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Successful integrated management of the invasive predatory sea lamprey (Petromyzon marinus) in the Laurentian Great Lakes of North America is owed largely to the long history of beneficial use of two lampricides: 3-trifluoromethyl-4-nitrophenol (TFM) and 2′,5-dichloro-4′-nitrosalicylanilide (niclosamide). Ensuring continued successful sea lamprey control necessitates consideration of possible next-generation lampricides to supplement or replace current lampricides. This review identifies fifteen hallmarks of success for current lampricides to be used as design criteria in a search for next-generation lampricides. A three-stage research approach is outlined. Targeted research using omics, computer modelling, and high-throughput technology to define molecular mechanisms and high probability molecular targets for sea lamprey selective toxic action is crucial to prioritizing chemical candidates. Targeted delivery or identifying synergists to existing or new lampricides can provide increased efficiency and reduced environmental impact. Ultimate development of next-generation lampricides will rely on traditional toxicity testing methodologies to ensure safety and regulatory compliance.

Beginning with the first lampricide treatments of Lake Huron streams in 1960, applying lampricides and maintaining barriers to migration in Lake Huron tributaries continue to be the primary methods of controlling sea lampreys (Petromyzon marinus). In years leading up to the new millennium, sea lamprey abundance in Lake Huron remained at high levels, yet well below the pre-control adult abundance estimate of 700,000 adults. Two decades later, a variety of sea lamprey control strategies have been developed and employed to further reduce sea lamprey abundance in Lake Huron. In recent years, record low numbers have been achieved and continue to be at or near target levels for sea lamprey abundance. The St. Marys River continues to be one of the largest producers of sea lampreys to Lake Huron and additional actions have been taken to address this challenge. Controlling larval abundance in the St. Marys River along with increasing lampricide treatments throughout the basin and focusing on the largest sea lamprey producing streams have contributed to the decline in sea lamprey abundance. Lake trout (Salvelinus namaycush) restoration efforts along with increased sea lamprey control has facilitated natural reproduction and increase in wild stocks across the basin. The fish community objective for sea lampreys in Lake Huron has not been entirely met and efforts to achieve this objective are ongoing.

Ecosystem managers confronted with newly invasive species may respond with a program of suppression or eradication. Suppression of an invasive species refers to management of a species such that its effect on other biota in the local ecosystem is acceptable. Eradication is the removal of all individuals of a species from a defined region. We examine the cost and benefit trade-offs between suppression and eradication of Laurentian Great Lakes sea lampreys (Petromyzon marinus) based on discussions at the 3rd Sea Lamprey International Symposium (held in 2019). Substantial effort has been expended annually since the 1960s to suppress sea lampreys in the Great Lakes basin. Choosing between suppression and eradication is a value judgement, ideally made jointly by scientists, decision-makers, stakeholders, and society. Successful large-scale eradications have been limited to a small number of cases for which the cost to human society justified and supported the long-term commitment necessary for success. The greatest challenge to successful eradication of sea lampreys from the Great Lakes may be a suitable social, political, legal, and institutional environment. Preparations could be made now for a transition in which public pushback on current control methods (pesticide applications and barriers to fish passage) leads to more extensive use of an alternative control method, such as genetic control.

From 2000 to 2019, sea lamprey control in Lake Erie focused primarily on deploying lampricides and completing important barrier and dam renovation projects in tributaries including the Big Creek barrier, and Harpersfield and Dunnville dams. The number of Lake Erie stream treatments doubled during 2000–2019 compared to 1986–1999 due to changes in larval assessment and stream selection protocols. Despite increased lampricide use and few infested tributaries relative to other Great Lakes, achieving program targets for Lake Erie has been challenging, but progress has been made. The St. Clair River is a potential significant source of sea lampreys to Lake Erie harboring an estimated 920,000 larvae; however, to date, no lampricide treatments have occurred. In 1986, sea lamprey control began in Lake Erie primarily to restore lake trout. Prior to control efforts, sea lamprey induced mortality on lake trout was too high to allow the establishment of adult lake trout stocks necessary to achieve fishery program goals. Sea lamprey marks on adult lake trout declined dramatically in Lake Erie following sea lamprey control. State and provincial agencies have prioritized stocking the Seneca strain of lake trout, thought to be more resistant to sea lamprey attacks. This change in stocking strategy, coupled with more aggressive sea lamprey control, has contributed to increasing adult lake trout survival. Protecting species at risk in Lake Erie adds to the challenges faced by the sea lamprey control program, but productive collaboration has proven an effective strategy in dealing with the increasing complexity of sea lamprey control.

Lake Ontario boasts a diverse fish community comprised of native and introduced species that support vibrant recreational, commercial and Indigenous fisheries. The effective delivery of a program to assess and control the sea lamprey (Petromyzon marinus) is crucial to achievement of Lake Ontario Fish Community Objectives of rehabilitating native fish stocks while protecting and maintaining the abundance of introduced salmonines. During 2000–2019, the Great Lakes Fishery Commission (GLFC) and its control agents, Fisheries and Oceans Canada (DFO) and the U. S. Fish and Wildlife Service (USFWS), delivered a consistent program of sea lamprey assessment and control. Beginning in 2004, rising sea lamprey abundance and marking rates on lake trout (Salvelinus namaycush) in Lake Ontario coincided with a decline of large lake trout in gillnet surveys. Efforts were undertaken to identify and control important sources of juvenile sea lampreys, including larvae that survived treatment, inhabited deepwater areas, or colonised previously uninhabited stream reaches and tributaries. A renewed reliance on proven conventional controls, including lampricide treatments and barriers, has resulted in consistent suppression of sea lamprey abundance and lake trout marking to prescribed targets in Lake Ontario during 2014–2019. This achievement is unprecedented in the 49-year history of Sea Lamprey Control Program delivery in Lake Ontario, and is attributable to the collaborative efforts of the GLFC, the control agents, and federal, provincial, state, and Indigenous partners.

The lampricides TFM and niclosamide are added to streams to control invasive larval sea lamprey (Petromyzon marinus) populations in the Laurentian Great Lakes. Lampricide effectiveness depends upon TFM and niclosamide bioavailability which is influenced by both abiotic and biotic factors. For example, at lower pH, TFM bioavailability is higher because a greater proportion exists as un-ionized TFM (TFM-OH), which easily crosses the gills. At higher pH, however, the negatively charged ionized species of TFM (TFM-O−) predominates, which is less easily taken-up, meaning more TFM must be applied. Although water alkalinity does not directly affect TFM speciation, as a buffer it influences how much expired water crossing the gills is acidified by CO2 and metabolic acid excretion. In poorly buffered waters, greater acidification of the expired water increases TFM bioavailability in the gill microenvironment than in better buffered, higher alkalinity waters where more TFM must be applied. Hence, sea lamprey and non-target fishes such as lake sturgeon (Acipenser fulvescens) are more sensitive to lampricides in low pH, low alkalinity waters. Differences in gill structure and microenvironment acidification might also explain why TFM sensitivity of young-of-the-year lake sturgeon approaches or exceeds that of sea lamprey in higher alkalinity waters. Other biotic factors such as body size and metabolic rate also contribute to differences in lampricide sensitivity. We conclude that better understanding of the abiotic and biotic factors influencing lampricide bioavailability can be used to refine treatment protocols to improve lampricide effectiveness and to better protect non-target fishes from lampricide toxicity.

Adaptive cluster sampling (ACS) provides researchers with an alternative technique to estimate the abundance of rare or spatially clustered organisms, but its application in field investigations has been limited to relatively few studies. We used ACS to estimate parameters of a spatially autocorrelated population of larval sea lampreys, Petromyzon marinus, in Wilmot Creek, a Lake Ontario tributary. When compared with simple random sampling (SRS), ACS significantly increased catch per sample as well as the percentage of samples that contained larvae. Although ASC-generated samples are spatially biased, the use of established formulae enabled us to calculate unbiased estimators of mean larval density and variance. With ACS, variance was reduced, improving the precision around estimates of mean density, however; increased precision came at the price of greater sampling effort. When variance was adjusted for effort, ASC provided equal or greater efficiency than SRS in 33% of sampling events, with no apparent site or seasonal bias. Based on the knowledge that larval sea lampreys are spatially aggregated, we anticipated that ACS would result in higher precision for a greater proportion of sampling events. Nonetheless, we consider ACS to be a useful technique for evaluating larval sea lamprey populations and anticipate increased application for investigating other spatially over-dispersed species.

Sea lampreys ( Petromyzon marinus ), first reported in Lake Erie in 1921, emigrated from Lake Ontario via the Welland Canal. It was not until the advent of pollution abatement, stream rehabilitation, and salmonid enhancement programs that sea lampreys proliferated. The Great Lakes Fishery Commission (GLFC), in co-operation with state, provincial, and federal fisheries agencies, implemented an integrated sea lamprey management (IMSL) plan for Lake Erie in 1986. Suppression of sea lampreys was nearly immediate, as indicated by declining larval-, parasitic-, and spawning-phase abundance, while survival of lake trout ( Salvelinus namaycush ) was markedly improved. Consistent with their vision statement, the GLFC began reducing lampricide use by the mid-1990s, while increasing reliance on alternative control methodologies. Reduction of treatment effort coincided with the development of new lampricide application techniques and treatment selection criteria, in addition to heightened regional concern for the impact of lampricide on non-target species. Subsequently, Lake Erie's sea lamprey numbers have rebounded, and marking rates on lake trout have approached pre-control levels. It is hypothesized that Lake Erie's rising abundance is primarily fuelled by untreated and residual larval populations, although some migration of parasitic-phase sea lampreys from Lake Huron is suspected. Model simulations infer that treatment effort on Lake Erie was sub-optimal from 1995 to 1998. Beginning in 1999, the GLFC enhanced measures to identify and control sources of sea lampreys. Based on historical abundance patterns and model results, it is anticipated that intensified management in Lake Erie will reduce sea lamprey numbers and provide an opportunity for lake trout restoration.

The Great Lakes Fishery Commission successfully coordinated fishery management and delivered an international program of sea lamprey ( Petromyzon marinus ) control and assessment that facilitated the rehabilitation of lake trout ( Salvelinus namaycush ) and development of a salmonid fishery in Lake Ontario during 1971 to 1999. During the 1980s and 1990s, the integrated management of sea lampreys (IMSL) strategy optimized control and assessment techniques and refined the stream selection process. The program focused primarily on the juvenile life stage (larval phase) through the application of the selective lampricide 3-trifluoromethyl-4-nitrophenol (TFM) to tributaries of the lake, although the addition of barriers to Lake Ontario tributaries appears to have reduced the reliance on TFM. By 1999, IMSL had eliminated sea lampreys in 20 of the 57 tributaries with historical records of production. Accordingly, the abundance of spawning phase sea lampreys showed a significant downward trend, and substantial declines in marking rates for large lake trout were observed. Targets for control were established through the lake trout rehabilitation plan, fish community objectives, and simulation models incorporating an economic injury level (EIL) approach. The EIL for sea lamprey control is the point in which more treatment is not economically justified. The updated IMSL model developed for Lake Ontario provided a tool to support trade-off analysis to estimate target levels of sea lamprey abundance relative to lake trout harvest. Comparison of model predictions and trends in spawning phase abundance revealed general agreement, but also pointed to inconsistencies that merit further investigation. The EIL of control based on the entire treatment history is 8,100 kg TFM per year, which produces a treatment residual of 4,100 adult sea lampreys. Given recent estimates of transformer production from the Niagara River, the annual contribution from untreated sources is estimated to be in the range of 10,000 to 15,000 adult sea lampreys. The target level (EIL) for adult sea lampreys in Lake Ontario is thus 14,100 to 19,100. Over the past 10 years, Lake Ontario TFM treatments have averaged 4,600 kg per year.

We developed two weight-length models from 231 populations of larval sea lampreys (Petromyzon marinus) collected from tributaries of the Great Lakes: Lake Ontario (21), Lake Erie (6), Lake Huron (67), Lake Michigan (76), and Lake Superior (61). Both models were mixed models, which used population as a random effect and additional environmental factors as fixed effects. We resampled weights and lengths 1,000 times from data collected in each of 14 other populations not used to develop the models, obtaining a weight and length distribution from reach resampling. To test model performance, we applied the two weight-length models to the resampled length distributions and calculated the predicted mean weights. We also calculated the observed mean weight for each resampling and for each of the original 14 data sets. When the average of predicted means was compared to means from the original data in each stream, inclusion of environmental factors did not consistently improve the performance of the weight-length model. We estimated the variance associated with measures of abundance and mean weight for each of the 14 selected populations and determined that a conservative estimate of the proportional contribution to variance associated with estimating abundance accounted for 32% to 95% of the variance (mean = 66%). Variability in the biomass estimate appears more affected by variability in estimating abundance than in converting length to weight. Hence, efforts to improve the precision of biomass estimates would be aided most by reducing the variability associated with estimating abundance.