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

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

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’s method of back calculation with Carlander’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.