The parasitic sea lamprey Petromyzon marinus occurs throughout North America's Great Lakes, where it has an immense economic impact on commercially and recreationally important fishes. Sea lampreys indisputably invaded Lake Erie and the upper Great Lakes from Lake Ontario in the mid‐1900s, but their official status as a nonnative species in Lake Ontario is based on circumstantial evidence and has long been subject to controversy. Presently, sea lampreys are considered by U.S. and Canadian government agencies to be an invasive species within the entire Great Lakes watershed, and millions of dollars are spent annually to suppress them. We sequenced 330 base pairs of the mitochondrial DNA control region of 224 sea lampreys collected from 10 locations (3 within the Lake Ontario drainage, 2 within the Lake Superior drainage, and 5 rivers between Quebec and New York that are tributary to the Atlantic Ocean). Eighteen haplotypes were revealed, of which 17 occurred in specimens from Atlantic coast rivers, 6 in specimens from Lake Ontario, and 2 in specimens from Lake Superior tributaries. Haplotype frequencies were not significantly different (P > 0.05) within regions, indicating low or no homing fidelity. But when haplotype frequencies were grouped within regions and compared along the hypothesized colonization pathway, significant differences were seen. Pronounced differences in haplotype frequency patterns between Atlantic coast and Lake Ontario drainage collections, together with arguments against the viability of canal passage, strongly support the idea of post‐Pleistocene natural colonization by one of at least three hypothesized zoogeographic pathways. If sea lampreys are indigenous to Lake Ontario, management policies aimed toward intense suppression might need reevaluation.

Anadromous fishes are believed to make regular circuits of migration in the sea before homing to their natal rivers. Sea lamprey Petromyzon marinus is an anadromous fish that is an exception to this life-history pattern. It also differs from other anadromous fishes in that its adult phase is parasitic, a feeding strategy that should make homing problematic for lamprey cohorts that become widely dispersed through transport by the diverse hosts they parasitize. We sequenced a portion of the mitochondrial DNA control region from sea lampreys collected from 11 North American east coast rivers to test for genetic evidence of homing. There were no significant differences (c 2 Z235.1, pZ0.401) in haplotype frequencies among them, with almost 99 per cent of haplotypic diversity occurring within populations. These findings, together with concordant genetic results from other geographical regions and ancillary information on pheromonal communication, suggest that sea lamprey does not home but rather exhibits regional panmixia while using a novel 'suitable river' strategy to complete its life cycle.

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Adult sea lampreys Petromyzon marinus parasitize a variety of recreationally, commercially, and ecologically important fishes in the north temperate Atlantic Ocean and some inland waters of North America, including the Great Lakes and Lake Champlain. This has resulted in the development of international, provincial, and state programs to suppress their abundance in some of these waters. This effort, in part, emanates from the nonnative status afforded sea lampreys in these lakes by management agencies. However, whether the sea lamprey is native to Lake Champlain has long been debated, and the outcome of this debate may impact the intensity of efforts to suppress their abundance there. We addressed this issue by using sequence analysis of the mitochondrial DNA control region. Resultant haplotype frequencies of sea lampreys from the Lake Champlain watershed were compared among sea lamprey collections made in multiple drainages within the Lake Ontario and Lake Superior watersheds and Atlantic coast rivers. We found no evidence of population structuring among tributaries within any of these four systems, but we did find highly significant genetic differentiation between systems. Haplotype diversity was low in the collections from the two Great Lakes and even more so in those from Lake Champlain compared with that in Atlantic coast rivers. Our genetic results are most consistent with the hypothesis that sea lampreys are native to Lake Champlain, having colonized the lake postglacially by one of several zoogeographic corridors.