Sea lampreys (Petromyzon marinus), native to the north Atlantic Ocean, invaded the Laurentian Great Lakes in the early 1900s. By the mid-1900s native lake trout (Salvelinus namaycush) populations, that once supported commercial harvests of millions of pounds, were decimated, due in part to the sea lamprey invasion. In response to this decimation, a treaty was struck between Canada and the United States in 1954, creating the Great Lakes Fishery Commission (Commission), whose job it is to control sea lampreys. The Commission implemented a control program that greatly reduced Great Lakes sea lamprey populations. Control continues on an annual basis; when control is relaxed, sea lampreys bounce right back, setting back fishery and ecosystem recovery by decades. Fishery managers and the Commission prioritize monitoring sea lamprey populations, to keep them in check, and to tailor and prioritize spending according to fishery needs on each lake. Annual indices of lake trout abundance, sea lamprey abundance, and sea lamprey-induced marking rates on lake trout are used to monitor success of past efforts and direct future efforts of sea lamprey control. A mark (or wound) on a host fish is an indication that a sea lamprey has attached to and potentially fed on host fish. Marking rates on lake trout are presumed to be largely driven by abundances of sea lampreys and lake trout. Occasionally, marking rates increase when they are expected to decrease (or vice versa), leaving decision makers puzzled about the true status of sea lamprey control in the Great Lakes.I investigated three possible reasons for these occasional inconsistencies in the status metrics: measurement error in each metric, survivor bias in marking rates, and alternative hosts. Inclusion of measurement error alone did not reproduce observed inconsistency rates, implying that a simple predator-prey model (a Type II functional response) lacked some important components. With survivor bias, the sea lamprey attack rate and sea lamprey-induced mortality rate were well indexed by Type A (piercing) marking rates if annual lethality rates were relatively constant. Other studies have demonstrated evidence of a relatively constant lethality rate, so survival bias is likely not a major contributor to observed inconsistencies. In Lake Ontario, sea lampreys exhibited a strong preference for lake trout, but switched to Chinook salmon when relative abundance of lake trout was low. Host switching impedes using a marking rate focused only on lake trout as an index of sea lamprey attacks. I also developed stock-recruitment relations for sea lampreys. I used a lake-specific adult-to-adult stock recruitment model to quantify how lampricide treatment effort reduced sea lamprey recruitment.This work eliminates measurement error as a primary cause of observed inconsistencies in sea lamprey status metrics and diminishes concern about survivor bias in Type A marking rates. Although I demonstrated that the presence of alternative hosts can affect sea lamprey-induced marking rates on lake trout, further research into this and other mechanisms contributing to variability in the status metrics is needed.
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