possible management schemes. Mitochondr ial DNA genotypes, determined through restriction fragment length polymorph isms (RFLP) analysis, were resolved in fish from several rivers throughout the geographical range of eulachon. Our data support the idea that extant e ulachon populations result from postglacial dispersal from a single Wiscons inan glacial refuge. Further, while three of the 37 haplotypes recovered ac count for approximately 79% of the samples, many private haplotypes were ob served, suggesting possible regional population structure. While a great de al of genetic variation was observed (37 haplotypes in 315 samples), an AMO VA 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 se em 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 now.