SHORT COMMUNICATION Population genetic structure of the salmon louse, Lepeophtheirus salmonis (Krøyer) on wild and farmed salmonids around the Pacific coast of Canada

Thepopulationgeneticstructureofthesalmonlouse(Lepeophtheirussalmonis:Caligidae)isofconsiderableinterest because of indirect evidence suggestingtransmission of this ectoparasite between wildsalmon and farmed salmon (reviewed by Boxaspen2006). Consequently several studies have looked forfarm-speci¢c molecular markers that would allowthe path of transmission to and from wild ¢sh tobe traced. Unfortunately, an early report of RAPDmarkers that were speci¢c to Scottish salmonfarms (Todd,Walker,Woli, Northcott,Walker, Ritchie,Hoskins, Abbott & Hazon 1997) was not found in asecond RAPD study (Dixon, Shinn & Sommerville2004) or in a more extensive study using six micro-satellite markers (Todd, Walker, Ritchie, Graves &Walker 2004). Nevertheless, interest in ¢nding farm-speci¢c molecular markers continues. It is still notpossible to follow the salmon lice larvae during the4^14-day period they spend in the plankton beforethey infect a host (Johnson & Albright 1991). Wethereforetestedtheresearchhypothesisthatpopula-tion genetic structure existed among salmon licepopulations collected from diierent geographicalregions of British Columbia, Canada, because ofrestrictedgene£ow.Samples of the salmon louse, L. salmonis,wereobtained from three diierent wild Paci¢c salmonspecies by sports ¢shing and during creel (sports¢shery catch) surveys and from farmed Atlanticsalmon by netting (Table 1). Sea lice were removedfrom each salmon host with forceps and preservedin 95% ethanol.To test for population genetic struc-turewiththelargestsamplesizepossible,wedecidedto use a fragment of the mitochondrial cytochromec oxidase subunit1 (COI) (Folmer, Black, Hoeh, Lutz& Vrijenhoek1994) that hasbeenextensively used forDNA barcoding (Hebert, Cywinska, Ball & deWaard2003).This allowedus to make use of a large numberof existing salmon louse COI sequences and to easilycompare the sequence divergence that we observedamong diierent clades with that of other species ofanimals(Hebertetal.2003).Template DNA was extracted from individualsalmon lice using modi¢ed proteinase K methods.A1mm