Sea Lamprey Control Research Articles

An Operating Model for the Integrated Pest Management of Great Lakes Sea Lampreys

Models of entire managed systems, known as operating models or management strategy evaluation (MSE) models, have been developed in recent years to more fully account for uncertainty in multiple steps of fishery manage- ment. Here we describe an operating model of sea lamprey management in the Great Lakes and use the model to compare alternative management strategies for sea lamprey control in Lake Michigan. Control of sea lampreys is mainly achieved through the application of chemical lampricides that target stream-dwelling larvae before they become parasites. The op- erating model simulated uncertainty due to process variation in larval population dynamics, the accuracy of population as- sessments used to direct selection of areas to be chemically treated, and the effectiveness of these treatments. We used the operating model to compare the performance of stream selection strategies that either rely on assessments to direct chemi- cal treatments or eliminate the assessment process altogether by relying on prior but uncertain knowledge of stream-level sea lamprey growth rates to specify a fixed schedule for chemical treatments. The fixed schedule strategy led to a modest improvement in expected suppression of parasitic sea lamprey abundance over the assessment-based strategy so long as assessment cost savings were allocated to chemical treatment when assessment was not used to select streams for treat- ment. We also evaluated the sensitivity of the assessment-based strategy to differing but plausible levels of assessment uncertainty. A moderate reduction in assessment uncertainty led to a large increase in suppression of parasitic sea lamprey abundance for the assessment-based selection strategy, emphasizing the importance of both accurately measuring and re- ducing assessment uncertainty.

Predation on emergent lake trout fry in Lake Champlain

The rehabilitation of extirpated lake trout ( Salvelinus namaycush) in the Great Lakes and Lake Champlain has been hindered by various biological and physiological impediments. Efforts to restore a lake trout fishery to Lake Champlain include hatchery stocking and sea lamprey control. Despite these management actions, there is little evidence of recruitment of naturally-produced fish in annual fall assessments. Spawning occurs at multiple sites lake-wide in Lake Champlain, with extremely high egg and fry densities, yet sampling for juvenile lake trout has only yielded fin-clipped fish. To investigate this recruitment bottleneck, we assessed predation pressure by epi-benthic fish on emergent fry on two spawning reefs and the subsequent survival and dispersal of fry in potential nursery areas. Epi-benthic predators were sampled with 2-h gillnet sets at two small, shallow sites in Lake Champlain throughout the 24-h cycle, with an emphasis on dusk and dawn hours. In total, we documented seven different species that had consumed fry, with consumption rates from 1 to 17 fry per stomach. Rock bass and yellow perch dominated the near-shore fish community and were the most common fry predators. Predator presence and consumption of fry was highest between 19:00 and 07:00. Predators only consumed fry when fry relative abundance was above a threshold of 1 fry trap − 1 day − 1 . We used an otter trawl to sample for post-emergent fry adjacent to the reef, but did not capture any age-0 lake trout. Due to the observed predation pressure by multiple littoral, species on shallow spawning reefs, lake trout restoration may be more successful at deep, offshore sites.

A rapid assessment approach to prioritizing streams for control of Great Lakes sea lampreys (Petromyzon marinus): a case study in adaptive management

We developed and evaluated an alternative method (rapid assessment or RA) for assessment of larval sea lampreys, Petromyzon marinus . We determined that using RA would result in at least as many, if not more, sea lampreys being killed than would using the current assessment method (quantitative assessment sampling or QAS) to select streams for lampricide treatment. Both assessment methods were carried out simultaneously throughout the entire Great Lakes basin from 2005 to 2007. RA required fewer resources than the current method and thus allowed for the chemical treatment of additional streams with lampricides, given a fixed overall budget for control. Population estimates generated from the QAS surveys showed that using RA would result in approximately equal numbers of metamorphosing lamprey and greater numbers of larval lamprey killed than by using QAS. Mark–recapture results indicated that prioritizing streams for treatment using RA may result in higher numbers of metamorphosing and larval lampreys killed than by using QAS. RA is currently being adopted throughout the Great Lakes as the method for assessing larval sea lamprey populations. Other fishery management programs may benefit from examining the value of their assessment programs relative to other uses of resources.

Lake Trout Population Dynamics at Drummond Island Refuge in Lake Huron: Implications for Future Rehabilitation

AbstractThe Drummond Island Refuge (DIR) was established in 1985 as part of the rehabilitation effort for lake trout Salvelinus namaycush in Lake Huron. Since then, several strains of hatchery‐reared lake trout have been stocked annually at the DIR. An intensive lampricide treatment of the St. Marys River during 1998–2001 was expected to lower the abundance of sea lamprey Petromyzon marinus within the DIR by 2000. We conducted annual gill‐net surveys during spring and fall to evaluate the performance of each of the strains of lake trout as well as that of the entire lake trout population (all strains pooled) in the DIR during 1991–2005. The criteria to evaluate performance included the proportion of “wild” fish within the population, spawner density, adult survival, growth, maturity, and wounding rate by sea lampreys. Wild lake trout did not recruit to the adult population to any detectable degree. During 1991–2005, the average density of spawning lake trout appeared to be marginally sufficient to initiate a self‐sustaining population. Survival of the Seneca Lake (SEN) strain of lake trout was significantly higher than that of the Superior–Marquette (SUP) strain, in part because of the higher sea‐lamprey‐induced mortality suffered by the SUP strain. However, other factors were also involved. Apparently SUP fish were more vulnerable to fishing conducted in waters near the refuge boundaries than SEN fish. The St. Marys River treatment appeared to be effective in reducing the sea lamprey wounding rate on SEN fish. We recommend that the stocking of SEN lake trout in the DIR, control of sea lampreys in the St. Marys River, and reduction of commercial fishery effort in waters near the DIR be maintained.

Modeling the sustainability of lake trout fisheries in eastern Wisconsin waters of Lake Superior

Lake trout Salvelinus namaycush stocks in Lake Superior collapsed in the 1950s due to overfishing and sea lamprey Petromyzon marinus predation. Stocks were rehabilitated through stocking, sea lamprey control, and fishery regulation, but concern over lake trout population sustainability still exists because commercial and recreational fishing demand is high relative to productivity, and the currently accepted 42% total annual mortality rate has not been tested for sustainability. Our objective was to estimate the maximum sustainable (i.e., likely not to cause extirpation) fishing mortality and total annual mortality rates for lake trout in eastern Wisconsin waters of Lake Superior. We developed a dynamic, age-structured population model to predict future lake trout abundance. The simulation model included parameter uncertainty and autocorrelated process error in the stock–recruitment relationship and in sea lamprey mortality, and accounted for assessment and implementation error. We used mean abundance, mean total annual mortality rate, mean catch for the commercial and recreational fisheries, and probability of decline as performance metrics to evaluate a range of commercial and recreational fishing mortality rates and different assessment and implementation errors. Increasing assessment error led to undesirable outcomes (e.g., low mean abundance). However, varying implementation error had almost no effect on performance metrics. Increased commercial fishing had larger effects on performance metrics than recreational fishing. Combinations of commercial and recreational fishing mortality rates corresponding to total annual mortality rates of approximately 40% may be sustainable, and so the currently accepted 42% total annual mortality rate is also likely sustainable. However, we recommend that the amount of assessment error be more accurately quantified, low spawner abundance be avoided through the use of threshold management strategies, and commercial fishing quotas be closely monitored and enforced.

Management inferences from a demographic analysis of sea lamprey (Petromyzon marinus) in the Laurentian Great Lakes

We use matrix models incorporating uncertainty in values of life history traits and density-dependent survival to assess pest management strategies implemented by the Great Lakes Fishery Commission to control nonnative sea lamprey (Petromyzon marinus) in the Laurentian Great Lakes. The primary method of sea lamprey control has been treatment of rearing tributaries with chemical lampricides, but release of sterilized males and deployment of migratory barriers and traps are important components of the management plan. Uncertainties in the effectiveness of alternative control remain, however. Our models demonstrated that the management target of reducing lampricide use by 20% while maintaining current levels of control could be achieved if alternative methods are used to suppress current lake-wide fecundity rates by 49%–65%, assuming equal lampricide efficiency on larvae and metamorphosing individuals, or by 42%–55% when lampricide mortality on larvae is assumed to be half of that on metamorphosing individuals. At current levels of lampricide use, reduction to 72%–88% of current fecundity rates is recommended to ensure long-term control of sea lamprey populations in the face of uncertainty in current estimates of population growth rates. New control options targeting additional vital rates, such as survival of the parasitic life stage, could further reduce reliance on lampricides while maintaining effective sea lamprey control.

New estimates of lethality of sea lamprey (Petromyzon marinus) attacks on lake trout (Salvelinus namaycush): implications for fisheries management

Sea lamprey (Petromyzon marinus) control in North America costs millions of dollars each year, and control measures are guided by assessment of lamprey-induced damage to fisheries. The favored prey of sea lamprey in freshwater ecosystems has been lake trout (Salvelinus namaycush). A key parameter in assessing sea lamprey damage, as well as managing lake trout fisheries, is the probability of an adult lake trout surviving a lamprey attack. The conventional value for this parameter has been 0.55, based on laboratory experiments. In contrast, based on catch curve analysis, mark–recapture techniques, and observed wounding rates, we estimated that adult lake trout in Lake Champlain have a 0.74 probability of surviving a lamprey attack. Although sea lamprey growth in Lake Champlain was lower than that observed in Lake Huron, application of an individual-based model to both lakes indicated that the probability of surviving an attack in Lake Champlain was only 1.1 times higher than that in Lake Huron. Thus, we estimated that lake trout survive a lamprey attack in Lake Huron with a probability of 0.66. Therefore, our results suggested that lethality of a sea lamprey attack on lake trout has been overestimated in previous model applications used in fisheries management.

Making Wise Decisions about Transferring Fish among Lakes within the Great Lakes Basin

The management of Great Lakes fisheries is guided by Fish Community Objectives defined for each lake by the lake committees under A Joint Strategic Plan for Management of Great Lakes Fisheries. The lake committees meet twice per year in a Council of Lake Committees (council) to discuss issues of basin-wide concern to the Great Lakes. To achieve these objectives and satisfy stakeholder expectations for healthy fish communities and fisheries, fishery managers use a variety of tools. These include direct measures such as harvest restrictions, sea lamprey control, stocking, and habitat protection and improvement, as well as indirect measures such as advocacy for better ballast water exchange programs and for effective policies to govern water withdrawals. Among the direct measures are tactics, such as stocking, that can involve the transfer of fish from one part of the Great Lakes basin to another. The purpose of this essay is to examine issues surrounding the transfer of fish among lakes within the Great Lakes basin and to suggest a general process for decision-making to manage this practice wisely. The decision to transfer fish is associated with uncertain benefits and risks. Fish transfers are considered or implemented for at least three different reasons: (1) to support restoration of depleted or extirpated native species, such as lake trout (Salvelinus namaycush); (2) to support economic opportunities for use of Great Lakes fisheries, through stocking of non-native salmonines; and (3)

Explicit consideration of uncertainty in Great Lakes fisheries management: decision analysis of sea lamprey (Petromyzon marinus) control in the St. Marys River

We used decision analysis to assist the Great Lakes Fishery Commission (GLFC) with sea lamprey (Petromyzon marinus) control in the St. Marys River, which connects Lake Superior to Lake Huron and until recently has been a major source of sea lamprey infestation of Lake Huron. The process involved decision-makers, managers, and experts in an analysis of management options, objectives, and critical uncertainties affecting the expected performance of alternative choices. We developed quantitative assessments of uncertainty about sea lamprey demographics and about the implementation effectiveness of three pest control options: trapping adult lampreys, sterile male releases, and lampricide treatment. We used a stochastic simulation model to forecast future parasitic sea lamprey abundance in Lake Huron for 10 management options. Management options that included enhanced levels of trapping and sterile male releases exhibited the best performance for all measures. Our analysis substantially influenced the GLFC strategy for the St. Marys River, providing an example of how explicit consideration of uncertainty using decision analysis can influence fishery management decisions.

Research to Guide the Use of Lampricides for Controlling Sea Lamprey

This paper is one of a series supported by the Great Lakes Fishery Commission that describe research priorities related to each facet of the sea lamprey control program (assessment, pheromones, barriers and trapping, sterile males and lampricides). The specific focus of this paper is research needs related to the use of lampricides. To that end, we first provide a brief history of the lampricide control program and its processes and operations emphasizing the progress that has been made over the last 50 years. We then articulate research priorities for the continued improvement of lampricide use under four major categories; improving the effectiveness of lampricide treatments, improving the understanding of the effects of lampricides on non-target species, gaining a better understanding the mode(s) of toxic action of lampricides, and how they differ between lamprey and non-target species, and finally, initiatives designed to find new and more effective methods of applying existing lampricides and to develop new lampricides, based on new knowledge of chemical vulnerabilities unique to larval sea lamprey. Research priorities are summarized at the end of the paper and sources of additional information concerning lampricide research are provided.

Research to Support Sterile-male-release and Genetic Alteration Techniques for Sea Lamprey Control

Integrated pest management of sea lampreys in the Laurentian Great Lakes has recently been enhanced by addition of a sterile-male-release program, and future developments in genetic approaches may lead to additional methods for reducing sea lamprey reproduction. We review the development, implementation, and evaluation of the sterile-male-release technique (SMRT) as it is being applied against sea lampreys in the Great Lakes, review the current understanding of SMRT efficacy, and identify additional research areas and topics that would increase either the efficacy of the SMRT or expand its geographic potential for application. Key areas for additional research are in the sterilization process, effects of skewed sex ratios on mating behavior, enhancing attractiveness of sterilized males, techniques for genetic alteration of sea lampreys, and sources of animals to enhance or expand the use of sterile lampreys.

UNDERSTANDING UNCERTAINTY IN THE EFFECT OF LOW‐HEAD DAMS ON FISHES OF GREAT LAKES TRIBUTARIES

Small dams represent one of the most widespread human influences on riverscapes. Greater understanding of how these structures affect aquatic organisms is needed to ensure that decisions regarding their construction and removal strike an appropriate balance between components of human and ecosystem services. Within the basin of the Laurentian Great Lakes, the effects that in-stream barriers (dams) used to control the non-native, parasitic sea lamprey (Petromyzon marinus) on the diversity of non-target fishes is a significant concern for fishery managers. A previous study indicated that upstream changes in the species richness of non-target fishes observed in 24 streams with a sea lamprey barrier relative to paired reference streams (a measure of effect size) was variable across the basin. We examined the degree to which the variance in effect size could be attributed to imprecision in the field sampling protocol used to estimate effect sizes, differences in catchment-scale landscape attributes between barrier and reference streams within pairs, and differences in landscape attributes at different spatial scales among barrier streams. Simulation modeling and analyses of repeated field measurements made for a subset of streams demonstrated that a large variance in effect size is expected for the field sampling design and that estimates of effect size measured for individual barrier streams are imprecise. Regression models and multimodel inference methods based on Akaike's Information Criterion provided less support for hypotheses linking effect size to landscape attributes. Mean effect size, adjusted for the influences of landscape characteristics within and across stream pairs, provides the most reliable and least biased estimate of the effect of sea lamprey barriers on the richness of nontarget fish species. With the information currently available, landscape characteristics of catchments cannot be used to help decision makers anticipate effects sizes for candidate streams being considered for future barrier construction. Our findings will help fishery managers in the Laurentian Great Lakes make more informed decisions regarding the use and placement of sea lamprey barriers and achieve their objective of delivering an integrated pest management plan for sea lamprey control that is environmentally and economically sound and socially acceptable.

Research to Guide Use of Barriers, Traps, and Fishways to Control Sea Lamprey

This paper provides a rigorous and directed research framework for fostering innovations in the design, implementation, and operation of barriers, traps, and fishways used to control the sea lamprey ( Petromyzon marinus) in the Laurentian Great Lakes. It was developed to support the Great Lakes Fishery Commission's milestone pledging to decrease reliance on chemical lampricides and achieve 50% of sea lamprey suppression through alternative control technologies, including barriers and traps. The paper first substantiates the need to develop a long-term research plan for barriers, traps, and fishways by summarizing (i) current management challenges, (ii) the barrier, trap, and fishway options being used to meet these challenges, and (iii) the key uncertainties in our knowledge regarding these options. The paper then proposes a long-term research strategy that envisions a transition from barriers designed to block the upstream spawning migrations of sea lamprey, to barrier and trap combinations that facilitate physical removal of sea lamprey and, in some cases, passage of non-target fishes, to barrier and trap designs that are specific to sea lamprey, transparent to non-target fishes, and safer for operators. Thirteen research needs are identified to support this strategy along with a general work plan on how they can be achieved. The research needs and work plan highlight the exceptional opportunity to develop the Great Lakes basin as a leading, international research center for fish migration and passage, and the development of environmentally friendly barriers.

Evidence That Lake Trout Served as a Buffer against Sea Lamprey Predation on Burbot in Lake Erie

AbstractThe population of burbot Lota lota in Lake Erie recovered during 1986–2003, mainly because of the control of sea lamprey Petromyzon marinus, which began in 1986. Burbot populations continued to grow during 1996–1998, when sea lamprey control was substantially reduced. We calculated mortality parameters for burbot in Lake Erie by estimating age at capture for 2,793 burbot caught in annual gill‐net surveys of eastern Lake Erie from 1994 to 2003. Based on catch‐curve analysis, annual mortality in Lake Erie during 1994–2003 was estimated as 33%. Annual mortality of the 1992 year‐class of burbot was estimated as 30%. The mortality of burbot during the years of reduced sea lamprey control was not different from that during the 3 years preceding reduced control and was significantly lower than that during the entire portion of the time series in which full sea lamprey control was conducted. These results suggest that the reduction in sea lamprey control did not lead to increased burbot mortality. The catch per gill‐net lift of large burbot (total length > 600 mm), the size preferred by sea lampreys, was lower than that of adult lake trout Salvelinus namaycush (age 5 and older; total length > 700 mm) before lampricide application was reduced. Although adult lake trout populations declined, the abundance of large burbot did not change during the period of reduced lampricide application. These results support a hypothesis that a healthy population of adult lake trout can serve as a buffer species, acting to reduce predation of burbot by sea lampreys when sea lamprey populations increase. Burbot attained sexual maturity at a relatively early age (3 or 4 years) and a total length (approximately 500 mm) that was smaller than the preferred prey size for sea lampreys. These characteristics and the buffering effect of the lake trout population enabled growth of the burbot population during the brief period when lamprey control was reduced.

Predicting Sea Lamprey ( Petromyzon marinus) Ammocoete Habitat Using Geographic Information Systems

The sea lamprey ( Petromyzon marinus) is an invasive, parasitic species with a long history of decimating fisheries in the Laurentian Great Lakes. The sea lamprey life cycle consists of stream larval ammocoete, open water parasitic, and adult spawning phases. Population control of sea lamprey is achieved mainly through the application of chemical lampricides that target the sedentary larval stage. The physical characteristics of preferred ammocoete habitat are well defined at the sub-reach scale (< 50 m). We tested whether the spatial distribution of beds of preferred ammocoete habitat depends upon a specific set of geomorphic variables (field-measured slope, and geographic information system (GIS)- derived curvature, radius of curvature, presence of a confluence, and valley wall type). We tested for this relationship at several spatial scales of stream length ranging from 50 m to 300 m, on the East Branch of the Chagrin River, Ohio, USA, a tributary stream to Lake Erie. Of the five geomorphic variables tested, field-measured slope and radius of curvature influence the probability of a stream segment containing preferred habitat at a stream segment length of 50 m. We found no relationships at longer stream segment lengths. GIS-estimated slopes were not sufficiently accurate at such short segment lengths, so the final model included radius of curvature only. These results are applicable to the Empiric Stream Treatment Ranking (ESTR) system, which ranks tributaries for treatment with lampricide based partially on the total amount of preferred ammocoete habitat. GIS-based estimates of the total amount of preferred ammocoete habitat may complement current field-based estimates, or provide a basis for nested sampling designs.

Evidence That Sea Lamprey Control Led to Recovery of the Burbot Population in Lake Erie

AbstractBetween 1987 and 2003, the abundance of burbot Lota lota in eastern Lake Erie increased significantly, especially in Ontario waters. We considered four hypotheses to explain this increase: (1) reduced competition with lake trout Salvelinus namaycush, the other major coldwater piscivore in Lake Erie; (2) increased abundance of the two main prey species, rainbow smelt Osmerus mordax and round goby Neogobius melanostomus; (3) reduced interference with burbot reproduction by alewives Alosa pseudoharengus; and (4) reduced predation by sea lampreys Petromyzon marinus on burbot. Species abundance data did not support the first three hypotheses. Our results suggested that the apparent recovery of the burbot population of Lake Erie was driven by effective sea lamprey control. Sea lamprey predation appeared to be the common factor affecting burbot abundance in Lakes Michigan, Huron, Erie, and Ontario. In addition, relatively high alewife density probably depressed burbot abundance in Lakes Ontario and Michigan. We propose that a healthy adult lake trout population may augment burbot recovery in some lakes by serving as a buffer against sea lamprey predation and will not negatively impact burbot through competition.

Effects of low-head barriers on stream fishes: taxonomic affiliations and morphological correlates of sensitive species

Low-head barriers used in the control of parasitic sea lamprey (Petromyzon marinus) in the basin of the Laurentian Great Lakes can alter the richness and composition of nontarget fishes in tributary streams. Identification of taxa sensitive to these barriers is an important step toward mitigating these effects. Upstream–downstream distributions of fishes in 24 pairs of barrier and reference streams from throughout the basin were estimated using electrofishing surveys. For 48 common species from 34 genera and 12 taxonomic families, 8–19 species, 5–16 genera, and 2–7 families showed evidence of being sensitive to barriers, with the variation in number depending on the statistical measure applied. Barriers did not differentially affect species from certain genera or families, nor did they affect species of certain body form. Therefore, taxonomic affiliation and swimming morphology are not useful for predicting sensitivity to barriers for fishes that co-occurred with sea lampreys but were not sampled adequately by our survey. Our estimates of sensitivity will help fisheries managers make sound, defensible decisions regarding the construction, modification (for fish passage), and removal of small, in-stream barriers.

A Practical Method for Obtaining Useful Quantities of Pheromones from Sea Lamprey and Other Fishes for Identification and Control

Pheromonally-mediated trapping is currently being developed for use in sea lamprey control in the Laurentian Great Lakes. To identify and test lamprey pheromones a practical procedure was needed to isolate relatively large quantities of pheromone from lamprey holding water. The present study developed such a technique. It employs Amberlite XAD7HP, an adsorbent resin which we found can extract over 80% of the sea lamprey migratory pheromone from larval holding water at low cost and with relative ease. This technique allowed us to collect tens of milligrams of all three components of the sea lamprey migratory pheromone, eventually permitting both identification and successful field testing. This technique might also be used to collect pheromones released by other species of fish.

Movement of Sea Lamprey in the Lake Champlain Basin

Sea lamprey ( Petromyzon marinus) are a nuisance aquatic species in the Great Lakes and Lake Champlain that have devastated native fish populations and hampered the restoration of sport fisheries. This study examined inter-basin movement of sea lamprey in Lake Champlain to identify tributaries that contribute parasitic-phase sea lamprey and provide information for prioritizing those tributaries for sea lamprey control. A total of 4,125 recently metamorphosed sea lamprey was captured in tributaries to Lake Champlain and marked using coded wire tags between the fall of 2001 and winter 2003. These sea lamprey migrated to the lake to prey on salmonids and other fishes and returned to tributaries to spawn about 12–18 months after migration. We recaptured 6 tagged sea lamprey from the lake from spring 2002 through winter 2004, and 35 from tributaries in spring 2003 and 2004. We noted no apparent trends in movement among basins. Sea lamprey were collected at distances up to 64 km from their natal tributaries. Tributary contributions of parasites were significantly different from expectations in the 2002 parasitic-phase cohort (χ 2 = 9.668, p < 0.011, 3 df), suggesting differential survival rates among out-migrating transformers from different tributaries. Estimates of the lake-wide out-migrating transformer population for the 2002 and 2003 parasitic-phase cohorts were 269,139 ± 55,610 (SD) and 111,807 ± 23,511 (SD). Results from this study suggest that sea lamprey movement is not inhibited by causeways dividing sub-basins, but movement among sub-basins is somewhat constrained. This indicates that management efforts to control sea lamprey should continue to treat the lake as a single system.

Ecology of the Lake Huron fish community, 1970-1999

We review the status of the Lake Huron fish community between 1970 and 1999 and explore the effects of key stressors. Offshore waters changed little in terms of nutrient enrichment, while phosphorus levels declined in inner Saginaw Bay. Introduced mussels (Dreissena spp.) proliferated and may have caused a decline in Diporeia spp. This introduction could have caused a decline in lake whitefish (Coregonus clupeaformis) growth and condition, with serious repercussions for commercial fisheries. Bythotrephes, an exotic predatory cladoceran, and other new exotics may be influencing the fish community. Sea lampreys (Petromyzon marinus) remained prevalent, but intensive control efforts on the St. Mary's River may reduce their predation on salmonines. Overfishing was less of a problem than in the past, although fishing continued to reduce the amount of lake trout (Salvelinus namaycush) spawning biomass resulting from hatchery-reared fish planted to rehabilitate this species. Massive stocking programs have increased the abundance of top predators, but lake trout were rehabilitated in only one area. Successful lake trout rehabilitation may require lower densities of introduced pelagic prey fish than were seen in the 1990s, along with continued stocking of hatchery-reared lake trout and control of sea lamprey. Such reductions in prey fish could limit Pacific salmon (Oncorhynchus spp.) fisheries.