Ascertaining whether an enhanced bubble curtain could deter Asian carp movement into small tributaries in a practical manner; immediate installation of sound deterrents in the Mississippi River
In 2009, the University of Minnesota developed an enhanced new bubble curtain design that reduces up- and down-stream movement of the invasive common carp by 70-80% (results under peer review). The primary advantage of this new technology is that it is very practical and inexpensive: a simple industrial blower connected to PVC pipes with holes drilled in a specific manner (that costs less than $2,000) can stop about 75% of all common carp. This technology also has the potential to be taxon-specific because it is based on sound and hydrodynamic fields generated by the bubbles and additionally will work safely and efficiently in shallow waters.
Because the silver and bighead carp are just as (and possibly more) sensitive to sound as the common carp, this technology could have great potential for stopping these new, highly invasive species in the hundreds of small tributaries to Minnesota's large rivers that are too expensive and difficult to protect with other methods, such as electrical or mechanical barriers.
In continuation of MAISRC efforts to use sound deterrents to control movement of bighead and silver carp in Minnesota's rivers, and response to the recent report of late-stage bighead carp embryos being found in Mississippi River Pool 9, MAISRC proposes to immediately purchase and install underwater transducers at Lock & Dam #8.
This activity installed the first sonic deterrent system in a lock system and clearly demonstrated that enhanced bubble curtains and sound alone can function as behavioral deterrents with potential to selectively control the movement of fish with high sensitivity to sound including the invasive carps. Due to their low cost, ease of installation, safety, and taxon-specific effects, we believe bubble curtains hold great promise for protecting the many low head tributaries connecting with the Mississippi River. In this particular study we investigated the effectiveness of a bubble curtain as a deflection screen which directed carp away from one channel into another and found this approach (vs. blocking) to be especially promising. Using a split passage experimental channel we determined that common carp, silver carp, and bighead carp passage could all be diverted away from a specific channel in the laboratory with a success rate of 82-90%. This rate was approximately 10-15% higher than we noted earlier with a design that simply blocked. It also used 1/10th of the air flow rate.
In addition to demonstrating the diversion functions more efficiently than blocking, we also demonstrated in a different experimental design for the first time in either a freshwater or invasive fish that carps detect and respond to sound in directional manners and thus sound could be used in directional and predictable manner to divert. It is very possible that sound alone produced by speakers could be highly effective in the natural world, especially if sound is engineered correctly. This is important because air curtain use is limited to shallower waters because of possible deflection by water currents, need to produce highly pressurized air, and limited sound pressures generated. Part of a separate grant continues this line of research by evaluating the response, or lack thereof, of native, non-hearing specialist species, lake sturgeon (Acipenser fulvescens) and brown trout (Salmo trutta), to an acoustic deterrent to quantify the species specific differences.
Blocking bighead, silver, and other invasive carp by optimizing lock and dams
Unknown numbers of invasive Silver and Bighead carp presently inhabit the Mississippi River below the Iowa border. This project worked to prevent their upstream spread by developing a scheme to modify lock and dam structures in Minnesota by enhancing their deterrent properties through key, linked steps, which included:
Activity 1: Installed a safe carp deterrent in front of the lock at Lock and Dam #8 located at the Iowa border while guiding efforts to enhance and optimize velocity fields to stop carp movement through its gates while having minimal effects on native fishes. The goal of this activity was to immediately and safely maximize water velocity through the gates of lock and dam #8 near the Iowa border while deploying a simple and safe acoustical deterrent system in its lock chamber as a stop-gap measure.
Activity 2: Quantified the swimming capabilities of both species of adult Bighead carps, thereby producing the data needed to optimize dam function. Swimming performance data for adult carps is essential to accurately forecast passage and optimize gate function so that velocities are not higher than needed.
Activity 3: Tested and developed new acoustical deterrent systems that best deter carp from entering lock chambers while having minimal effects on native fishes. Lock chambers present a potential way for Bigheaded carps to pass upstream, irrespective of gate function. Sound deterrents have special promise because carps are hearing specialists.
Activity 4: Developed numeric solutions to eventually optimize dam operation at all Minnesota lock and dams (#2 through #8) to prevent Bighead carp invasion statewide. The purpose of this activity was to identify potential weaknesses (scenarios by which carp might swim through the lock and dams) in Lock and Dam #2 in Hastings. This Lock and Dam was of special interest because it maintains higher velocities than other dams, is ideally situated far from the invasion front, and is located downstream of the Minnesota River.
Activity 5: In order to test and validate the models previously developed, researchers radio-tagged invasive common carp (as a surrogate to Asian carp) as well as 250 native fish specimens. They monitored the tendency and ability of these fish to challenge the increased flow from the dams as well as how they move through or around the dam. This work occurred at Lock and Dam #2.
Activity 6: Researchers used high-resolution imaging sonar to capture the location of all fish in the lock area when the acoustical deterrent system was turned on and off, which showed whether and how their behavior is affected by the sound. This work occurred at Lock and Dam #8.
Activity 7: In order to test upstream-migrating silver and bighead carp (instead of common carp), researchers designed and helped install an underwater speaker system on the lock gates at Lock and Dam #19 in Iowa.
Activity 8: Developed solutions to address weaknesses in Lock and Dam #4 and optimize its gate operation to prevent passage of invasive carp. This lock and dam system maintains a high velocity than other dams, is situated far from the invasion front, and is located just upstream of Lock & Dam #5, so the two systems can be used in conjunction. The project will include developing a 3D statistical model to calculate water velocities in and around the dam under a variety of conditions; measuring velocities near the dam to validate the model; developing and implementing a computation tool to search through the 3D velocity fields to identify specific swimming pathways that carp could take; and pairing this information with already-known swimming performance data to determine how best to block carp passage while having minimal effect on native fishes.
The overall objective of this work was to make explicit recommendations with (and to) the U.S. Army Corps of Engineers (USACE) for optimization of all Minnesota lock and dams (#2 through #8) to block the invasion of Bigheaded carps while still serving USACE needs and having minimal effects in native fishes.
Researchers successfully collaborated with the United States Army Corps of Engineers (USACE) and developed new ways and technologies to impede the upstream movement of invasive (bigheaded) carp through their locks and dams in the Mississippi River. These approaches have now been implemented at Lock and Dam #8, which is the southernmost Lock and Dam in Minnesota and has thus been a focus. At this structure, dam spillway gate operating protocols were adjusted by the USACE to optimize their ability to stop carp and speakers added to the lock gates to deter carp with few effects on native fish. This is the first structure in the world to be so modified and our calculations suggest it now stops twice as many carp as it once did (well over 90%). Tentative plans for similar modifications to Lock and Dams #2 and #5 (the other most promising structures in Minnesota) have also been presented to the USACE for future deployment at their discretion. This progress was possible because we met all four objectives of this project: 1) added speakers to Lock and Dam #1; 2) quantified and published how well bigheaded carp swim (and thus what flows might stop them); 3) developed and tested several new acoustic systems in the laboratory and field that stop carp but do not affect native fish; and 4) developed new solutions for the gates at Lock and Dam #2-8 and provided specific data (specific solutions) for Locks and Dams #5 and #2, the most promising structures of these.
Evaluating zebra mussel spread pathways and mechanisms in order to prevent further spread
Activity 1: Identification of the sources of inland invasions throughout Minnesota, and the pathways through which zebra mussels have spread throughout the state
Researchers completed genotyping and analyzing the first data set, which included 43 geographic sites, 16 lakes and 3 river systems, and 1,281 mussels at 9 microsatellite marker loci (samples gathered in the fall of 2016). Using genetic methods to study invasion sources and pathways at the scale of a U.S. state is novel. Researhcers then expanded the genotyping further to investigate regional spread and the lack of contribution from perceived “super-spreader” lakes. In total, we have now genotyped 2,050 mussels from 40 lakes and 4 rivers at 9 microsatellite markers with a total of 69 geographic sites included. Findings include:
- Genetic diversity in lakes is high. That means that lakes are colonized by large numbers of mussels or larvae.
- Several lakes are genetically distinct which allows for the study of invasion sources.
- Genetic clusters identify regional patterns of spread. Minnesota contains three large regions of many lakes – Detroit, Alexandria, and Brainerd Lakes regions – in which infested lakes are clustered. Genetically, lakes within each reach fall into one or more localized genetic clusters found nowhere else in the state. Since mussels in lakes in each region are closely related and genetically unique, we conclude that vectors active within the region have spread genetically similar “colonizing” mussels, from the first lake(s) infested, then from lake to lake. This calls for more efforts to identify and block regional spread vectors.
- “Super-spreaders” Mille Lacs and Prior Lake have, surprisingly, not infested other lakes in Minnesota. Their super-spreader status is inferred due to high boater traffic. Mille Lacs watercraft inspection data shows that departing boaters next visit a large number of both recently infested and uninfested lakes. We evaluated 35 lakes that were infested after 2005, testing an invasion model in which Mille Lacs served as the source for the new lake, and have not yet detected a single case in which Mille Lacs was selected by the model to be the source.
This extreme discord between boater movements and genetics must mean that watercraft inspection and decontamination has been effective on Mille Lacs and should be continued — on this and other high traffic lakes.
Activity 2: Clarifying downstream drift as a mechanism of spread of zebra mussels between lakes
In order to address the risk of spread through downstream drift (the phenomenon of veligers naturally moving in lakes and streams, settling, and founding new populations), samples were been gathered, counted, and photographed using our CPLM-image analysis system. Findings include:
- Settlement occurs immediately downstream of the headwater lake and declines to zero, typically within the first kilometer downstream.
- Small streams (< 10 m wide) are not good habitat for zebra mussels, so the management issue is not colonization of the stream bottom for most of its length, rather the concern is dispersal of larvae.
- The number of larvae leaving infested lakes can range from 10 million to more than a billion per day.
- Downstream larval dispersal sharply declined with distance, and this decline becomes steeper as summer progresses.
- Early in the season, it may be possible for veligers to disperse downstream as far as 40 miles in the Pelican system. This could explain the infestation of Dayton Hollow and Orwell Reservoirs below Fergus Falls.
Population genomics of zebra mussel spread pathways
Phase II of this effort focused on preventing zebra mussel invasions by developing genetic evidence of spread sources and pathways so that they may be interrupted. It also lays the groundwork for potential biocontrol through genetic modification technologies. Learn more about Phase II of this project here.
Estimating overland transport frequencies of invasive zebra mussels
Zebra mussel invasions of inland lakes in Minnesota are on the rise. In order to develop prevention and control methods, it’s crucial to understand the pathways and mechanisms that are enabling this spread. One suspected source of human-assisted zebra mussel transport is through residual water in recreational boats. The lack of data around this concern (to support it or to rule it out) has led to challenges related to statewide inspection practices and even recreational boat design.
Therefore, the goals of this study were to:
Estimate the relative contributions of different surfaces and compartments on and in recreational boats and trailers to the transport of zebra mussels and their larvae (veligers), focused on measurements of the concentrations of veligers in residual water across a full range of vessel types in Minnesota.
Identify “high-risk” vessel types and “high-risk” areas of watercraft that are likely to transport large volumes of residual water, and evaluate where boat redesign can be targeted to most effectively reduce residual water volumes.
Develop a refined model to assess the risk for residual waters to transport – and thereby spread – live veligers within the state.
This study partnered with the DNR’s watercraft inspection program to collect data on presence and location of adult zebra mussels on seven different watercraft types leaving two popular zebra mussel infested lakes in Minnesota. In addition, a subset of the boats inspected had residual water – that which is left in a boat after a user has attempted to fully drain it – sampled from various compartments of the watercraft, including live wells and bait wells, bilge areas, ballast tanks (if present), motors and any other location that may potentially transport a zebra mussel. The water was analyzed for the presence of veligers. Additionally, lab tests and field experiments determined the ability of veligers to survive in some of these high-risk compartments.
In phase one of this project, DNR watercraft inspectors collected residual water from boats leaving two different Minnesota lakes. Additional engine samples were collected by mechanics at Tonka Bay Marina. Roughly half of these samples contained no veligers and 75% contained five or fewer veligers. Sterndrive engines and ballast tanks ranked first and second for volumes of residual water. Ballast tank samples contained the largest median number of veligers per sample, and sterndrive engines contained the highest maximum number of veligers. Compartments that contained more residual water on average had larger numbers of veligers compared to the compartments with smaller volumes of water and had a greater likelihood of finding one or more veligers. Based on the results of this study, recreational equipment that contains one or more ballast tanks poses the greatest likelihood of moving high numbers of veligers given our findings that ballast tanks were the compartment with the greatest likelihood of containing veligers and had the greatest mean number of veligers per sample. Wakeboard boats are the type most likely to contain ballast tanks, and some boat models can have multiple ballast tanks. Given the amount of veligers ballast tanks may contain, extra steps (such as hot water decontamination) should be taken by boat owners to minimize the likelihood of introducing live veligers to new water bodies via overland transport.
In the second phase, researchers performed experimental mortality trials on larvae in two common boat compartments: live wells and ballast tanks. Both compartments were exposed to various temperatures (20, 27, 32, and 38°C for live wells and 20 and 32°C for ballast tanks). For veligers in the residual water of live wells, > 95% mortality was observed after five hours of exposure to all temperatures. The same level of mortality was reached in ballast tanks at 48 hours. Based on our results, watercraft equipped with ballast tanks are at a greater risk of transporting live veligers to new water bodies over short trips (e.g. 6 hours). This tells us that additional prevention steps (i.e. using hot water) should be taken to reduce the risk of transporting living veligers in residual water.
The results of this study can help resource managers evaluate the potential risks that watercraft in compliance with state laws may pose. Providing on-site or on-call decontamination services to watercraft users can help reduce the risk of transporting aquatic invasive species. These systems are often expensive, however, and not all agencies or organizations have the funding necessary to offer these services. Based on the results found in this study, the nationally recognized suggestion to dry your watercraft for five days or more (Stop Aquatic Hitchhikers!) drastically reduces the risk of transporting living zebra mussel veligers to new water bodies. For boaters planning on moving between water bodies within shorter periods of time, extra precautions such as hot water decontamination could be done to minimize the risk of moving living veligers.
Eco-epidemiological model to assess aquatic invasive species management
MAISRC researchers are working to develop a first-of-its-kind eco-epidemiological model that will forecast the potential risk of spread of zebra mussels and starry stonewort across Minnesota. The model will take into account introduction probability, establishment probability, and levels of management interventions. This model will be used as a decision-making tool to generate effective intervention strategies and design cost-effective surveillance programs to mitigate and prevent the spread of AIS.
To establish introduction probability, pathways among lakes will be evaluated based on water connectivity, boater movement, and geographic proximity. To understand the establishment probability, researchers will use next-generation ecological niche modeling techniques with remote sensing data. Cumulatively, this will identify lakes or areas of the state that are at higher risk for AIS, including lakes that are highly vulnerable and lakes that may be “super-spreaders,” both of which will help prioritize management efforts.
Data from this project is now available for download and use:
This project resulted in a predictive risk model for zebra mussels and starry stonewort that estimates the probability of a lake becoming infested by 2025.
The model evaluated all 25,000 bodies of water in Minnesota that are recognized by the Minnesota DNR and took into account the three abovementioned factors for each. The simulation was run 10,000 times to produce a percentage probability of whether a lake will become infested with either invasive species. For example, a score of 0.3245 means that when the model was run 10,000 times, the lake became infested 3,245 times by 2025 – a 32.45% chance.
- Click here to download the starry stonewort risk scores.
- Click here to download the zebra mussel risk scores.
Note that each county is on a separate tab of the Excel spreadsheet.
Final report summary:
Ecological Niche Models: We created ecological niche models for starry stonewort under current and future climate scenarios, zebra mussels, and Heterosporis sutherlandae. These models provide projections of theoretically suitable lakes in Minnesota, based on known environmental conditions of the species in the native and invaded ranges. While the potential range varies for each species, it is clear from this project that there are many suitable, but not yet invaded, lakes in Minnesota. Efforts to prevent spread should remain a high priority for managers. It is also important to note that not all lakes are considered suitable and efforts to more strategically target intervention is warranted.
Network Models: We created network matrixes for watercraft movement, water connectivity and geographic proximity. These matrixes provide a robust dataset from which we can estimate connectivity through known high-risk human-mediated (watercraft) and natural (water and proximity) pathways. The watercraft and water connection data both provide directionality and weighted edge (e.g. number of boats and river distance) and were further developed for use in other parts of the project. Significant effort was made to adjust the sampling bias inherent in the watercraft inspection data to account for unequal sampling effort and sampling locations. This is achieved through a series of statistical models, including random forest (biased effort), logistic regression (biased location), gamma regression (number of boats) and linear regression (number of boats staying on the same lake, e.g. self-loop). Understanding and evaluating connections between lakes will help managers to prioritize prevention and early detection efforts. These data are now being used to inform the MAISRC project Decision-making tool for optimal management of AIS.
We successfully built predictive risk models for zebra mussels and starry stonewort to estimate the probability of a lake getting infested by 2025. The risk models quantified the risk by incorporating both watercraft and water connectivity and lake suitability (see above for data). The simulation allowed an infested lake to spread the AIS to uninfested lakes through these pathways following a stochastic process. Even with introduction, an uninfested lake could only become infested if the lake was suitable for the specific AIS. Once a lake switched from uninfested to infested, the lake became a new source from which the AIS could continue to spread in the model. Using known location data (confirmed zebra mussel infestations as of 2011 or starry stonewort infestations as of 2015) and a Bayesian modeling approach, the model was calibrated for the time period 2012-2017 (zebra mussels) or 2016-2017 (starry stonewort) with 26-time steps per year. The outputs were the number of infested lakes for each species each year. The model calibration performed very well and was used to project future risk scores for uninfested lakes from 2018-2025 using 10,000 simulations.
In addition to overall trends, the results provided a lake-level breakdown of likelihood of introduction via boats or water, and combined for both. For example, a score of 0.3245 means that 3,245/10,000 times the lake became infested by 2025. While the model is not perfect (no models are), the results are robust and provide useful information from which to make decisions. In isolation, this may not be entirely useful, but when considered across a watershed, county or state, the ability to rank waterbodies based on actual, not perceived, risk is a game changer. These data will also be useful overtime to assess the trends of AIS invasion (i.e. what happens if we do nothing?). In both cases, the scenarios project a future with more infested lakes, albeit not equally distributed. For example, NE Minnesota remains at relatively low risk, while Central Minnesota increases significantly.
Determining highest-risk vectors of spiny waterflea spread
Spiny water fleas are an invasive zooplankton that pose a serious threat to the ecology and recreational value of Minnesota’s waters. Previous studies have shown that over 40% of northern Minnesota lakes provide suitable habitat for spiny water fleas, and human recreational activity is believed to be the primary vector of spread. However, little is known about the specific pathways by which dispersal occurs. This can lead to unclear messaging and directions for recreationalists to prevent further spread.
To learn more about spread and prioritize prevention efforts, researchers measured the relative risk of spiny water flea attachment on commonly used recreational equipment including stationary anchor ropes, trolled fishing lines, trolled bait buckets, trolled downrigger cables, and trolled simulated livewells. Researchers sampled in the middle of the day and in the evening to account for spiny water fleas’ tendency to migrate closer to the water’s surface at dusk.
Researchers ranked the threat of each type of gear tested to help recreationalists prioritize their cleaning efforts in order to prevent further spread of spiny water fleas.
All research has been completed for this project. Researchers conducted 36 sampling events on Island Lake Reservoir (near Duluth, MN) and 36 sampling events on Lake Mille Lacs (near Garrison, MN).. Researchers compared the number of spiny water fleas on the deployed recreational equipment to the natural abundance of the fleas in the surrounding lake water.
In total, researchers processed 216 anchor ropes; 72 bait bucket samples; 72 livewell samples; 72 downrigger steel cable samples; 72 downrigger fishing line samples (monofilament); and 216 shallow-running fishing line samples (72 monofilament, 72 fluorocarbon, and 72 braided).
Researchers found that the downrigger and shallow-running fishing lines accounted for 87-88% of all ensnared spiny water fleas on the gear tested. They did not find a difference between twilight and daytime ensnarement of the fleas on gear. As expected, higher numbers of spiny water fleas in the lake water resulted in greater ensnarement of spiny water fleas on gear, particularly angling line.
While few spiny water fleas were found in bait buckets or simulated livewell samples, these items are still risky because they can retain residual water in which the spiny water fleas could remain alive longer than on other gear. Residual water in any gear or boats carries this risk.
Researchers found almost no spiny water fleas on anchor ropes. However, these ropes were left stationary in the water for several hours. They were not exposed to currents or flowing water. Researchers do not know if flowing water containing spiny water fleas would result in greater ensnarement of these fleas on anchor ropes. This is something that needs future research as researchers have received anecdotal reports of such ensnarement in invaded rivers and flowages.
More detail: Researchers tested different types of angling line (fluorocarbon, monofilament, and braided), but there were not strong or consistent differences in the type of line.
To reduce the risk of spreading spiny water fleas, researchers recommend that when fishing in infested lakes, anglers wipe down their line as they reel it in from their final cast of the day or as they reel in their downrigger line and cable. A small sturdy cloth or towel works well for this.
At the dock, recreationalists should be very careful to drain all water from their boat and all gear and wipe out water holding areas such as livewells and bait buckets. From other studies, researchers know that spiny water fleas and their eggs cannot survive being completely dry for longer than six hours. Thus, researchers also recommend that all gear and the boat are dried in the sun until everything has been completely dry for more than 6 hours before moving to another water body.
What does sampling lakes for spiny water flea look like? Find out in this video!
Decision-making tool for optimal management of AIS
This project will develop a decision-making tool to help AIS managers, counties, and other agencies prioritize their resources for optimal prevention and intervention of AIS, specifically zebra mussels and starry stonewort. The tool will answer two major questions:
- Can it get here? To assess this risk, researchers will take into account the geographic proximity to an infested lake, boater movement in Minnesota, and water connectivity.
- Can it survive here? This will be answered using species-specific ecological niche models. These suitability models take into account lake and landscape variables such as temperature, precipitation, pH, conductivity, and chlorophyll.
A static version of this model has already been created by a previous MAISRC project. This new model will take that, integrate new data, and build it so it can incorporate up-to-the-minute changes. Once input from counties and other stakeholders is taken into account and the model is finalized, it will be put online in a user-friendly format for AIS managers and agencies to use.
Once the decision optimization model is created, reports will be created and distributed to counties to help them prioritize their resource allocations in order to have the biggest impact on reducing the risk of spread of AIS.
Preventing the spread of AIS through human-associated pathways is a priority for many state and local agencies. A science-based tool to inform planning and decision-making is urgently needed.
The first step of developing AIS risk estimates for each lake in Minnesota is completed, with the creation of a hydromorphological network model. The model suggests that while water connectivity is important, other factors are also clearly influencing the spread of AIS. Researchers have now begun to evaluate optimal management scenarios based on the data available for lake connectivity and suitability.
We used a big data approach to combine hydrologic connectivity and boat movement to create a multiplex metacommunity model for both zebra mussel and Eurasian watermilfoil. We found that the hydrological corridors are important pathways of spread, even more so that previous research has suggested. While overland dispersal of AIS via boater movement is still a significant factor, additional management strategies should be developed to include intervention of hydrological pathways.
Using connectivity networks of boater movement, we developed county-based AIS management optimization models that prioritize inspection locations that will intercept the highest number of ‘risky boats’ (e.g. moving from infested to uninfested lakes). We piloted the models in Crow Wing, Ramsey, and Stearns Counties and had a very productive collaboration with county managers and citizen advisory boards during the development and evaluation for each. Ultimately, the application of this approach was well received and helped inform allocation of their inspection hours at the county level (for example, Crow Wing County).
Dissemination and usability of the models was a priority of this project. We created online tools to 1) visualize the spread risk for zebra mussels and Eurasian watermilfoil based on model predictions made in Activity 1, and 2) visualize and modify the decision optimization model at the county level based on management thresholds or funding availability.
Recognizing high-risk areas for zebra mussels and Eurasian watermilfoil invasions in Minnesota
The early detection of invasive species such as zebra mussels and Eurasian watermilfoil is crucial to the success of control efforts. However, detecting these species early can be very challenging due to several factors, such as the absence of a surveillance program, relying on public reporting, and limited resource availability, which can result in reporting bias and underreporting.
The goal of this project was to improve the decision-making process and prevent the spread of AIS by implementing risk-based prevention and mitigation management strategies. This project combined clustering detection, network analysis, and probability co-kriging to recognize dispersal patterns and estimate the risk of zebra mussel and Eurasian watermilfoil invasions while attempting to account for the reporting bias and for underreporting.
To evaluate the areas of highest risk for zebra mussel infestations, researchers looked at distance to the nearest zebra mussel infested water body, boater traffic, and road access. The Eurasian watermilfoil model was similar, looking at connectivity to infested water bodies instead of road access. Results confirmed that zebra mussel and Eurasian watermilfoil invasions are potentially confounded by human densities, which is explained by varying human impact on either or both dispersal and reporting of invasions. Considering this impact of human density, this research suggests that a combination of passive and targeted surveillance, where the magnitude of efforts are stratified by human densities, may provide insight into the true invasion status and its progression in the Great Lakes region.