Methods

Study area
The experimental (McDermott Lake; 46.00299280, -90.16081610) and reference (Sandy Beach Lake (46.10614350, -89.97131020) systems are located in Iron County in Northern, Wisconsin. McDermott Lake has a surface area of 33.1 ha and a mean depth of 3.0 m, while Sandy Beach Lake has a surface area of 44.5 ha and a mean depth of 2.1 m. Maximum depth in McDermott Lake is 5.7 m and in Sandy Beach Lake is 4.0 m. Both lakes include a variety of substrates (e.g., rock, gravel, and sand) and areas of submerged and emergent vegetation. At the start of the study, McDermott and Sandy Beach Lake fish communities were similar with high Centrarchidae spp. (i.e., centrarchid) abundances, few adult walleye, and a history of natural walleye recruitment. Other species present include yellow perch, northern pike, muskellunge, black bullhead, and white sucker (Table 1).
Fish sampling
Standardized surveys
From 2017-2020, we conducted fish population sampling on the experimental (McDermott) and reference (Sandy Beach) lakes. Every year of the study, we have conducted standardized monitoring surveys employing numerous sampling techniques to detect changes in the fish community relative to 2017-baseline information (Fig. 1). Sampling began immediately following ice-out (~mid-April) with the deployment of five fyke nets for one week. The fyky surveys served two purposes: 1) to serve as the capture method for marking walleye as part of the mark-recapture survey to attain a population estimate, and 2) to estimate other focal species (i.e., black crappie, yellow perch, muskellunge, northern pike) relative abundances. During these surveys, all collected walleye were measured (total length (TL); mm), sexed, checked for a Passive Integrated Responder (PIT) tag, and if one was not present, marked with a unique PIT tag. We also removed a dorsal spine sample for aging. Adult (mature) walleyes were defined as all fish 381 mm and all fish for which sex could be determined (regardless of length). Walleye of unknown sex <381 mm were classified as juvenile (immature). McDermott and Sandy Beach Lakes have both had walleye population estimates previously conducted by the Wisconsin Department of Natural Resources (WDNR) therefore the goal was to mark 10% of the anticipated spawning population (based off of previous population estimates). Marking continued until the target number was reached or spent females began appearing in the fyke nets. Walleye were recaptured using an AC boat electrofishing survey within one week (typically 1-4 days) after netting and marking were completed. In each lake, the entire shoreline was sampled. All captured walleyes were measured and examined for marks. Based on electrofishing mark-recapture data, population estimates were calculated using the Chapman (1951) modification of the Petersen Estimator as:
N=((M+1)(C+1))/((R+1))
where N was the population estimate, M was the number of fish marked and released, C was the total number of fish captured and examined for marks in the recapture sample, and R was the total number of marked fish observed in C. The Chapman Modification method was used because it provides more accurate population estimates in cases when R is relatively small (Ricker 1975).

From early-May to mid-June, we sampled larval fishes using a 1,000m mesh conical ichthyoplankton net towed for five minutes immediately below the lake surface. Weekly samples were taken at night at five sampling locations in each lake. Each lake was divided into five quadrats and sites were established at a randomly selected nearshore (<100 m of shore) location in each quadrat on each sampling date. Once selected, locations remained fixed throughout the study. Volume of water filtered during each tow was estimated using a General Oceanics© model 2030R flowmeter mounted in the center of the net frame. Samples were transferred to containers and stored in 90% ethanol. Collected fishes were identified to species according to Auer (1982) and enumerated.
In addition to the adult walleye population, we were interested in estimating the size of the adult largemouth bass population. We performed early summer (late-May) mark-recapture surveys using AC boat electrofishing (Wisconsin‐style; AC; 2.0–3.0 amps, 200–350 V, 25% duty cycle with two netters) to sample largemouth bass. Collected largemouth bass were measured, checked for a top caudal fin clip, and if not present, marked with a top caudal fin clip and released. Adult (mature) largemouth bass were defined as all fish 203 mm. We aimed to recapture 10% of the marked population. Largemouth bass are in very low abundance in Sandy Beach Lake therefore a population estimate was not possible. In McDermott Lake, due to the small population size of largemouth bass we completed multiple marking surveys from late-May to early June to achieve this recapture rate. From electrofishing mark-recapture information, population estimates were calculated using the Schnabel (1938) modification of the Lincoln-Petersen method:
where N was the population estimate, M was the number of fish marked and released in sample t, C was the number of fish captured in sample t, and R was the number of fish already marked when caught in sample t.
To obtain centrarchid population demographic data, current standardized WDNR surveys of inland lakes consist of early summer (water temperature range = 13.0–21.0°C) AC boat electrofishing surveys or mid‐summer (18.3–26.7°C) mini‐fyke net (Simonson et al. 2008). To encompass this range of water temperatures, we performed a combination of surveys starting at the end of May with an AC boat electrofishing survey. Then, fish were sampled once monthly when lake surface water temperatures were ≥13.0°C in both lakes (June–September). Both lakes were sampled during 1‐week each month using three gears (AC boat electrofishing, mini-fyke nets, cloverleaf traps). Lakes were sampled on consecutive nights in each 1‐week period but only one gear type was employed per night.
All gears sampled shallow shorelines (0–5 m from bank, depth ≤2 m) and were deployed in fixed locations following standard approaches (Bonar et al. 2009). Sampling locations were evenly distributed along the shoreline of the lake, and all gears were deployed in similar habitat types. Five 10‐min nighttime boat electrofishing (Wisconsin‐style; AC; 2.0–3.0 amps, 200–350 V, 25% duty cycle) transects were conducted using two dipnetters. Five mini‐fyke nets (0.9‐m × 0.61‐m frames, 3.2‐mm mesh [bar measure], 7.6‐m‐long lead, and a double throat) were deployed in areas where the net frames would be in 1.0–1.5 m of water, and leads were fixed onshore. Five cloverleaf traps (three lobed, height = 41 cm, 50 cm diameter, 6.0‐mm bar wire mesh with 12.7‐mm‐wide openings between lobes, and an attractant [liver]) were deployed in littoral habitats. Both mini‐fyke nets and cloverleaf traps were set in early afternoon, fished overnight, and retrieved the following afternoon (~24‐h soak time). All catches were standardized according to gear-specific effort. For boat electrofishing, catch per unit effort (CPUE) is presented as fish/hr. For mini-fyke nets and cloverleaf traps, CPUE is calculated as fish/net night or fish/trap night.
To quantify walleye recruitment in each lake, we employed multiple gears throughout the sampling season including micromesh gillnets, beach seines, and boat electrofishing. In late July/early August, we deployed four 46-m x 1.2-m gillnets with 0.95-cm bar mesh. Sampling locations were evenly distributed along the shoreline and locations were fixed each year. Gillnets were set at night and at depths ranging from 0-5 m. Set duration ranged 1-2 hours to minimize bycatch, thus catches were standardized to age-0 walleyes collected per 10 hours of soak time. In late August, we pulled .24-m long beach seines with 0.64-cm mesh at five sites in each lake. Sites were chosen to represent a variety of habitat types and based on ability to effectively use the seine. Seining sites remained fixed for the duration of the study. Seines were used during daylight hours on each lake. Catch per unit effort was calculated as the number of individuals per seine haul. When water temperatures fell below 21°C (early September), we sampled age-0 walleyes using nighttime boat electrofishing (Wisconsin‐style; AC; 2.0–3.0 amps, 200–350 V, 25% duty cycle, two netters). The entire shoreline of each lake was sampled. Surveys were conducted prior to walleye fingerling stocking. All collected walleye were measured (TL, mm). Catch per unit effort was calculated as the number of age-0 walleyes per meter shoreline.
Removal efforts
In addition to standardized surveys in our experimental lake, in 2018 we began centrarchid removal efforts using a variety of techniques including fyke nets, boat electrofishing, mini-fyke nets, and cloverleaf traps (Fig. 1). Following annual spring fyke net surveys, fyke nets remained in the experimental lake to remove centrarchids. In 2018, we sampled 10 fyke nets from May 14 to June 7 when centrarchid catches started to decline. Due to personnel limitations, in 2019 and 2020 only five fyke nets were used from late spring (May 9, April 30) until late June (June 27, June 25). Additionally, we sampled five mini-fyke nets and 21 cloverleaf traps from late May through mid-August. All gears were emptied every 1-2 days and sites were rotated to maximize centrarchid catches. Collected fish were identified to species and measured (TL, mm). Centrarchid species were retained while other species were returned to McDermott Lake.

Methods

Each lake was sampled in nine locations. Each sample received four qPCR replicates.
Each lake was accompanied by a field (FLD), filter (FIL), and qPCR no template control
(NTC) blank. Lakes with eDNA samples extracted together in a batch share the extraction
(EXT) blank. Lakes listed without extraction blanks take the previous lake's extraction
blank in the order listed in the data. <br/> Each lake was sampled in nine locations. Each sample received four qPCR replicates.
Each lake was accompanied by a field (FLD), filter (FIL), and qPCR no template control
(NTC) blank. Lakes with eDNA samples extracted together in a batch share the extraction
(EXT) blank. Lakes listed without extraction blanks take the previous lake's extraction
blank in the order listed in the data. <br/> Each lake was sampled in nine locations. Each sample received four qPCR replicates.
Each lake was accompanied by a field (FLD), filter (FIL), and qPCR no template control
(NTC) blank. Lakes with eDNA samples extracted together in a batch share the extraction
(EXT) blank. Lakes listed without extraction blanks take the previous lake's extraction
blank in the order listed in the data. <br/>

Methods

The state of Wisconsin is comprised of about 15,000 inland lakes ranging from 0.5 to 53,394 ha (WDNR 2009). Most lakes occur in the northern and eastern part of the state as a result of glaciation. about 3,620 lakes are greater than 20 ha and together comprise about 93% of the state's inland lake surface area (Wisconsin Department of Natural Resources 2009). Wisconsin lakes constitute a wide range of physical and biological characteristics. Wisconsin inland lakes support valuable recreational fisheries for a variety of species, including Walleye (Sander vitreus), Northern Pike (Esox lucius), Muskellunge (Esox masquinongy), Yellow Perch (Perca flavescens), Largemouth Bass (Micropterus salmoides), Smallmouth Bass (Micropterus dolomieu), Lake Sturgeon (Acipenser fulvescens), and a variety of sunfish species (Lepomis spp.).

Methods

All methods describing the calculation of these data can be found in Embke et al. (in review)

Methods

Fisheries surveys of inland lakes and streams in Wisconsin have been conducted by the Wisconsin Department of Natural Resources (WDNR) professionals and its predecessor the Wisconsin Conservation Department for >70 y. Standard fyke net and boat electrofishing surveys tend to dominate the fisheries surveys and data collected. Most fyke net data on certain species (e.g., Walleye Sander vitreus and Muskellunge Esox masquinongy) originates from annual spring netting surveys following ice-out. These data are used for abundance estimates, mark and recapture surveys for estimating population sizes, and egg-take procedures for the hatcheries. Boat-mounted boom and mini-boom electrofishing surveys became increasingly common in the late 1950s and 1960s. Boat electrofishing surveys have typically been conducted during early summer months (May and June), but some electrofishing survey data are also collected in early spring as part of walleye and muskellunge mark-recapture surveys. Summer fyke netting surveys have been collected more sporadically over time, but were once more commonly used as a panfish survey methodology. Surveys were largely non-standardized. Thus, future users and statistical comparisons utilizing these data should acknowledge the non-standard nature of their collection. More in-depth description of these data can be found in Rypel, A. et al., 2016. Seventy-Year Retrospective on Size-Structure Changes in the Recreational Fisheries of Wisconsin. Fisheries, 41, pp.230-243. Available at: http://afs.tandfonline.com/doi/abs/10.1080/03632415.2016.1160894

Methods

Fisheries surveys of inland lakes and streams in Wisconsin have been conducted by the Wisconsin Department of Natural Resources (WDNR) professionals and its predecessor the Wisconsin Conservation Department for >70 y. Standard fyke net and boat electrofishing surveys tend to dominate the fisheries surveys and data collected. Most fyke net data on certain species (e.g., Walleye Sander vitreus and Muskellunge Esox masquinongy) originates from annual spring netting surveys following ice-out. These data are used for abundance estimates, mark and recapture surveys for estimating population sizes, and egg-take procedures for the hatcheries. Boat-mounted boom and mini-boom electrofishing surveys became increasingly common in the late 1950s and 1960s. Boat electrofishing surveys have typically been conducted during early summer months (May and June), but some electrofishing survey data are also collected in early spring as part of walleye and muskellunge mark-recapture surveys. Summer fyke netting surveys have been collected more sporadically over time, but were once more commonly used as a panfish survey methodology. Surveys were largely non-standardized. Thus, future users and statistical comparisons utilizing these data should acknowledge the non-standard nature of their collection. More in-depth description of these data can be found in Rypel, A. et al., 2016. Seventy-Year Retrospective on Size-Structure Changes in the Recreational Fisheries of Wisconsin. Fisheries, 41, pp.230-243. Available at: http://afs.tandfonline.com/doi/abs/10.1080/03632415.2016.1160894

Methods

BOOM SHOCKING1987:A standard WDNR electrofishing boat was used on Lake Mendota set at 300 volts and 2.5 amps (mean) DC, with a 20 % duty cycle and 60 pulses per second. On all sampling dates two people netted fish, the total electrofishing crew was three people. Shocking was divided into stations. For each station, the actual starting and ending time was recorded. Starting and ending points of each station were plotted on a nap. A 7.5 minute topographic map (published 1983) and a cartometer was used to develop a standardized shoreline mileage numbering scheme. Starting at the Yahara River outlet at Tenney Park and measuring counterclockwise, the shoreline was numbered according to the number of miles from the outlet. The length of shoreline shocked for each station was determined using the same maps. The objectives of the fall 1987 electrofishing was: to gather CPE data for comparison with previous surveys of the lake; develop a database for relating fall electroshocker CPE to predator density; collect fall predator diet data; make mark-recapture population estimates of YOY predators; and determine year-class-strength of some nonpredators (yellow perch, yellow bass, and white bass).1993: Electrofishing was used to continue marking largemouth and smallmouth bass (because of low CPE in fyke nets), to recapture fish marked in fyke netting, and to mark and recapture walleyes ( less than 11.0 in.) on Lake Mendota. Four person crews electrofished after sunset from May 05 to June 03, 1993. A standard WDNR electrofishing boat was used, set at about 300 volts and 15.0 amps (mean) DC, with a 20 % duty cycle at 60 pulses per second. On all sampling dates two people netted fish; thus, CPE data are given as catch per two netter hour or mile. Shocking was divided into stations. For each station the actual starting and ending time and the generator s meter times was recorded. Starting and ending points of each station were plotted on a map. 7.5 minute topographic maps (published in 1983) were used in addition to a cartometer to develop a standardized shoreline mileage numbering scheme. Starting at the Yahara River outlet at Tenney Park and measuring counterclockwise the shoreline was numbered according to the number of miles from the outlet. The length of shoreline shocked for each station was determined using these maps. The 4 person electroshocker crews were used again from September 20 to October 19. Fall shocking had several objectives: to gather CPE data for comparison with previous surveys of the lake; develop a database for relating fall electroshocker CPE to piscivore density; and make mark recapture population estimates of young of year (YOY) piscivores.1997:5/13/1997-5/20/1997: Electrofishing was completed at night on lakes: Mendota, Monona, and Waubesa. A standard WDNR electrofishing boat was used, set from 320-420 volts and 16-22 amps DC, with a 20 % duty cycle at 50 pulses per second. Two netters were used for each shocking event. At a particular station, starting and ending times where shocking took place were recorded. The location of the designated shocking stations is unknown.9/23/1997-10/14/1997: Electrofishing was completed at night on Mendota, Monona, Waubesa, and Wingra. A standard WDNR electrofishing boat was used, set from 315-400 volts and 16-24 amps DC, with a 20% duty cycle at 60 pulses per second. Two netters were used for each shocking event. Starting and ending time at each shocking station was listed. The location of the designated shocking stations is unknown.1998:Electrofishing was completed at night on Mendota, Monona, Wingra, and Waubesa from 5/12/1998- 10/28/1998. A standard WDNR electrofishing boat was used, set from 240-410 volts and 15-22 amps DC, with a 20% duty cycle at 50-100 pulses per second. Two netters were used for each shocking event. Starting and ending time at each shocking station was listed. The location of the designated shocking stations is unknown. FYKE NETTING1987:Fyke nets were fished daily from March 17 to April 24, 1987 on Lake Mendota. The nets were constructed of 1.25 inch (stretch) mesh with a lead length of 50 ft. (a few 25 ft. leads were used). The hoop diameter was 3 ft. and the frame measured 3 ft. by 6 ft. Total length of the net was 28 ft. plus the lead length. Nets were set in 48 unknown locations. Initially, effort was concentrated around traditional northern pike spawning sites (Cherokee Marsh, Sixmile Creek, Pheasant Branch Creek, and University Bay). As northern pike catch-per-effort (CPE) declined some nets were moved onto rocky shorelines of the lake to capture walleyes. All adult predators (northern pike, hybrid muskie, largemouth and smallmouth bass, walleye, gar, bowfin, and channel catfish) captured were tagged and scale sampled. Measurements on non-predator species captured in fyke nets were made one day per week. This sampling was used to index size structure and abundance, and to collect age and growth data. In each net, total length and weight of 20 fish of each species caught was measured, and the remaining caught were counted.1993:Same methods as 1987, except fyke nets were fished from 4/8/1993-4/29/1993 on Lake Mendota. The 1993 fyke net data also specifies the &ldquo;mile&rdquo; at which the fyke net was set. This is defined as the number of miles from the outlet of the Yahara River at Tenney Park, moving counterclockwise around the lake. In addition, abundance and lengths of non-gamefish species captured in fyke nets were recorded one day per week. Six nets were randomly selected to sample for non-gamefish data. This sampling was used to index size structure and abundance, and to collect age and growth data. In each randomly selected net, total length and weight was measured for 20 fish of each species, and the remaining caught were counted.1998:There is no formal documentation for the exact methods used for fyke netting from 3/3/1998-8/12/1998 on Lake Mendota. However, given that the data is similar to data collected in 1987 and 1993 it is speculated that the same methods were used.MINI-FYKE NETTING1989:There is no formal documentation for the exact methods used for mini-fyke netting on Lake Mendota and Lake Monona from 7/26/1989-8/25/1989. However, given that the data is similar to data collected from 1990-1993 it is speculated that the same methods were used. In the sampling year of 1989, mini-fyke nets were placed at 22 different unknown stations.1990-1993: Mini-fyke nets were fished on Lake Mendota and Lake Monona during July-September at 20, 29, 13, and 15 sites per month during 1990, 1991, 1992, and 1993, respectively to estimate year-class strength, relative abundance, and size structure of fishes in the littoral zone. Nets were constructed with 3/16 in. mesh, 2 ft. diameter hoops, 2 ft. x 3 ft. frame, and a 25 ft. lead. Sites were comparable to seine sites used in previous surveys. Sites included a variety of substrate types and macrophyte densities. To exclude turtles and large piscivores from minifyke nets, some nets were constructed with approximately 2 in. by 2 in. mesh at the entrance to the net. Thus, mini-fyke net data are most accurate for YOY fishes, and should not be used to make inferences about fishes larger than the exclusion mesh size. 1997:There is no formal documentation for the mini-fyke methods which were used on Lake Waubesa and Lake Wingra from 9/16/1997-9/18/1997. However, given that the data is similar to data collected in 1989, and 1990-1993, it is speculated that the methods used during 1997 are the same. SEINE NETTING1989, 1993: Monthly shoreline seining surveys were conducted on Lake Mendota and Lake Monona during June through September to estimate year class-strength, relative abundance, and size structure of the littoral zone fish community. Twenty sites were identified based on previous studies. Sites included a variety of substrate types and macrophyte densities. Seine hauls were made with a 25ft bag seine with 1/8 inch mesh pulled perpendicular to shore starting from a depth of 1 m. Twenty fish of each species were measured from each haul and any additional fish were counted. Gill Netting (1993)Experimental gill nets were fished in weekly periods during June through August, 1993. Gill nets were used to capture piscivores for population estimates of fish marked in fyke nets. All nets were constructed of five 2.5-4.5 in. mesh panels, and were 125 ft. long. Nets set in water shallower than 10 ft. were 3ft. high or less; all others were 6ft. high or less. Sampling locations were selected randomly from up to three strata: 1) offshore reef sets, 2) inshore sets, 6.0-9.9 ft. deep, and 3) mid-depth sets, 10-29.9 ft. deep. The exact location at which the gill nets were set on the lake is unknown because the latitude and longitude values which were recorded by the WDNR are invalid. Temperature and dissolved oxygen profiles were used to monitor the development of the thermocline and guide net placement during July and August. After the thermocline was established nets were set out to the 30 ft. contour or to the maximum depth with dissolved oxygen greater than 2 ppm. Walleye Age: Scale and Spine Analysis (1987) Scales were taken from walleye that were shocked during the fall of 1987 electrofishing events on Lake Mendota. Scales were taken from 10 fish per one-inch length increment. The scales were removed from behind the left pectoral fin, and from the nape on the left side on esocids. In addition, the second dorsal spine was removed from 10 walleyes per sex and inch increment (to age and compare with scale ages for fish over 20 inches). CREEL SURVEYS1989:Fishing pressure, catch rates, harvest, and exploitation rates were estimated from a randomized, access-point creel survey. The schedule was stratified into weekday and weekend/holiday day types. Shifts were selected randomly and were either 07:00-15:00 h or 15:00-23:00 h. In addition, two 23:00-03:00 h shifts and two 03:00-07:00 h shifts were sampled per month to estimate the same parameters during night time hours. During the ice fishing season (January-February) 22 access points around Lake Mendota and upstream to the Highway 113 bridge were sampled. The clerk counted the number of anglers starting and completing trips during the scheduled stop at each access point. During openwater (March-December) 13 access points were sampled; 10 were boat ramps and 3 were popular shore fishing sites<strong>. </strong>At each of these sites, an instantaneous count of shore anglers was made upon arrival at the site, continuous counts of anglers starting and completing trips at public and private access points were made. Boat occupants and ice fishing anglers were only interviewed if they were completing a trip. Both complete and incomplete interviews were made of shore anglers. Number caught and number kept of each species, and percent of time seeking a particular species were recorded. All predators possessed by anglers were measured, weighed, and inspected for finclips and tags. We measured a random sample of at least 20 fish of each non-predator species per day.1990-1993: Same as 1989, except 23 access points were used during the ice fishing season. In addition, 13 access points were sampled during the openwater (May-December) season; 9 sites were boat ramps and 4 sites were popular shore fishing sites. 1994-1999: No formal documentation exists, but given the similarity in the data and consistency through the years; it is speculated tha tthe methods are the same.

Methods

We sampled the Sparkling Lake littoral fish community bimonthly during summer months using electrofishing, angling, and fyke nets were from 2009-2010. All fish were measured (total length, mm), weighed (g), and tagged with Floy tags or fin clips. Diets were collected from each species at each electrofishing sampling event using gastric lavage and preserved in 95% EtOH. Scales were also collected from some fish.

Methods

To control for sampling methodology and allow comparisons among native and invasive species, we only included data where both invasive and native species from a taxonomic group were sampled using the same methods across multiple sites. Exceptions were made to include rusty crayfish (Orconectes rusticus) in its native range and zebra mussel (Dreissena polymorpha) data. Native rusty crayfish data were obtained from (Jezerinac 1982). Zebra mussel data were mainly obtained from a meta-analysis (Naddafi et al. 2011) which compiled data from 55 European and 13 North American sites from 1959-2004. Additional densities from North America were compiled from multiple primary literature sources (Table S3). All zebra mussel records were presented as number per m2 and are from their invaded range; we did not include native mussel data.Crayfish data were obtained from multiple sources. Crayfish were collected in Wisconsin, USA during summers of 2002-2010 from lakes in the Northern Highlands Lake District following their protocol for crayfish collection. Crayfish were sampled in Wisconsin streams tributary to Lake Michigan from 2007-2010 using 10 gee-style minnow traps per site baited with chicken livers and set overnight. Swedish crayfish were sampled using 30 minnow traps baited with frozen fish in lakes and streams of southern Sweden from 2001-2003 as described in (Nystrom et al. 2006). Washington crayfish were collected from 100 lakes in the Puget Sound Lowlands region of Washington State, USA between 2007 and 2009 from mid-June to early October of each year. At each lake, the investigators set 20 minnow traps baited with fish-based dog food. Traps were deployed in four clusters of five traps each and recovered the following day. All crayfish densities are presented as number per trap per day, with the exception of native range rusty crayfish data, which were reported as number per site (Jezerinac 1982) and excluded from all comparisons that depend on sampling units.Wisconsin fish data were collected from streams throughout the state from 2005-2010 using either a backpack or towboat electrofisher with pulsed DC current in wadeable (less than1m depth) streams for a minimum of 15 minutes. For Wisconsin trout species, locations sampled within 10 years following a stocking event of that species were excluded. Lamprey data were collected from 2008-2010 from Great Lakes tributaries using backpack electrofishers following standardized methods as a part of the sea lamprey assessment program of the United States Fish and Wildlife Service and Department of Fisheries and Oceans, Canada. North American fish densities are presented as number per minute of sampling. Swedish fish data were collected using backpack electrofishing between 1980 and 2010 from streams in Vasterbotten county, northern Sweden, and were obtained from the Swedish Electrofishing REgister (SERS), www.fiskeriverket.se, and are reported as number per 100 m of stream.Snail data were collected in 2006 from lakes in the Northern Highlands Lake District in Wisconsin as described by (Solomon et al. 2010), and densities are presented as number per two m2. Aquatic plant data were collected using a systematic grid-based point-intercept sampling methodology to record macrophyte frequency of occurrence in 242 Wisconsin lakes from 2005-2008. Aquatic plant presence absence was recorded from a boat using a double-sided rake sampler at each point on a sampling grid as described in (Mikulyuk et al. 2010). Density data are presented as proportion of sites within lake littoral zone where a species was present.For all data, if multiple records existed from the same location, we used the most recent record. If replicate samples existed within the same site on the same sampling date, the mean value was used.

Methods

We sampled largemouth bass between June and August of 2006 primarily via electrofishing along the lake perimeter. Fish were collected via angling when lake conductivity was not suitable for electrofishing. Thirty fish were collected from each lake to determine size-specific growth rates. Fish length (total length; mm) was recorded, and 5 to 10 scales were collected from each fish from the area posterior to a depressed pectoral fin. We removed young-of-the-year fish from the analysis owing to the lack of annuli, and as a result, sample size varied between lakes. Scales from yearling fish and older were sonicated and pressed between two slides. Nonregenerated scales were read into a digital imaging system. Annual growth rates (mm per year)1) were determined using Fraser-Lee&rsquo;s method of back calculation with Carlander&rsquo;s recommended constant of 20 mm for largemouth bass. It is possible that LRD could have changed during the lifetimes of the longer-lived bass in our study, especially because LRD boomed during the 1990s but slowed substantially during the 2000s. To eliminate any potential effects of changing LRD levels, only the annual growth estimates from 2001 to 2005 were included as repeated measures of annual growth for each fish.