Category Archives: Reports

Diets of Larval Walleyes in Northern Wisconsin Lakes

Nathan Jaksha1, Daniel Isermann2, & Daniel Dembkowski1

University of Wisconsin-Stevens Point1, USGS-Wisconsin Cooperative Fishery Research Unit2


  • Walleye are an important species in Wisconsin that support both recreational hook-and-line and tribal spear fisheries.
  • Walleye recruitment has declined in some lakes that previously supported natural recruitment yet has remained stable in other lakes.
  • Recent studies suggest that a recruitment bottleneck may be occurring at or before the larval stage in lakes exhibiting declines in natural recruitment.
  • Reasons for the contrasting trends in recruitment among populations are unclear, as are mechanisms underlying the recruitment bottleneck in lakes with declining recruitment.
  • Differences in prey availability among lakes with different recruitment histories could contribute to the observed trends in recruitment.
  • Assessment of larval walleye diets from lakes with different recruitment histories could provide important insight to the role of prey resources in explaining trends in walleye recruitment in Wisconsin lakes.
Photo of an adult walleye sampled by the Wisconsin DNR


Describe diet composition of larval walleyes in northern Wisconsin lakes displaying two different recruitment histories: sustained natural recruitment (S-NR) and declining natural recruitment (D-NR).

Methods – Sampling

  • Larval  walleye sampling occurred during 2016 & 2017 on 13 lakes
    • Five of these lakes were resampled in 2018
  • Sampling started mid-May each year
    • Continued at 7-10 d intervals until early June
    • Surface temps = 11-16˚C
  • Larval walleyes were collected using a 1,000-µm mesh conical ichthyoplankton net.
    • Towed at surface (5 min) at both nearshore (within 100-m) & offshore locations (≥100-m) at night
  • Percid larvae identified to species using dichotomous keys
    • Random subset selected for genetic verification of visual species identification using PCR techniques

Methods – Diets

  • Gut contents of each fish were removed and diets were quantified using mean percent composition by number of individual diet items.
  • Zooplankton items classified to order for adult copepods and genus for cladocerans
  • Larval fish encountered in diets identified genetically using qPCR techniques
Magnified image of a zooplankton (Daphnia spp.) diet item
Magnified image of a larval Walleye

Results – All Lakes

  • Larval walleye collected from 6 S-NR lakes and 4 D-NR lakes (diets summarized in Table 1).
  • Guts were removed from 115 larval walleyes (mean TL = 11.4 mm; range = 6.4 –22.0 mm).
  • 27% of diets (31 of 115) were empty
  • 37% of diets (42 of 115) contained larval yellow perch
  • 16% of diets (18 of 115) contained zooplankton
Table 1. Mean percent composition by number and counts of prey items observed in larval walleye diets from all lakes combined.
Diet Item% CompositionCount
Daphnia spp.15.041
Calanoid Copepods1.83
Cyclopoid Copepods5.816
Bosmina spp.0.53
Larval Fish63.349

Results – S-NR vs. D-NR Lakes

  • S-NR Lakes = 86 larval walleye (diets summarized in Table 2)
    • Larval walleye mean length = 11.3 mm (range = 6.5 -22.0 mm)
    • 38 % of diets (33 of 86) were empty
    • 31% of diets (27 of 86) contained larval yellow perch
    • 20% of diets (17 of 86) contained zooplankton
  • D-NR Lakes = 29 larval walleye (diets summarized in Table 2)
    • Larval walleye mean length = 12.4 mm (range = 7.8 -21.0 mm)
    • 28% of diets (8 of 29) were empty
    • 52% of diets (15 of 29) contained larval yellow perch
    • 3% of diets (1of 29) contained zooplankton
Table 2. Mean percent composition by number and counts of prey items observed in larval walleye diets from S-NR and D-NR lakes.
Recruitment History:S-NR LakesD-NR Lakes
Diet Item% CompositionCount% CompositionCount
Daphnia spp.19.1404.81
Calanoid Copepods2.530.00
Cyclopoid Copepods8.1160.00
Bosmina spp.0.730.00
Larval Fish60.13471.415


  • Larval yellow perch were the dominant prey item for larval walleyes in lakes exhibiting both sustained and declining natural recruitment.
    • Extent of piscivorymuch greater than previously assumed
  • In general, larval walleyes in S-NR lakes consumed a greater diversity of prey items and more zooplankton than larval walleyes in D-NR lakes.
  • Further statistical analyses required to determine if diets and prey availability differed between S-NR & D-NR lakes.


  • This study was funded by the Wisconsin Department of Natural Resources through the Fisheries Analysis Center at the University of Wisconsin-Stevens Point.
  • Jason Gostiaux
  • Walleyes for Tomorrow: Research Fellowship

Flexible Classification of Wisconsin Lakes for Improved Fisheries Conservation and Management

Successful fisheries management practices developed for one ecosystem can often be used in similar ecosystems. We developed a flexible lake classification framework in collaboration with ~100 fisheries biologists for improved fisheries conservation management in Wisconsin, USA. In total, 5,950 lakes were classified into 15 lake classes using a two tiered approach. In tier-one, lakes were clustered into “simple” and “complex” sportfish assemblages. In tier-two, lakes were further clustered using accumulated degree days, water clarity, and special cases. We focus on temperature and clarity because these factors often drive fisheries change over time — thus a lake’s class can change over time. Lake class assignments were refined through a vetting process where fisheries biologists with expert knowledge provided feedback. Relative abundance, size-structure, and growth rates of fishes varied significantly across classes. Biologists are encouraged to utilize class interquartile ranges in fisheries metrics to make
improved fisheries assessments. We highlight hard-won lessons from our effort including: (1) the importance of co-developing classification frameworks alongside fisheries biologists; and (2) encouraging frameworks where lakes can shift classes and fisheries expectations over time due to factors like climate change and eutrophication.

2017-18 Winnebago System Walleye Report

Attached is the 2017 Winnebago Walleye Report. The report covers a variety of topics including 2017 spring water level and walleye hatch results, 2017 spring adult spawning stock results, walleye exploitation and reward tag study results, a walleye population outlook, and a regulation change discussion update.

Walleye anglers have enjoyed some productive walleye fishing on the Winnebago System over the last few months, particularly in May and June. Many anglers have also been wondering how the 2018 walleye hatch will stack up after the historical spring weather events that swept through the area (you may remember the 30 inch snow event in April). DNR staff and a host of local volunteers completed the first round of the Annual Lake Winnebago Bottom Trawl Assessment last week.

Preliminary results indicated a measurable 2018 walleye hatch (5.7 young of year/trawl), but we will have to wait until the September and October trawling rounds are completed for official results. The survey is also vital for evaluating the year class strength of other sport and forage fish species in Lake Winnebago. Stay tuned for the annual trawling report once the survey concludes in fall. I hope you enjoy the walleye report and good luck with your summer fishing adventures. We are committed to service excellence.

Visit our survey at to evaluate how I did. Adam D. Nickel Phone: (920) 424-3059

Click here for 2017-18 Winnebago System Walleye Report

2016 Lake Winnebago Bottom Trawling Assessment Report

2016 Lake Winnebago Bottom Trawling Assessment Report
Adam Nickel, Winnebago System Gamefish Biologist, January 2017

The 2016 Winnebago bottom trawling survey results are in and it was a great year to be on the boat as the survey revealed strong year classes for crappie, walleye, and forage base species. Over 36 volunteers (a mix of new and veteran) boarded the Calumet in 2016 and donated over 400 volunteer hours of labor. The bottom trawl assessment is the most critical fisheries assessment conducted on the Winnebago System and simply could not be conducted without the help of our dedicated volunteer base.

The objectives of the trawling assessment are to:
1) provide critical information on year class strength of game and nongame fish species,
2) monitor trends in the forage base,
3) monitor general population trends of game and nongame fish species. The survey also provides volunteers with a hands-on experience with
conducting survey work on the system.

Full report details are here

Walleye Movement in the Winnebago System (2011-2013)

I attended the Berlin chapter meeting last night and had some interesting conversation. Today I called Adam Nickle our Fisheries Biologist. The attached PDF document is a report on a walleye tracking study DNR conducted with WFT funding. WFT purchased the sonic tags. Much of the Berlin conversation centered around lack of walleye using the Fox River as a spawning area.

As you can see from the text, the Winnebago System has 35 listening devices installed. These are used to track sturgeon primarily but are also used to track walleye, flathead catfish and musky. The System is so large and dynamic that this system has been very beneficial to understand how important fish species move around on an annual basis.

Walleye Movement in the Winnebago System (2011-2013)



Report from Green Lake

image1After several outboard motor issues, we set seven nets the evening of the twenty-second. In cooperation with the local DNR biologist who are currently performing their five year warm water fish survey, we are running three of their nets so we actually have ten nets to work.

We got three net-nights under our belts. We have handled 137 walleyes, with 38 of which were “workable” females (nothing over 26″ do we strip, this also accounts for green females – all these fish were documented then released) to 61 lively always ready boys – decent ratio.

We have 3’072’000 future walleye dinners/wall-hangers incubating. Hatching of those eggs will begin about 5/12 with full hatching on 5/16. Target is around 9’300’000 eggs, with a target release number of 7’500’000. We’ll see how it all goes, we’ve got a great hatchery attended – should go fine.

Peshtigo Flowage – Walleye Spawning Reef

Here are a bunch photos for the Peshtigo Flowage – Walleye Spawning Reef project

Lakeshore Woody Habitat in Review (Sawyer County)

Lakeshore Woody Habitat in Review

by Max Wolter

WDNR Fisheries Biologist

Sawyer County


Coarse woody habitat (CWH) in lakes is classified as trees, limbs, branches, roots, and wood fragments at least 4 inches in diameter that enter a lake by natural (beaver activity, toppling from ice, wind, or wave scouring) or human means (logging, intentional habitat improvement, flooding following dam construction, Guyette and Cole 1992, Christensen et al. 1996; Engel and Pederson 1998). CWH serves many functions within a lake ecosystem including erosion control, as a carbon source, and as a surface for algal growth which is an important food base for aquatic macroinvertebrates (Engel and Pederson 1998; Sass 2009). Presence of CWH has also been shown to prevent suspension of sediments, thereby improving water clarity (Sass 2009). CWH serves as important refuge, foraging, and spawning habitat for fish (Hanchin et al 2003, Lawson et al. 2011), aquatic invertebrates, turtles, birds, and other animals (Engel and Pederson 1998; Sass 2009).

The amount of CWH occurring naturally in lakes is related to characteristics of riparian forests and likelihood of toppling (Marburg et al. 2006). However, humans have also had a large impact on amounts of littoral CWH present in lakes through time. During the 1800’s the amount of CWH in lakes was increased beyond natural levels as a result of logging practices. But through time changes in the logging industry and forest composition along with increasing shoreline development have led to reductions in coarse woody habitat present in many northern Wisconsin lakes. CWH is often removed by shoreline residents to improve aesthetics or recreational opportunities (swimming and boating). Jennings et al. (2003) found a negative relationship between lakeshore development and the amount of CWH in northern Wisconsin lakes. Similarly, Christensen et al. (1996) found a negative correlation between density of cabins and woody habitat present in Wisconsin and Michigan lakes. While it is difficult to make precise determinations of natural densities of CWH in lakes it is believed that the value is likely on the scale of hundreds of logs per mile. The positive impacts of woody debris on fish communities have been well documented by researchers, making the loss of these habitats a critical concern. Fortunately, remediation of this habitat type is attainable on many waterbodies, particularly where private landowners and lake associations are willing to partner with county, state, and federal agencies. Benefits of these habitats to the fish communities (with particular emphasis on fisheries issues in northern Wisconsin) and notes on the placement, maintenance, and degradation of these woody structures are detailed in the following sections.

The Influence of Coarse woody Habitat on Fish Communities

Woody structure in lakes and ponds has been shown to be an important and preferred habitat for many fish species. Newbrey et al. (2005) observed 16 different species occupying coarse woody habitat in a Wisconsin lake. Woody structure likely has an increased importance in northern Wisconsin lakes where aquatic vegetation is often absent or limited.

CWH has been shown to be important for various life stages of many different species. Johnson et al. (1988) found that artificial structures simulating wood attracted 5-10% of bluegill present in ponds despite accounting for less than 1% of the area of the ponds. Black bass species (smallmouth and largemouth) often build spawning nests in proximity to CWH, particularly large logs (Hunt and Annett 2002; Lawson et al. 2011; Weis and Sass 2011). Research suggests that addition of supplemental CWH may improve reproduction of black bass in lakes where CWH is lacking or has been removed. Newly hatched smallmouth bass will often inhabit littoral wood, and declines in this habitat type have been linked to reduced abundance of young smallmouth (Brown and Bozek 2010). Yellow perch need submerged wood and vegetation on which to lay eggs (Hanchin et al 2003). Accordingly, Helmus and Sass (2008) and Sass et al. (2006b) found a decline in a yellow perch abundance following an artificial reduction in CWH.

Woody habitat can play an important role in predator prey relationships, particularly between largemouth bass and bluegill. Bluegill are attracted to woody debris as a refuge from predation and also because these habitats are often colonized by insects that feed on periphyton growing on the surface of wood. Presence of largemouth bass has been shown to alter the types of woody structure preferred by bluegill, which will select smaller interstices in the presence of this predation threat (Johnson et al. 1988). Largemouth bass are by nature a generalist species capable of shifting foraging mode to suit experienced habitats. Ahrenstorff et al. (2009) found largemouth bass had smaller home ranges, lower activity rates, and greater occupancy of the nearshore littoral zone (<2m deep) following an increase in littoral CWH. Additionally, bass in lakes with more CWH had higher consumption rates and a more general diet, indicative of a sit-and-wait foraging strategy (Sass et al 2006b). Both sets of authors suggest that reduced activity but increased consumption rates by largemouth bass in the presence of CWH could lead to faster growth rates in comparison to open habitats. Other studies of bass and bluegill interactions support this hypothesis (Schindler 2000, Sass 2006a). Sass et al. (2006a) found predation rates on prey fish were highest in the areas immediately adjacent to woody littoral habitat and decreased with increasing depth and distance from these areas. Predation rates within a refuge of nearshore woody habitat were also lower than on the edge of the refuge. Bevelhimer (1996) showed that smallmouth bass will select these types of structure even when water temperature is above optimal, suggesting that bass of both species will continue to use littoral woody habitat even through warm summer months.

Placement, Design, and Degradation of Woody Debris in Lakes

CWH decomposition rate increases with water temperature, pH, and the abundance of shredding invertebrates (Engel and Pederson 1998). While large pieces of CWH can have a residence time of several centuries (Guyette and Cole 1999), Christensen et al. (1996) estimated that it could take up to 200 years for developed lakes to naturally replenish woody debris that had been removed. These findings justify littoral wood additions, particularly additions of whole trees harvested from upland areas and placed on shorelines, as a means to restore these habitat types to levels at which they occurred naturally.

The design of woody habitat additions should be given some consideration to meet the goals of a restoration project. Johnson and Lynch (1992) evaluated the fish attraction capabilities of several types of structures including evergreen trees (upright and prone), brushpiles, and stakebeds. Evergreen trees provided as good or better fish attraction than the other two structures and were considered to be less costly to construct. Woody structures in this study were placed at four meter depth; littoral tree drops were not evaluated. In an evaluation of habitat structure by Bryant (1992), young and adult smallmouth and largemouth bass utilized both a uniformly-dense and dense-with-open-pockets CWH structure design. These results suggest that the preference of these species for occupying woody habitat over open areas is stronger than the preference among specific types of woody habitat. In this study, adult bass utilized structure located in up to three meters depth. Johnson (1993) found that bluegill selected both horizontal and vertically oriented structure over open water, with a slight preference for vertical over horizontally oriented artificial structures. Newbrey et al. (2005) found increasing complexity of branching to be positively associated with fish species richness, diversity, and abundance, but preference for branching complexity varied by species. Generally, most studies conclude that fish have a strong preference for some degree of branching over simple, unbranched logs, and striving for a variety of branching densities may be an optimal strategy to benefit a wide range of species (Sass et al. 2011). Similarly, CWH additions that cover a range of depths can be expected to be beneficial to a wider variety of species over a greater seasonal timeframe. It should be noted that individual pieces of structure appear to have a ‘carrying capacity’ of fish that they are capable of supporting (Johnson et al. 1988). Therefore, density of structure being placed into a waterbody should be considered carefully to balance habitat needs and human effort.

Limitations to these projects exist. Trees can become dislodged from shore as a result of ice sheering or wind and wave action. Dislodged trees can pose a serious danger to boaters and may also result in property damage if swept into docks or boat lifts. Additionally, lakes that experience water level draw-downs will pose major challenges to placing and maintaining trees. The species of trees selected for these projects is also an important consideration. Hardwood species such as cedar, oak, and ironwood will have increased residence time in comparison to softwood species such as basswood and pines. Spruce and hemlock trees should be avoided for use in fish habitat work as the bark of these species has been shown to be toxic to some taxa of aquatic animals (Buchanan et al. 1976).

Rogers and Bergersen (1999) evaluated four woody structure types and concluded that additions of structure could be expected to have the most benefit to fish communities and anglers in lakes with little existing structure and sparse aquatic vegetation. There is some debate as to whether the presence of woody debris stimulates aquatic plant growth in the immediate area by stabilizing sediments and seed beds (Bryant 1992; Sass 2009) and reducing wave scouring, or, hinders growth by shading out plants (Engel and Pederson 1998). Regardless, additions of wood should not be expected to have significant or far reaching influences on aquatic plant communities in northern Wisconsin lakes.

Potential benefits to angling and management

There has been considerable debate as to whether the placement of structure into lakes results in an actual increase in fish production or merely congregates populations. Some researchers argue that fish attraction occurs because these areas provide an advantage in prey acquisition or predator avoidance (Johnson et al. 1988). Other authors have argued that added structures, particularly in lakes with little existing structure, only congregates populations making them more accessible to anglers, merely giving the impression of increased abundance. In situations where fish populations show a strong affinity for added structure and these areas are targeted heavily by anglers it may be possible for overfishing to occur. This debate has largely been centered on the addition of offshore structure or “fish cribs” that are utilized by fish throughout winter months. Additions of nearshore wood in the form of tree drops are more natural and are largely considered to be positive. Nearshore woody structure is typically thought to promote ecosystem balance, particularly in highly developed lakes where this habitat has been previously removed. Project designers should be conscientious that large-scale wood addition projects will be more likely to provide positive and far-reaching benefits to aquatic ecosystems than small-localized projects.

Previous studies suggest that additions of littoral wood could have several positive affects on fish community interactions, particularly between largemouth bass and walleye in northern Wisconsin lakes. However, it should be noted that population level effects may not be immediate (Sass et al. 2011). Three specific potential benefits to management programs and fish community structure resulting from additions of CWH are detailed below:

Reduced predation of juvenile walleye by adult largemouth bass through habitat partitioning:

There is evidence within the scientific literature suggesting that dense populations of bass prey heavily on walleye thereby limiting walleye recruitment. In Wisconsin, Nate et al. (2003) and Fayram et al. (2005) both found negative relationships between measures of walleye abundance and largemouth bass abundance (for a full review of this issues see “Interactions between Walleye and Black Bass in Lakes: A Literature Review” by Dave Neuswanger, WDNR). Pratt and Fox (2001) assessed habitat use of young of year walleye and found a strong preference for occupying mid-depth weedbeds in mid-summer (June and early July) and shallower weedbeds later in the summer (mid-July to late August). In this study largemouth bass were shown to occupy shallower weedbeds at the same time as walleye, suggesting that predation on walleye by bass may occur during this time. Increases in nearshore CWH will create an alternate and preferred habitat for largemouth bass where predation could be expected to shift to bluegill and other centrarchid species that also have a strong preference for CWH (Johnson 1988). These additions of CWH may be more effective at partitioning habitat use by largemouth bass and walleye in lakes where CWH is currently lacking.

Increased abundance of yellow perch, an important walleye forage item:

Yellow perch need woody or vegetative structure on which to drape their eggs when spawning. Studies have shown decreases in perch abundance when littoral wood is

removed (Sass et al. 2006b). Restoring woody habitat may have positive effects on yellow perch spawning success and abundance. Because yellow perch have been documented as an important diet item for walleye it would follow that increases in juvenile yellow perch abundance would lead to improved walleye growth rates and potentially recruitment. Yellow perch could also provide an alternative forage item for largemouth bass which may relieve predation pressure on walleye to some extent.

Improved largemouth bass growth rates:

Several studies have shown that in the presence of littoral woody habitat largemouth bass are able to devote less energy into searching for and capturing prey, thereby increasing growth rates (Sass et al 2006b). This may also be true for other ambush predators including pike and muskie which may use littoral woody habitat on a more limited basis.


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