Natural Lakes Research Team

Iowa’s muskellunge stocking program started in 1960 when 40 fingerlings were stocked in two natural lakes. The goal was to help anglers catch trophy-sized fish without leaving Iowa. There was some early opposition to stocking a large predator, like muskellunge, into Iowa’s natural lakes. This was particularly true when other fish populations like yellow perch or walleye were in decline, even though muskellunge were not responsible for those downturns.

Managers quickly learned that for successful muskellunge management in Iowa, specific population density targets needed to be established and that other important population parameters, such as growth and mortality rates, must be monitored closely. Low population density targets were set to allow for trophy growth potential and satisfy most anglers’ concerns of muskellunge and other sport fish species interactions.

The number of natural lakes stocked grew as the popularity of the program grew. currently, muskellunge are managed in Clear Lake, Black Hawk Lake, North Twin Lake, Spirit Lake, East Okoboji Lake and West Okoboji Lake. Since muskellunge are a long-lived species, even slight changes in death or emigration rates can substantially influence population density and size structure. Muskellunge population densities are closely monitored in these lakes by inserting tags into each fish stocked or captured during broodstock spring gillnetting and then using computer models to estimate adult population abundance from individual fish capture histories. With this information, managers can adjust stocking densities or evaluate the effects of various length-limit regulations to keep population densities within the objectives set for each lake.

Muskellunge provide an important fishery to Iowa, but they must be managed in a way that is sustainable and beneficial to all sport fish populations and users. Monitoring muskellunge population densities each year is essential to ensure muskellunge fisheries in Iowa’s natural lakes stay consistent with management objectives. Yearly assessments combined with adaptive management allow managers to adjust stocking rates as needed to properly manage muskellunge in multi-species fisheries.

Natural reproduction of walleye in Iowa’s natural lakes is very limited. Annual stockings of fry and fingerlings are needed to sustain these fisheries. Consistent survival of stocked fry and fingerling walleye is key to increase walleye numbers in many of Iowa’s natural lakes. These lakes often have high walleye harvest deaths, so stocking alone would not produce the walleye numbers needed to meet management goals. A combination of improved stocking survival and proper harvest regulations is needed to increase walleye populations. Since hatchery costs are dependent on the size of fish produced, most research in Iowa has focused on evaluating the survival of fall stocked walleye fingerlings. Several years of research examining age-0 and age-1 walleye electrofishing catch rates and population numbers concluded that large (>7 inches) walleye fingerlings are needed to meet adult walleye population goals.

In addition to stocking survival, managers also use harvest regulations to improve adult walleye abundance, size structure, and growth rates. In 2007, a protected slot limit of 17-22 inches (1 fish over 22 inches, daily bag of 3 fish) was implemented on the Iowa Great Lakes and Storm Lake. The minimum length limit of 14 inches was kept on Clear Lake. Angler surveys each year and spring broodstock gillnetting provide information about walleye numbers, walleye size structure and growth rates, as well as walleye harvest and release rates before and after the regulation change.

The protected slot limit has provided more consistent numbers of adult walleye in the Iowa Great Lakes and Storm Lake since the regulation change. The regulation change has experienced periods of high adult male walleye abundance within the protected slot that provide little benefit to anglers or broodstock production. In 2020, a study that evaluated the current 17-22 inch protected slot found that although this regulation was working, other combinations of protected slot limits could be used that allow harvest of slow growing adult males, yet maintain ever valuable populations of female walleye broodstock.

This study found that special regulations, such as protected slot limits, may improve the walleye fishery at Clear Lake. Based off these findings, in January of 2021, a protected slot limit of 19-25 inches (1 fish over 25 inches, daily bag of 3 fish) will be implemented on the Iowa Great Lakes and Storm Lake. At Clear Lake, the walleye regulation will change from a minimum length limit of 14 inches to a protected slot limit of 17-22 inches (1 fish over 22 inches, daily bag of 3 fish).

Further research monitoring broodstock densities, life history features of the broodstock populations, stocking success with creel surveys, mark and recapture tagging, and age and growth analysis is needed to understand the impacts of harvest regulations and stocking strategy changes. Findings from this research will guide walleye management decisions and strategies for Iowa’s natural lakes.

The Iowa DNR has stocked walleye into almost every natural lake in Iowa over the past 50 years, but only a few of these natural lakes consistently sustain high-quality walleye fisheries. Maintaining these high-quality walleye fisheries requires constant management and evaluation of stocking products. Understanding recruitment dynamics for stocked walleye fisheries is important to evaluate stocking contribution and identify lapses in recruitment, which may be partially remedied by stocking fall fingerlings.

Hatchery produced walleye fingerlings (≥ 6 inches) are expensive and production quotas are unpredictable, often causing a situation in the fall where biologists are required to prioritize fall fingerling stockings within their management district. Prioritizing attempts have been made by conducting night electrofishing surveys prior to stocking. Since stockings may occur as early as mid-September, surveys have been ineffective at catching adequate numbers of young walleye to make informed decisions.

Identification of past recruitment patterns can guide fisheries management walleye stocking decisions. For example, a stocked walleye fishery that has had two successful year-classes in the past three years may not be a good candidate for fall fingerling stocking the following year. Conversely, a stocked walleye fishery that has shown several years of poor fry stocking success may need higher prioritization of fall stocked fingerlings to sustain a viable walleye fishery. Information regarding the success or failure of past walleye stockings needs to be incorporated into fall fingerling stocking decisions so that the use of this product is maximized.

In this study, fall electrofishing surveys will be conducted on at least eight natural lakes each year. Data obtained will allow researchers to evaluate recruitment, growth, and abundance patterns that can be used as tools to make informed management decisions, such as prioritizing fall fingerling stockings.

Iowa’s muskellunge program was started in 1960 by stocking 40 fingerling muskellunge each into West Okoboji Lake and Clear Lake. Since those first stockings, muskellunge culture and management have advanced considerably thanks to nearly continuous stocking evaluations. These evaluations found that stocking muskellunge yearlings in the spring greatly improved adult population densities. By 2013, survival of spring-stocked yearling muskellunge was found to be extremely variable and resulted in declines in adult muskellunge densities in some lakes. Other factors, such as hauling stress, stocking technique, initial size at stocking, body condition, or physical deformities, were identified that may influence yearling survival. The focus of this study was to evaluate those factors and try to maximize yearling muskellunge survival in Spirit Lake and the Okoboji lakes.

Understanding the individual factors that influence yearling muskellunge survival is necessary to effectively manage these fisheries. All yearling muskellunge stocked into the Iowa Great Lakes have been tagged before stocking since 2011 to identify factors such as total length or condition that may contribute to increased or reduced survival. In addition to tagging all stocked fish, a subset of yearling muskellunge stocked into Spirit Lake in 2016 and 2017 were tagged with radio transmitters to help estimate post-stocking survival and to determine if different stocking techniques were responsible for increased or decreased survival rates.

Results from two years of tracking in Spirit Lake found that yearling muskellunge survival to 100 days was 50-65 percent and was related to size at the time of stocking, with larger fish surviving at a higher rate. Using this information, yearling muskellunge stocked in 2018-2020 were required to be at least 13 inches before stocking in May. Those that were less than 13 inches were held for an additional 30 day grow-out period to try to improve their size. On average, the grow-out period resulted in about a 1-inch larger fish at time of stocking in late June. A subsample of grow-out fish were implanted with telemetry tags and their movements and survival rates were recorded for 100 days. In 2018 and 2019, survival rates of grow-out yearling muskellunge was ≥75.0 percent. In 2020, the survival rate decreased to 50 percent, primarily from increased predation by great blue herons and fish predators.

Collectively, we found that the grow-out period improved yearling muskellunge survival by at least 30 percent each year. In 2021, all yearlings stocked in Spirit and the Okoboji lakes were stocked using the grow-out technique. Finding the most efficient stocking strategies for Iowa’s lakes reduce production costs and provide desired muskellunge populations that meet the needs of anglers and hatchery production.

Recent research has found that movement of walleye and muskellunge among the Iowa Great Lakes (chain of six natural lakes in northwest Iowa) can be substantial. Movement of these species in connected lakes is not uncommon, but can result in population imbalances within a connected lake chain or direct loss of fish into connected rivers. For successful individual management of these lakes, it is important to understand the extent to which populations of each species move among and out of lakes. If movement is significant, managers may be able to reduce loss by installing fish barriers at outlet connections or use other adaptive management techniques to counteract population imbalances. Recent research found that a pulsed direct current barrier was successful in deflecting walleye from a controlled, simulated barrier experiment. However, no information exists on how effective a barrier of this type would be for muskellunge or the effectiveness for either species in a field experiment.

Since walleye and muskellunge are top-level predators and take several years to reach maturity, immediate losses of large quantities of fish through a spillway or outlet structure may have devastating long-term effects for that fishery. Specifically, these population imbalances and declines have been most prevalent in the Iowa Great Lakes muskellunge fishery, but periods of walleye escapement have also been observed.

This study will focus on evaluating the effectiveness of a low-pulse electric barrier to reduce both walleye and muskellunge loss into Milford Creek. One component of this evaluation includes recapturing fish within the river and using tag information to determine the approximate timing of movement over the barrier. Another component of this study is to use acoustic telemetry to determine long-term (> 4 years) movements and escapement of individual muskellunge that are fitted with acoustic tags. Field experiments will be conducted to evauluate individual fish response to different electric barrier settings to mazimize barrier effectiveness. Ultimately, this study with provide movement information that will be critical to determine the long-term effectiveness of such a barrier.

Most-fished species in Iowa and bass (largemouth and smallmouth bass combined) tied walleye as the single species they most often fish for. This same study identified that monitoring fisheries populations is important to Iowa anglers.

Relatively little is known about bass population dynamics across Iowa’s natural lakes. Iowa’s standard comprehensive survey sampling protocols for natural lakes seldom yield catch rates sufficient enough to collect the number of bass needed to estimate population parameters. In Iowa’s natural lakes, targeted bass population assessments are uncommon since bass are typically not stocked in natural lakes and are managed more as a function to manage water quality and habitat. Managers still desire information about bass populations so that trends in population abundance, size-structure, growth, and mortality can be examined and compared.

Targeted night electrofishing during the spring typically provides the best index of bass density and size structure. For this study, natural lakes were inventoried using historical data to determine if bass populations were previously detected. Eighteen natural lakes were identified as having past bass populations. In 2019-2020, bass populations in these natural lakes were sampled capturing a total of 1,557 largemouth bass and 86 smallmouth bass. Highest catch rates occurred at Black Hawk Lake and the largest fish (20.7-inch largemouth bass) was captured from West Okoboji Lake. Dorsal spines were collected from a subsample of fish to determine age-and-growth characteristics and death rates each year for each lake.

Starting in 2021, less frequent (once every five years) and more intense (at least 75 bass sampled) sampling of bass populations will occur in natural lakes to evaluate population characteristics and determine if management options can be implemented that may improve bass populations in Iowa’s natural lakes.

• Distribution, population dynamics, and interspecific competition of Yellow Bass in Iowa’s shallow natural lakes (Summary)
• Distribution, population dynamics, and interspecific competition of Yellow Bass in Iowa’s shallow natural lakes (Full Report)
• Short-term stocking survival of yearling Muskellunge raised in a recirculating aquaculture system (Summary)
• Short-term stocking survival of yearling Muskellunge raised in a recirculating aquaculture system (Full Report)
• Population Densities, Biomass, and Age-growth of Common Carp and Black Bullheads in Clear Lake and Ventura Marsh (Summary)
• Population Densities, Biomass, and Age-growth of Common Carp and Black Bullheads in Clear Lake and Ventura Marsh (Full Report)
• Development of an Optimal Stocking Strategy for Walleye in Spirit, East Okoboji, and West Okoboji lakes
• Evaluation of Special Regulations for Managing Walleye in Iowa's Natural Lakes
• The Relationship of Catch per Unit Effort Data to Estimated Density of YOY and Yearling Walleyes in Spirit, East Okoboji, Clear and Storm lakes, and an Evaluation of the Use of Trend Data for Managing Natural Lakes in Northwest Iowa
• The Production and Consumption Dynamics of Major Fish Species, and the Estimated Carrying Capacity of Major Fish Piscivores in Spirit Lake, Iowa
• Shallow Lake Renovation Based on Alternative Stable Trophic States