Mitochondrial DNA evidence for high levels of gene flow among populations of a widely distributed anadromous lamprey Entosphenus tridentatus (Petromyzontidae) D. H. GOODMAN*†, S. B. REID‡, M. F. DOCKER§k, G. R. HAAS{ AND A. P. KINZIGER* *Department of Fisheries Biology, Humboldt State University, 1 Harpst Street, Arcata, CA 95521, USA, ‡Western Fishes, 2045 East Main Street, Ashland, OR 97520, USA, §Great Lakes Institute for Environmental Research, University of Windsor, Windsor, ON, N9B 3P4 Canada and {Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, AK 99775-7220, USA (Received 18 February 2007, Accepted 3 October 2007) Mitochondrial DNA variation among 1246 individuals of Pacific lamprey (Entosphenus tridentatus) from 81 populations spanning 2600 km from the Skeena River, British Columbia, to the Ventura River, California, was surveyed using five restriction enzymes. A total of 29 composite haplotypes was detected in two gene fragments (ND2 and ND5). The three most common haplotypes, occurring in 91% of all samples, were present at similar frequencies in all regions. Samples were divided into six biogeographic regions based on sample distribution and geographical landmarks to assess geographic genetic structure. Analysis of molecular variance indicated that 99% of the genetic variation was explained by variability within drainages. The lack of geographical population structure is likely related to a life-history pattern that includes a prolonged larval freshwater stage, migration to oceanic feeding and return to fresh water to spawn. The lack of strong natal homing apparently promotes gene flow among drainages and regions. # 2008 The Authors Journal compilation # 2008 The Fisheries Society of the British Isles Key words: Entosphenus tridentatus; genetic variation; Lampetra tridentata; mitochondrial DNA; Pacific lamprey; phylogeography. INTRODUCTION Fishes that migrate between marine and freshwater environments exhibit differ- ent levels of genetic differentiation among populations. Many anadromous salmonids (Oncorhynchus spp.), for example, exhibit relatively high levels of among-population genetic differentiation (Allendorf & Seeb, 2000; Waples †Author to whom correspondence should be addressed at present address: US Fish and Wildlife Service, Arcata Field Office, 1655 Heindorn Road, Arcata, CA 95521, USA. Tel.: þ 707 822 7201; fax: þ 707 822 8411; email: damon_goodman@fws.gov kPresent address: Department of Zoology, University of Manitoba, Winnipeg, MB, R3T 2N2 Canada Journal of Fish Biology (2008) 72, 400–417 doi:10.1111/j.1095-8649.2007.01731.x, available online at http://www.blackwell-synergy.com 400 # 2008 The Authors Journal compilation # 2008 The Fisheries Society of the British Isles