Are Sea Lamprey Native to Lake Ontario? Debating the Evidence

Although already discussed thoroughly—most recently by Eshenroder (2009; Trans Am Fish Soc 138:1178–1189) and Waldman (2009; Trans Am Fish Soc 138:1190–1197), the question of whether Sea Lamprey Petromyzon marius are native to Lake Ontario still keeps me up at night.  It is clear that Sea Lamprey invaded the upper Great Lakes following construction (or subsequent modification) of the Welland Ship Canal, but whether they entered Lake Ontario in historic (within the past 200 years through manmade canals) or prehistoric (as a result of post-Pleistocene natural colonization) times is still debated.  The arguments supporting the “invasion-by-canal” hypothesis (e.g., that Sea Lamprey were not recorded in Lake Ontario prior to 1836) and the “native-but-rare” hypothesis (e.g., significant gene frequency and life history differences between the Lake Ontario and Atlantic Ocean populations) are both convincing.  It is indeed hard to imagine that Sea Lamprey could have been present and not recorded, but—given that, even today, only 9% of its tributaries support Sea Lamprey larvae—it is also conceivable that ecological conditions (e.g., before the end of the “Little Ice Age” in the mid-1800s) kept their numbers low.  I will review the evidence, and welcome discussion.  Cast your vote!