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Iowa Fisheries Research

Fisheries Research Highlights

Fisheries research is the foundation of smarter, more efficient and effective fisheries management. The DNR’s Fisheries Research Program focuses on evaluating existing fisheries management practices, innovating new techniques, and advancing the science and technology used to improve fishing in Iowa’s lakes, rivers, and ponds. This focus helps the Fisheries Bureau ensure that fishing license dollars are spent wisely and as efficiently as possible.

DNR’s Fisheries Research Program accomplishes all this with a relatively small group of highly skilled fisheries scientists. Seven research stations are located across the state. Each has a particular field of investigation. Five teams focus on fisheries in a specific Iowa water resource: natural lakes, reservoirs, small impoundments, interior streams, and the Mississippi River. The other two teams improve fish culture practices used in DNR fish hatcheries and investigate new ways to use technology to get information and data from fisheries managers to Iowa anglers.


Large River Investigations

24143 Hwy 52, Bellevue, IA 52031, 563-872-4976
Rebecca Krogman; Gene Jones; Royce Bowman

The Large Rivers team conducts research to answer critical questions to help our Fish Management Teams and partners manage the fisheries and habitat of large rivers and tributaries in Iowa.


Fish Culture Investigations

15053 Hatchery Place, Moravia, IA 52571, 641-647-2406
Alan Johnson; Steven Pecinovsky

The Fish Culture Research team evaluates potential solutions to problems hatchery staff face through carefully designed experiments. Culture research increases the efficiency and productivity of DNR hatcheries through research.


Interior Rivers and Streams Investigations

22693 205th Ave, Manchester, IA 52057, 563-927-3276
Greg Gelwicks; Greg Simmons; Jordan Vetter

The Interior Rivers and Streams team gather and share information needed to better manage Iowa’s stream and river fishery resources and maintain and improve fishing opportunities for Iowa anglers.


Completed Projects:


Technology and Data Management

1436 255th St, Boone, IA 50036, 515-204-8021
Jeff Kopaska

Technology and Data Management provides computer-oriented technical help to field personnel by developing, modifying, installing and maintaining a statewide database and other software and hardware for fisheries applications, software and hardware training, and updating and maintaining systems. Technology and Data Management strives to make information about fish, fishing, and fisheries more available and easily accessible to the public.


Completed Projects:

Reservoir Investigations

24570 US Hwy 34, Chariton, IA 50049, 641-774-2958
Madeline Lewis; Mark Richardson; Josh Goff

The Large Reservoir Research Team works with fisheries managers to identify and resolve issues affecting the fishery resources of large reservoirs. Large reservoirs provide many recreational opportunities throughout Iowa and include some of the state’s most popular fishing and boating destinations, such as Lake Red Rock, Rathbun Lake, the Creston area lake chain, and many others. Ongoing research projects ensure that Iowa’s reservoirs are managed with the best available science.


Completed Projects

Interior 1 + Response of Fish and Habitat to Stream Rehab

Stream habitat is a key factor influencing the health of stream fish populations. Iowa’s river and stream fish resources have been greatly impacted by habitat degradation. Concerned with the continued degradation of river and stream habitats and fisheries, Iowa resource managers are interested in using stream rehabilitation practices to effectively improve these resources. This study began in 2010 to evaluate Iowa river and stream rehabilitation practices, and develop management guidelines to improve river and stream habitat, as well as fishing opportunities for Iowa anglers.

Rock arch rapids on the Turkey River at Cresco.

The first project being evaluated is the modification of the Vernon Springs Dam on the Turkey River at Cresco. The dam was converted into a series of rock arch rapids in late July 2010 to address safety and fish passage concerns. Pre-construction fish community and habitat sampling was done at three sites above the dam and two sites below. Over 3,900 game and non-game fish were marked below the dam to monitor fish movement over the new structure. Fish community and habitat sampling was also done at three sites on the Volga River to serve as control sites for the three upstream sites on the Turkey River. Post-construction sampling upstream of the project found 16 Black Redhorse, 11 Golden Redhorse, 3 Walleye, and 1 Northern Hog Sucker that moved upstream over the structure. Smallmouth Bass and Black Redhorse were sampled post-construction above the dam at sites on the Turkey River and N. Branch Turkey River where they were not found pre-construction.

Pre-project fish and habitat data were collected in 2012 and 2013 for a dam removal on the Shell Rock River in Rockford. The dam was removed in the winter of 2014 and four years of post-project sampling have been completed. Golden Redhorse and Northern Hog Sucker were collected for the first time at sites above the dam in 2014, and increasing numbers of these species were found upstream in 2015, 2017, 2018, 2019 and 2021.

Channel Catfish numbers also increased at sites above the former dam. The project has restored riverine habitat in the former impoundment and improved the fish community (increased number of species, adult Channel Catfish abundance, and Smallmouth Bass abundance). Habitat conditions in the former impoundment are similar to the downstream site and the upstream control site in terms of stream width, mean depth, mean water velocity, and substrate composition. Dam removal negatively impacted downstream habitat initially, but habitat conditions largely recovered within 3-4 years post removal.

A whitewater park and habitat improvement project was completed in spring 2015 at the site of the Marion Street Dam on the Maquoketa River in Manchester. Pre-project fish and habitat sampling was done at sites upstream and downstream of the dam in 2012-2014. Over 19,600 fish of 19 species were marked downstream of the dam to monitor fish movement over the new structures. Sampling in 2015-2021 found 826 marked fish representing 12 species that had moved upstream over the structures. Continued monitoring of these projects and investigations of additional stream rehabilitation projects will help guide future decisions and lead to improved methods, designs, and sharing of resources to improve Iowa’s river and stream fisheries.

Interest in modifying and removing aging, low head dams on Iowa’s interior rivers has increased over the past several years. This interest is driven by safety/liability concerns, deterioration of existing dams, and a desire to increase river recreation opportunities. Areas below dams often are popular fishing spots. A common concern is that removing or modifying a dam will negatively impact angling, particularly below the dam. The impact of removing or modifying a dam on angling has not been studied in Iowa, and little information is available from other states.

The newly constructed whitewater park on the Maquoketa River in Manchester.

A whitewater park and habitat improvement project was completed in spring 2015 at the site of the Marion Street Dam on the Maquoketa River in Manchester. The dam was removed and six structures were built to create whitewater features, while also letting fish to pass upstream. The project was expected to improve angler access and fish habitat at the site. A roving angler survey was started in April 2012 to collect pre-project data on angler use, catch, and harvest on the Maquoketa River upstream and downstream of the dam. The survey also assessed other recreational uses like boating, tubing, and swimming. The survey was conducted in April-October for three years before and three years after construction. Total fishing effort (angler hours) was variable ranging from 4,232-6,797 pre-project and 3,770-7,597 post-project, and average fishing effort was similar before (5,267) and after (5,180) project construction. Average catch rates (number of fish/hour) were also similar before (1.47) and after (1.64) project construction, and varied annually from 1.14-1.95 pre-project and 0.92-2.56 post-project. Hours spent on the river by other recreational users in June-August averaged five times higher overall and 20 times higher in the project area during the post project period. Despite this significant increase in other recreational uses, our study found that angler effort and catch rates before and after construction were not significantly different and angler satisfaction has increased.

Measuring the impacts of removing or modifying a dam in Iowa will provide information to help managers address angler concerns with future projects. This information may also help identify project features which benefit anglers that can be incorporated into future projects.

Walleye fingerling stocking has greatly increased Iowa’s interior river Walleye populations over the last 20 years. This has created an increasingly popular fishery that has brought Walleye fishing opportunities close to home for many Iowa anglers. The success of this program has also increased demand for two inch long, Mississippi River strain Walleye fingerlings. Limited hatchery space has made it difficult to consistently produce enough fingerlings of the size and genetic strain requested for the program. Providing information needed to more efficiently use our limited hatchery production capacity, and exploring the potential of other fish culture systems to meet the demands of the river Walleye program is the focus of this study.


Available pond culture space has been a limiting factor for producing Mississippi River strain fingerling Walleye to stock in interior rivers. Recent research at the Rathbun Fish Culture Research Facility has shown promising results raising Walleye fingerlings using an alternative method, intensive fry culture. Intensively reared walleye fry are stocked into recirculating tanks and trained to eat formulated feed from day 1 post-hatch, instead of stocking them into ponds where they eat zooplankton (extensive culture). Evaluating the relative contribution of intensively reared fingerlings to interior river Walleye fisheries will determine if this production method could help further improve river Walleye fisheries.


Study sites were selected on four Iowa rivers to evaluate the contribution of intensively reared Walleye fingerlings to interior river Walleye populations. Extensively reared fingerlings were marked, hauled, and stocked alongside intensively reared fingerlings to serve as a control. Walleye fingerlings produced by this culture method are known to survive and contribute to river Walleye fisheries if river conditions are favorable. Intensively cultured Walleye fingerlings were marked with a circle freeze brand, and extensively cultured fish were marked with a bar brand. Over 61,000 marked intensively and extensively cultured walleye fingerlings were stocked in the Wapsipinicon, Maquoketa, Shell Rock and Cedar rivers in June 2016. Study sites were sampled during late-September and October to determine survival and growth of walleye fingerlings. This process will be repeated for several years. The results will help guide Walleye fingerling production and stocking methods to provide the greatest benefits for sustaining and improving Walleye fisheries in Iowa rivers.

Wild trout have played an increasingly important role in trout management in Iowa over the last 20 years. Recent increases in self-sustaining trout populations have expanded and diversified opportunities for Iowa anglers to pursue trout. A major factor in this increase is the use of fingerling stocks derived from wild and local parents to establish wild trout populations in other streams. Fisheries managers have had great success establishing self-sustaining populations of non-native Brown Trout by stocking fingerlings of French Creek origin. This has diminished the need for hatchery production and stocking of this popular species, and provided new recreational fishing opportunities for Iowa trout anglers. Self-sustaining populations of Brown Trout have expanded so rapidly in Iowa that their full extent is currently unknown. Wild populations of native Brook Trout have also been successfully restored to several northeast Iowa streams by stocking fingerling Brook Trout of South Pine Creek origin.

The South Pine Creek Brook Trout population provides a limited resource for propagation of fish for stocking, so it is important that restoration stocking is done on streams where there is the greatest probability that Brook Trout populations will be successfully established. The recent expansion of wild Brown Trout populations in northeast Iowa has raised concerns for fisheries managers, due to potential negative impacts of the species on Brook Trout restoration efforts. It is important to know the distribution of wild Brown Trout populations for managing Brown Trout populations and planning wild Brook Trout restoration efforts.

The goal of this project is to assess the status and distribution of Brook Trout and Brown Trout in northeast Iowa, and identify cold water streams where wild Brook Trout restoration has a high probability of success. Our general approach is to identify likely cold water streams using winter satellite images taken during very cold periods to find stream reaches that do not freeze over and are likely to have good cold water spring flow. We then select sites from these likely cold water reaches where we can sample fish and habitat conditions.

We have sampled 20 sites in four sub-watersheds of the Upper Iowa River. Brown Trout were sampled at 16 of these sites. Brook Trout were sampled at one site, and a Brook Trout X Brown Trout hybrid was sampled at one site where Brown Trout were also collected. The four sites where no trout were collected had the lowest stream flows (<0.2 cubic feet/sec) of our sample sites. Several sites identified from winter satellite imagery were either completely dry, or had very limited spring flow which was not sufficient to support fish during late summer. Conditions at 15 of these sites were documented with georeferenced photographs of dry reaches, beginning and end points of reaches with water, barriers to fish movement, and spring sources. This information will be used to help further refine site selection using remote imagery.

Technology and data management efforts are used to make information easily available to the public and help fisheries staff be more efficient and effective in their work. Fisheries staff collect information, such as fish numbers and sizes in lakes and rivers or numbers of fish stocked. This information is stored and analyzed in many data systems that the public can view online. To make all this work, the technology and data management team creates and modifies data systems, trains staff in their use, and regularly updates the computer systems that provide information to anglers and the public.

This work is important because the internet is the tool agencies use to share information with the public. More than 40% of Iowa anglers visit the Iowa DNR web site often. It is necessary to continuously improve the web site and the information it provides to keep current with new technologies.

Activities associated with this work have been started to develop, modify and maintain databases which store large fisheries data sets. Interpretation of these data provides the basis for making effective management decisions. Products of this work include the following:

  • the ability to immediately buy a fishing license on your mobile device
  • a search tool that integrates with a mobile device to tell you the closest store-front location to buy a hunting or fishing license.
  • lake, river, and trout stream web pages which include stocking data, waterbody specific fishing regulations, fishing reports, fishing forecasts, lists of amenities, contact information for fisheries biologists and conservation officers, and links to maps and fish survey data
  • online tool to submit Master Angler-qualifying fish
  • an online Fishing Atlas for mobile devices that combines updated contour maps and habitat structure maps for individual lakes
  • a statewide fish habitat structure location file that can be loaded onto a GPS
  • online maps that show statewide data sets of trout streams, paddling routes, boat ramps, fishing access points, fish habitat structures and fish cleaning stations

The DNR Fisheries Bureau is funded only by the sale of fishing and hunting licenses and equipment. Continued license sales are critical to provide the funding needed to manage Iowa fisheries. After years of simply selling fishing licenses, the DNR began a partnership with the Recreational Boating and Fishing Foundation (RBFF) in 2005 to assess fishing promotional efforts. These efforts were started in response to declining or fluctuating fishing license sales. Fishing promotional campaigns have varied from localized efforts to statewide and from specific target populations to all anglers. Using new strategies to sell more fishing licenses makes financial resources available to maintain and improve fishery resources.

Radio and television advertising, live events, movie theater advertising, magazines, letters, postcards and emails have been used to promote fishing to potential license buyers. An evaluation of fishing promotion efforts from 2005 to 2013 showed that lift (or increase in license sales associated with the marketing approaches) ranged from 0.1% to 4.6%. License sales patterns show that weather and economic conditions may strongly influence fishing license sales.

The results from these analyses support recommendations and suggestions for future fishing promotional efforts and highlight the uncertainty of major drivers of license purchase rates. Results from these assessments include:

  • inexpensive “renew your fishing license” postcards can be effective
  • email reminders following mailings yield results and is cost-effective
  • thousands of inconsistent anglers would fish more often if a friend or relative invited them
  • weather often has a major influence on a person’s decision to buy a fishing license

Future assessment activities will continue to look at promotional efforts, as well as community fishing opportunities like the urban trout stocking program. The DNR is also starting an aggressive Recruitment, Retention, and Reactivation (R3) program for hunters and anglers, in partnership with sporting goods vendors and local conservation groups. The research done thus far, and building better partnerships, should provide more and better fishing opportunities and fishing locations closer to home for more Iowans.

In 2016, the Iowa Department of Natural Resources (DNR) conducted a comprehensive mail and online survey to evaluate the trout fishing activities and preferences of anglers fishing for trout. The Iowa DNR conducts this survey about every five years; similar surveys were conducted by telephone in 1975, 1980, 1986, 1991, 1996 and 2001, by mail in 2006, and by mail and online in 2011. A total of 3,605 angler surveys were completed, equaling 7.7% of the 46,604 anglers who purchased trout fees in 2016. Mean age of all trout anglers was 43.8 years, which is similar to what was observed in 2011.

Licensed trout anglers spent an estimated 489,455 days trout fishing in Iowa and made 720,611 trips to individual trout fisheries in 2016. Total annual angler trips were determined for each catchable, special, urban winter pond, and put-and-grow trout fishery in Iowa. North Bear (21% of trout anglers), South Bear (17% of trout anglers), Trout Run (13% of trout anglers) and Bloody Run (12% of trout anglers) were the top four most heavily used fisheries. Put-and-grow streams had the least angling use and ranked the lowest including Turner (94th), Monastery Creek (91st) and White Pine Hollow (88th). Streams with the highest number of angler trips per mile of stream open to public fishing were Baileys Ford (30,836 trips), Trout Run (Winneshiek County) (21,450 trips), Joy Springs (14,804 trips), Richmond Springs (13,808 trips), Turkey River (13,210 trips) and Twin Springs (13,048 trips).

The average trout angler spent 11 days fishing Iowa’s trout waters. Overall, trout fishing activity days, angler trips, and mean days and trips per angler were at or above 2011 levels and comparable to previous years. The percent of anglers fishing and total trips taken to special urban trout fisheries have increased significantly since 2001. Fishing pressure on the urban winter trout fisheries in 2016 increased to 99,444 trips from 70,202 in 2011, 48,868 in 2006 and 12,920 in 2001. Trips to urban winter trout fisheries increased to 13.8% of all trout angler trips in 2016 from 12% in 2011 and 9% in 2006. The number of urban fisheries available expanded to 8 locations in 2006 and 17 in 2011 and 2016. Heritage Pond, Prairie Park Pond, Terry Trueblood Lake and Ada Hayden Lake ranked the highest in estimated angler trips to winter urban trout fisheries. Thirty percent of trout anglers purchased a trout fee specifically for an urban trout fishery.

Angler satisfaction with the trout program was ranked at 8 on a scale of 1 to 10, exactly the same as the 2011 survey. Angler responses to questions about the published stocking schedule broke down in a geographic pattern. Anglers from northeast Iowa’s trout zone and non-resident trout anglers were least likely to check the announced stocking schedule (39%), while anglers in areas with only winter stocking were most likely to check the stocking schedule (58%). Anglers who do check the stocking schedule generally use this information to fish the stocked water body (72%-84%). Most anglers are satisfied with the current amount of announced stockings (59%-67%). Anglers from the trout zone are more likely to avoid streams that were recently stocked than other resident anglers. While the majority of anglers prefer that the stocking calendar remain the same, anglers from the trout zone prefer fewer announced trout stockings. This information, combined with the budgetary issues of maintaining a rigid stocking schedule, could be used to justify reducing the number of announced trout stockings on northeast Iowa streams.

Harvest regulations are commonly used to manage fish populations and achieve specific fishery goals set by managers and stakeholders. Such regulations are most often used to manage sport fish populations, such as black bass, walleye, and muskellunge, but may also be used to protect commercial fisheries (e.g., channel catfish).

Management goals often differ by lake and species, with some populations managed to achieve large, trophy-sized fish, some to achieve a high catch rate by anglers, and others for biological control of forage species. Regardless of management objectives, harvest regulations are an invaluable tool to help managers achieve population-specific goals, provide quality fishing for anglers and stakeholders, and ultimately protect fish populations and ensure their viability in the future.

The Iowa DNR recently adopted a new database system for fisheries managers based around a statistical program called R Studio. With the transition to this new system, fisheries managers will be able to complete analyses in a more streamlined way. Without a regulation simulation program compatible with R Studio, however, assessments of potential management options cannot be integrated into the new system. By developing a simulation program specifically for R Studio, we aim to fill this gap and provide our fisheries managers a tool that fits seamlessly into their existing database system and helps facilitate data-driven decisions.

This study will allow fisheries managers to easily calculate population dynamics and simulate harvest regulations for individual fish populations, facilitating more responsive, science-based decisions about fishery management. Recently, Iowa DNR established greater flexibility for fishery managers to adjust harvest regulations at specific waterbodies. This study will provide a useful analytical tool for fisheries managers to justify changes.

By programming the software in R Studio, our analysis tool will not only be usable for Iowa DNR staff, but also be publicly available, allowing for a wide range of fisheries professionals to benefit from the study’s output.  Iowa DNR recently converted to Cloud-based standard data storage. This study’s product will be designed to use standard datasets already being collected by fisheries managers, minimizing extra data manipulation and custom programming by individual fishery managers. This makes evaluation of harvest regulations more efficient and standardized in methodology across the state, contributing to defensible, responsive decision-making.

Iowa DNR regularly evaluates lake-based recreation to better understand how anglers and other recreational users use our public natural resources. Mailed surveys were conducted about lake-based recreation beginning in 2002, with the most recent survey in 2019. Unfortunately, mail-based survey response rates have been declining, so alternative approaches must be investigated before the next survey to get better information.

Evaluation of lake and reservoir recreation, especially fishing and boating, guides fisheries management in Iowa by establishing baseline expectations by lake type; highlighting key waterbodies in need of fisheries management action such as renovation; highlighting key waterbodies in need of protection and maintenance of stellar fisheries; measuring effects of renovations; and understanding the dynamics of visitation in socio-ecological systems. The objectives of this study are two-fold: 1) to evaluate historical data and understand any trends, and 2) to design the next survey to maximize effectiveness and data quality.

The Iowa DNR has been restoring lakes using a watershed approach since the early 1990s, starting with Lake Ahquabi. The program’s impact increased when Iowa’s legislature approved $8.6 million in yearly funding in 2006. Restored lakes in Iowa usually have strong public support and benefit from increased recreational use. Unfortunately, the new lake effect can wear off quickly as fish populations stabilize, and aquatic vegetation control issues start taking more time and money to manage.

This project was started in 2014 to study which lake restoration factors, planned and unplanned, lead to the fastest fishery recovery and popularity with anglers, as well as which fishery management strategies work best. Creating or improving habitat, new fish stocking strategies, and adding public access or recreational facilities must be evaluated. For example, although constructed in-lake habitat helps to group fish for anglers, which type of structure is most often used for fishing has not been identified. This information would let managers install the type of structures that anglers use the most.

The Iowa DNR uses angler surveys to measure the success of constructed habitat and other pieces of lake restoration. These on-lake surveys can help determine fishing success, angler satisfaction, recreational use and economic impact before and after a restoration. Angler surveys are being done at Easter Lake (major renovations began in 2015), Green Valley Lake (restored in 2009), Twelve Mile Lake (restored in 2006), Three Mile Lake (fishery renovated in 2016), Hickory Grove Lake (scheduled for future restoration), and Thomas Mitchell Pond (dredged in 2011).

As the Lake Restoration program grows and continues to invest in Iowa’s lakes and reservoirs, the method to prioritize and select lakes becomes very important. The first list of program priority lakes was based on potential for public benefit, ecological health, and overall project feasibility. These categories are important, but several new or greatly expanded datasets are available that were not before. These datasets include the Iowa Lakes Information System, Iowa Lakes Survey, and improved geographic information from high-resolution photography (LiDAR) and detailed lake mapping. A decision-making model that incorporates data on ecological health (measured by water quality, biological integrity, and habitat quality), potential for public benefit, and project feasibility is being developed.

Creel surveys have been a valued source of fisheries information guiding management since the earliest days of formal fishery management in the U.S. In Iowa, creel surveys have been used for estimating fish population size, measuring impacts of fishery restructuring and habitat restoration, and determining the effectiveness of and compliance with fishing regulations. Creel surveys are also used for long-term fishery monitoring to determine overall catch, harvest, fish size, and angler satisfaction. In some cases, creel surveys have yielded long-term datasets at important fishing locations like Clear Lake, Spirit Lake, West Okoboji Lake and Rathbun Lake. Iowa DNR has consistently used an established method for analyzing creel data, but never had a standardized analysis tool.

The Iowa DNR must update its approach to standardized creel data collection, storage, and analysis. Without this effort, the current system could become defunct without warning and hinder ongoing creel surveys (e.g., if the data collection application stops working, or a Microsoft update alters the object library used by Access). Its difficulty for fisheries managers to access may reduce interest in conducting standardized creel surveys, resulting in a relapse to reliance on outdated, non-standard analytical tools.

Without developing a new tool with a more robust programming language, we are unable to improve upon or expand the analysis to align with current guidelines for creel surveys derived from the literature (Nieman et al. 2021; Lynch et al., in revision). The data collection and storage issues are already being addressed outside of the Sport Fish Restoration grant program, making developing an analytical tool compatible with the new system extremely timely.

This study will facilitate the continued standardization of quality creel surveys across Iowa by providing a useful tool for fisheries biologists. By making analysis and reporting easier, the tool will encourage biologists to conduct surveys following the standard design, analyze data using identical expansion and summary methods, and report findings in similar formats. Although in many ways, Iowa DNR staff have aligned with each other in methodology and much of their reporting for years, this analytical tool will automate that process, rather than forcing each biologist to accomplish the same work independently. This makes the creel survey analysis and reporting process more efficient, allowing biologists to spend more time on other work.

Walleye fisheries are mainly sustained in Iowa’s reservoirs through stocking, rather than natural reproduction. Unfortunately, even intensive stocking of high numbers of fish can fail due to weather and water conditions at the time of stocking, competition with other young fish, high overwintering deaths, and lots fish escaping over the dam and downstream. Low survival of stocked fry and fingerling-sized Walleye can be a major barrier to maintain the adult densities that fisheries management biologists target.

A DNR fisheries research biologist holding a large walleye.The success of fish stocked may depend on physical and behavioral characteristics unique to the genetic strain. Lake-strain Walleye, adapted to lake and reservoir life, and river-strain Walleye, adapted to moving waters, may perform differently in different environmental conditions. A study, started in 2019, compares the two strains when stocked into reservoirs with varying physical and environmental characteristics. The first year was dedicated to careful study design, with many reservoirs across the state examined for their fit as a Walleye stocking location. A laboratory method for identifying genetic strain from tissue samples was also developed. Subsequent years will involve stocking Walleye fry of both genetic strains, followed by field sampling to determine survival and growth over time.

If Walleye strains differ in long-term survival and recruitment to the fishable stock, then strain choice becomes an important factor in stocking requests for lakes and reservoirs. Guidance about optimal strain choice will be developed, with clear indication of when specific stocking choices are untenable. This guidance will be provided in formal stocking recommendations designed to maximize the effectiveness of Walleye stocking in inland waterbodies. This reduces hatchery production costs while maximizing the availability of Walleye fishing opportunities across Iowa, providing the most preferred fishing opportunity to as many people as possible for as little cost as possible.

Dams are substantial barriers to fish movement in both directions, and most dams in Iowa have no fish passage infrastructure (e.g., fish ladders) to help fish move upstream. Large dams impounding major rivers, such as Red Rock Dam and Saylorville Dam, prevent most upstream movement with a sheer drop of up to forty feet. Shorter dams, like Ottumwa Hydropower Dam, may allow passage upstream during high flows. The true extent of fish movement upstream and downstream by sport fishes is unknown, and the potential for death caused by dam passage or passage through hydropower facilities is unknown.

A new hydropower facility will be working soon (estimated 2020) at Red Rock Dam, and additional hydropower facilities are possible at other locations across the state. A substantial change from hydropower facilities is the deviation of some flow into large turbines to produce power. Fish movement out of Red Rock Reservoir may then happen through the dam’s regular operations, flood operations, or the power plant. Turbine passage has been shown to cause physical damage and disorientation of fish passing through, resulting in increased deaths. Determining the potential effect of dam and turbine passage on sport fish species, especially larger-bodied fish, is important to understand the relative impact of hydropower development on fishery management.

At a broader level, large-scale fish movement in the Des Moines River is not well-studied. Tracking fish movement from the upper river downstream can help quantify the risk of fish encountering each dam and passing through various habitat types.

The information gathered from this study will help fisheries managers determine the current and potential impacts on their reservoir sport fisheries of hydropower development and of large dams in general. Fish escaping is a common and challenging issue faced by most fisheries managers, and a better understanding of conditions with high escapement can help them take mitigative actions (e.g., recovering fish in the tailwater at targeted times of the year) or adjust their fishery development tactics (e.g., stocking plans). Most importantly, the Department as a whole will be able to provide more scientific input about dam and hydropower facilities proposed in the future.

Tailwaters are the areas immediately downstream of dams affected by dam outflow. Tailwater fisheries provide high-quality fishing opportunities below large reservoirs including Lake Red Rock and Saylorville Lake. Lake Red Rock supports Walleye, White Bass, and catfish fisheries in its tailwater and is the largest upstream barrier to Shovelnose Sturgeon in the Des Moines River.

Environmental conditions related to water flow and quality sometimes lead to fish kills. Previous fish kills in the lower Des Moines, including a significant sturgeon kill in 2012, were caused by rapid reductions in reservoir water levels, high dissolved gases, high tailwater temperatures, and low flow rates downstream. Nonetheless, fish kills have occurred during high flows as well, and more information is needed to identify water level scenarios and tailwater conditions that contribute to large-scale fish mortality downstream.

This ongoing assessment is designed to monitor environmental conditions and help identify situations with high risk of causing fish death downstream. Evaluation of fish kills would help identify specific environmental conditions that create higher risk for fish death, and water level management can be adjusted to reduce that risk. Improved tailwater management can support a healthier fishery with fewer large-scale losses of sport fish and forage species. Some tailwater fisheries, such as Walleye, are also greatly dependent on stocking, and reduction of fish kills directly supports a long-lived hatchery product. Recommendations for water level management can be applied not only to current reservoir projects, but to future additions such as new hydropower facilities. The assessment was started in 2019, but will require ongoing, annual monitoring.

Large River 2 + Completed Projects

ManMade 1 + Factors Affecting Mercury Concentration in Iowa

Food consumption advisories are more common now than 10 or 20 years ago. People are more aware of what they put into their bodies and what can be harmful. Most people have heard about mercury, and usually associate it with marine fish species. This pollutant can also be found in freshwater fish species in Iowa and may affect the at-risk population of people consuming these species. Children under 12 years old and women that might become pregnant, are pregnant, or breastfeeding should be aware of fish consumption advisories.

Iowa DNR promotes eating fish and studies show fish should be part of a healthy diet. Iowa anglers consumed an estimated 4.59 million meals of Iowa caught fish in 2018. Currently fish consumption advisories are issued on a lake by lake basis. A small portion of lakes and rivers in Iowa are sampled each year and results are used to post consumption advisories about how many meals a week anglers should eat from the specific waterbody.

In 2012, a more robust sampling schedule was set as part of a study to look at mercury contamination in fish found in Iowa. The goal was to sample a variety of lakes and rivers with all of the fish species in question and provide statewide advisories if needed. This study provided more information about mercury contamination in Iowa and itnalso generated more questions to answer before simple and easy to follow statewide advisories can be implemented.

The study found mercury contamination is higher in predator species like largemouth bass, flathead catfish, and muskie. Lower trophic level fish that eat mostly plankton and invertebrates, such as bluegill and perch, have little or no accumulated mercury and are safe to eat.

The goal of the new study started in 2019 was to sample 20 fish species from lakes and rivers. The sampling schedule was designed to collect 2,400 fish in three years. Targeted lengths were selected based on results from the earlier study. So far, 1,612 tissue samples have been collected from lakes and rivers. The additional samples collected in this follow up study will provide Iowa anglers with safe fish consumption recommendations for at-risk populatons.

ManMade 2 + What is the Annual Sediment Accumulation in Iowa Lakes and Why is it Important?

Advancements in technology, specifically Global Information Systems (GIS), have provided new tools and methods to measure sediment accumulation in Iowa lake basins. All of Iowa’s significant public owned lakes (SPOLs) were surveyed between 2006 and 2016 to measure lake depth at a known elevation or lake level. Contour maps, similar to topographic maps, were created and provided for several user groups working and recreating on Iowa lakes. Engineers, management biologists, and anglers are some of the users that request the available depth data found on the Iowa DNR website. The known elevation is a bench mark allowing multiple surveys to be compared over time to compare apples to apples and measure accumulated sediment in lake basins.

Surveys completed in 2012 can be compared with more recent surveys to calculate accumulated sediment in lake basins. Why is this important? Accumulated sediment can negatively affect water quality, change fish composition and size structure, and increase vegetation growth that have negative effects on recreational activities.

During the last five years, 35 lakes have been surveyed for a second time. The initial surveys were completed about 10 years ago. We found sediment accumulation in these lakes to be between .35 percent and .64 percent each year. This means a 100 acre lake with a volume of 700 ac-ft can gain between 430 and 650 dump truck loads of soil each year depending on the size of its watershed and land use types. The accuracy of these sediment accumulation values is between 4 percent and 6 percent based on our results from this study. After this study is finalized, we will evaluate models used to estimate accumulated sediments and help improve longevity of restoration projects and benefit anglers.

ManMade 3 + Best Management Practices for Community Fishing

Iowa’s Community Fishing Program started in 2016 to increase angling participation and opportunities within Iowa’s larger cities. New initiatives in Iowa, such as the Community Fishing Program, aim to bridge the gap between natural resources and urban populations and increase angler participation. Over 200 ponds can be found in these populated areas ranging in size from 0.5 to 232 acres, all of which are located within 38 communities. Several ponds are added to this list each year as new developments are constructed. Managing all of these systems for fishing requires a different set of tools than traditional methods used for rural ponds.

During 2022, we collected trip information about anglers using urban ponds. Urban ponds were located in business parks, near schools, incorporated into parks with amenities, and part of neighborhoods. Preliminary analysis showed more fish were released than harvested, meaning people just wanted to catch fish to have a successful trip. The majority of angling in neighborhood ponds was by kids less than 16 years old. The average trip length was shorter in these community fisheries compared to rural ponds, urban lakes were between .7 and 1.1 hours. Many of these urban ponds are in backyards or only a short walk away from homes, therefore short trips would be expected. Angling effort can also be up to 10 times higher in urban ponds compared to rural locations. Urban ponds were similar to rural ponds regarding sport fish quality ranging from poor to excellent.

Even though sport fish population collections are similar between rural and urban locations, fish sampling gear types may need to be changed. Some pond basins in urban locations were built as water control structures and didn’t consider angler access. Next year the Small Impoundment research team will collect additional data and provide managers with the appropriate tools to provide urban anglers with quality experiences.

ManMade 4 + Age and Growth of Bluegill in Iowa

In a recent survey, 76 percent of Iowa anglers rated the Iowa DNR as excellent or good in managing fishing and fisheries in Iowa. Fisheries managers need up-to-date information on fish populations to maintain this high level of angler satisfaction.

In the early 2000’s, the Iowa DNR collected bluegill age and growth information statewide from Significant Public Owned Lakes (SPOLs). Since then, a statewide bluegill bag limit was established and numerous fisheries were renovated through the Iowa DNR’s Lake Restoration Program. Bluegill age-growth information was collected sporadically during this period, primarily for special projects; but, a similar statewide assessment has not been completed for almost 20 years. This information is needed to evaluate the many management actions during this period.

Measuring bluegill length.


Bluegill populations were surveyed in 23 lakes in the fall between 2020 and 2022. Age structures were collected from approximately 1,725 fish between 3- and 10-inches. These fish were also weighed, measured for length, and sex was determined for mature individuals. Results show males grow slightly faster than females, and the maximum age collected to date for this study is 11 years with the majority reaching 8 years. Length-at-age data also suggested a difference in fish size from northern latitude to southern latitude, fish were larger in southern impoundments than northern climate impoundments.

Lakes sampled in the next two years will add to these results and assist management biologists in making science based decisions for managing bluegill in Iowa.


ManMade 5 + Relative Contribution of Bluegill Stocked into New and Renovated Lakes

Bluegill are a popular sport fish in Iowa with 58 percent of anglers indicating they fished for bluegill in a 2019 survey. Bluegill are found in most Iowa impoundments and are usually part of the first stocking in new or renovated lakes and ponds. When the Iowa DNR restocks bluegill after a lake renovation, the goal is for 8-inch bluegill to be available to anglers within four years after restocking.

This goal is achieved in some cases when water quality is improved, but there are exceptions. When these exceptions occur and are not explainable, fisheries managers question if the broodstock source might be a contributing factor. Bluegill stocking rates have been reduced in recent years to keep the initial year-class following a renovation from getting overabundant before the first year-class of largemouth bass can be established. If this initial bluegill stocking is supplemented from another source (e.g., fish escaping from ponds in the watershed or illegal stockings), it may result in density dependent growth that prevents bluegill from achieving the desired growth rate. Some bluegill sources may have genetics that lend themselves to faster growth.

A boy holding a string of bluegill.


During the 2020, 2021 and 2022 sampling seasons, bluegill genetic samples were collected from fin clips.These fish were collected from multiple lakes representing five different bluegill populations and progeny from two fish hatcheries. Results showed bluegill populations were different between the three lakes. There is also evidence that certain genes may contribute to growth. Additional testing will be conducted to confirm these results.

Information from this study will be used to guide future bluegill restocking efforts following lake renovations. The ultimate goal for bluegill fisheries after renovation is to provide anglers with desirable-sized fish as soon as possible after a fishery renovation.

ManMade 6 + Evaluating the Relative Contribution of Advanced Fingerling Walleye Produced in a Recirculating Aquaculture System

Walleye are a popular sport fish in Iowa, ranking in the top two most fished for species. Walleye can be caught in lakes, interior rivers, and large rivers. Walleye are stocked into lakes as fry, fingerling, and advanced fingerling sizes to maintain quality populations across the state in lakes and rivers. Advanced fingerling fish are the largest of the three sizes ranging between 6-inches and 12-inches long and cost more to raise than the other two sizes of fish. When these fish are stocked, it is important to see them survive and grow so anglers have an opportunity to catch them or they become part of the broodstock population in Iowa lakes.

New fish rearing methods have been developed with the nationwide spread of aquatic invasive species (AIS), such as zebra mussels. These invasive species can cause significant damage to native ecosystems and infrastructure. Fish hatcheries can combat AIS species by switching from a flow-through system to using a recirculating aquaculture system (RAS). This type of system uses clean water free of AIS species which reduces hatchery costs and problems associated with hauling fish to stock in lakes.

Rathbun Fish Hatchery, located in Southern Iowa, can raise walleye in a RAS from egg to advanced fingerling size and avoid AIS problems, but these fish have not been evaluated after stocking into a lake to see if their survival is similar to advanced fingerling walleye raised in a traditional flow-through system. In 2021, walleye were implanted with radio tags to monitor survival over 8 months. Results showed walleye reared in a RAS had similar survival rates to walleye reared in a flow-through system. This radio telemetry project also showed RAS fish moved greater distances between tracking events. 

In 2019, 2020, and 2021 three different advanced fingerling walleye products were stocked into lakes in October and then sampled in 2020, 2021, and 2022 to evaluate how each product survived after stocking. Sampling results in 2020 and 2021 for age-1 fish were dominated with fish grown in a flow-through system. Walleye reared in a RAS and collected in 2022 at age-1 were sampled at a significantly higher number than in 2020 and 2021. Sampling results for age-2 Walleye were dominated by fish grown in a flow-through system and RAS fish were sampled in low numbers. These results suggest a quality RAS product can survive in lakes and more work is still needed to improve our understanding about the contribution of RAS reared Walleyes in Iowa lakes.

FCI 1 Better Disease Management in Fish Production

Effective disease management and treatment at Iowa fish hatcheries is essential in raising quality fish for Iowa fisheries. Diseases can lead to fish mortality and reduce growth rates. In 2006, the Iowa DNR initiated a new project to dedicate more attention to obtaining approved hatchery drugs. Efforts will focus on improving prevention and management of Ich, a commonly occurring disease effecting our walleye production.

The Rathbun Fish Hatchery uses raw lake water that has not been disinfected in the tanks used to grow-out walleye. Since the water has not been disinfected, Ich infestations are common. Preventative treatments with formalin were used in the past to manage infestations. Formalin treatments are very effective, but increase the cost of walleye production. Beginning in 2009, research was conducted in production tanks to evaluate the reduced use of preventative treatment and the impact on Ich infection and the amount of formalin applied. A nine-hour formalin treatment every other day when Ich is first detected at less than 15 Ich cells per arch then daily treatments when more than 15 cells are detected is the most effective and economical treatment plan tested.

In 2014 a continuous 24-hour formalin treatment at 30 to 40 ppm was compared to the standard treatment tested in 2009. When 15 or more Ich cells were detected, continuous treatment began and continued until fish were resampled 4 to 7 days later and no Ich cells were observed. This treatment plan eliminated Ich infestations in seven days or less and compared to the standard treatment that was applied for two or more weeks. However, the final cost and amount of formalin applied between treatment plans was similar.

This research established a monitoring and treatment system used by hatchery staff to apply formalin judiciously resulting in cost savings and production of healthy fish. This study continues to garner knowledge to help hatchery staff reduce the cost of producing large walleye fingerlings.

FCI 2 Walleye Culture Research

Currently, all of Iowa’s large walleye are produced in a tandem pond to tank culture method where fry are stocked in ponds and grown on natural prey items, then converted to dry feed and grown to eight inches. Raising fry on dry feeds in tanks without first starting in a pond has been evaluated at the Rathbun Fish Culture Research Facility; however, the success of these techniques needed to be tested in production-scale tanks. Culture methods and tank designs may need changes before hatchery staff use this technology.

In 2016, we compared phase I fry production in 275-L tanks using Mississippi River brood stock sources and fed fry pelleted feed at 5-minute and 10-minute intervals until 38 days after hatch. All tanks of fry, regardless of strain, had ongoing deaths from cannibalism, either attempted or successful, because feed rates did not meet the appetite of fast growing fry. River source fry fed at 5-minute intervals survived at a rate of 53.8% while fry fed at 10-minute intervals had a 64.0% survival rate. This difference in survival was significant. The final size of fry was similar at 40 mm and 0.60 g. All river source fry were freeze branded to identify them in a comparison of pond rearing and tank rearing walleye fingerlings as part of another fisheries research study.

The training of pond fingerlings to eat commercial feed had been a challenge to production of eight-inch fingerlings until 2006. However, in the past five years hatchery staff observed poor survival during the feed training time which was believed to be from an unknown change in the Walleye Grower 9206 diet. In 2016, we compared survival and growth of pond-reared walleye trained to eat Otohime feed and then converted to WG 9206, BioVita, BioPro2, or Gemma Silk feeds. Fish fed BioVita had a survival rate of 76% while fish fed the other diets had a survival rate of 60.2% to 76.3%, but the difference was not significant. Final length of fish fed BioPro2 and BioVita was much greater than that of fish fed Walleye Grower 9206 and Gemma Silk.

Diets and tank shape were compared in a growout study in the research facility. All fish were graded for uniformity before phase III tank stocking. Three round tanks and rectangular tanks were fed BioOregon diets (BioOlympic followed by BioTrout) and three round tanks and rectangular tanks were fed WG 9206. Death rate was much lower in rectangular tanks (2.4%) compared to round tanks (10.0 to 12.2%). Death was caused by Columnaris disease that infected eroded caudal fins of fish in round tanks. Caudal fin condition was scored on a scale of 0 to 3 with three being an intact tail with lower scores progressively more eroded. The lowest caudal fin score at the end of the study was in round tanks fed the BioOregon diets. Death due to caudal fin erosion was not seen in raceways. Fish fed WG 9206 in round tanks were significantly longer and heavier than fish in the other tank shape and diet combinations. More research is needed to determine the dietary link to higher rates of caudal fin erosion in walleye.

FCI 3 Best Management Practices for Hybrid Striped Bass Production

Iowa DNR staff gets hybrid striped bass fry from other states to improve fisheries and culture methods to offer Iowa anglers more opportunities. However, the supply of fry is limited and raising pond fingerlings has been inconsistent and below expectations. This study will examine concerns of moving fry and fish production techniques to develop a management plan to raise hybrid striped bass in plastic-lined (Rathbun Fish Culture Research Facility) and earthen (Mt. Ayr Fish Hatchery) ponds. The plan will include best management practices for: 1) moving fry, 2) timing of first stocking, 3) recommended stocking densities, 4) pond fertilization treatments, and 5) water quality management. In 2016, current best management practices were compared in earthen ponds at the Mt Ayr Hatchery. Production performance of Sunshine Bass fed once daily by hand or four times daily by feeder was compared in plastic-lined ponds at Rathbun Fish Culture Research Facility (Rathbun) with similar best management practices.

Sunshine bass fry were stocked at a rate of 140,000 fry/acre at Rathbun. Ponds 1, 4, and 5 at Mt Ayr were stocked on 10 May with Sunshine Bass from Keo Fish Farms at a rate of 248,306 fry/acre counted by volumetric method. An error in fry estimates in the stocking barrel resulted in those ponds being stocked with 77% more fry than the standard 140,000 fry/acre stocking rate. Ponds 2 and 3 were stocked on May 11 with Palmetto Bass fry at a rate of 144,927 fry/acre. A mixed fertilization treatment of alfalfa and soybean meal was used to increase pond productivity throughout the culture period in ponds that were not fed fish feed. In ponds that were fed fish feed, fertilization was stopped after day 14 when feeding began.

Survival rate at the Mt Ayr Hatchery was excellent with one pond having a survival rate of 29% and the other four ponds with survival rates between 42% and 95%. Feeding ponds at Mount Ayr resulted in larger fish size compared to ponds that were only fertilized. The higher stocking rate of ponds resulted in 889,331 fingerlings harvested, the highest hybrid striped bass fingerling production to date at the Mt Ayr Hatchery.

Harvest at Rathbun was delayed to allow for a 35-day period of offering pelleted feeds so that any trend in growth or survival between feeding methods would become apparent. Feeding rates were increased from the initial feeding rate of 8 lbs/acre to 16 lbs/acre during the last week of feeding. Survival ranged from 40.7% to 58.0% among all ponds. Ponds fed by hand had a survival rate of 57.1% while those ponds of fish fed by automatic feeder had 47.3% survival which was not a significant statistical difference. Mean length was similar between feeding methods but the variation in length was significantly greater for ponds of fish fed by automatic feeder four times daily compared to ponds of fish fed once daily by hand. This finding suggests that fish size varies more which leads to cannibalism and reduced production with four daily feedings using an automatic feeder.

Production plans for 2017 include evaluating the best management practices at the Mt Ayr Fish Hatchery with pond feeding to increase fish size. Rathbun Fish Culture Research staff will evaluate the relationship of feeding methods and fish performance to produce a better product for stocking into Iowa’s fisheries.

FCI 4 Recirculating Aquaculture Systems for Sport Fish

The Iowa Department of Natural Resources produces over 200,000, 6 to 9-inch walleyes each year intensively on formulated diets at the Rathbun Fish Hatchery (RFH) or Spirit Lake Fish Hatchery. Both hatcheries use surface water sources to produce these fish in culture systems that only use water once before being discharged. The surface water sources may carry viral, bacterial, and protozoan pathogens which can cause fish deaths. Additionally, surface water sources for both hatcheries are threatened by aquatic invasive species such as zebra mussels. Disinfection systems to stop the spread of pathogens could be expensive using current culture systems. Though surface water sources at both hatcheries are plentiful, water quality and the presence of undesirable organisms are challenges to fish production. One fish pathogen, Ichthyophthirius multifilis, costs $25,000 to $35,000 each year to control during walleye growout at RFH.

One solution to these problems may be use of recirculating aquaculture system (RAS) technology which uses a small amount of makeup water to replace water lost during waste processing. When compared to our traditional culture systems, which replaces 100% of its tank water every one to two hours, the RAS may replace only 5 to 10% of its water each day. To continually reuse water while raising fish at high densities requires special components to remove waste products. These components are: 1) self-cleaning circular fish tanks; 2) microscreen filter for solids removal; 3) water pumps; 4) biofilter for ammonia and nitrite removal; 5) CO2 stripping column; 6) oxygen and ozone contactor; and 7) ultraviolet disinfection unit for reduction of bacterial counts.

The use of RAS for sport fish production is a new trend among state agencies. Egg incubation to food size fish production in RAS has been well documented for many food fish species like trout and salmon. However, few studies have evaluated RAS for walleye production. A pilot-scale RAS was built at the Rathbun Fish Culture Research Facility in 2014-15 with the goal of testing walleye and other sport-fish production in this technology. Fish performance, system performance, and economics information gained will help the IDNR develop these systems for production scale use.

In July 2016, feed trained walleye fingerlings were stocked into three culture tanks in the RAS to test grow-out performance to the nine-inch fall fingerling size. After four weeks in the system, the caudal fins of some walleye became eroded and those fish later died. Fish samples were sent for diagnosis and a Flavobacterium species of bacteria was found on the caudal fin. The condition of caudal fins was scored on a scale of 0 to 3 with three being a complete tail with lower scores reflecting increasingly worse conditions of tail erosion. At the end of the study, 38% of walleye had a score of 3 and 62% had a score of 0 to 2. Final survival rate was 78% and fish were 8.1 inches long and shorter than fish produced at RFH because of the disease issue. Future research should evaluate disease treatments to stop bacterial fin erosion (e.g. hydrogen peroxide).

Walleye fingerling growout occupied the RAS for about three months, leaving enough time to disinfect the system, repopulate the biofilter, and restock the system with cold-water species to culture overwinter. Walleye were harvested in October, the system was disinfected, and rainbow trout were restocked in December. The RAS was restocked with eight-inch rainbow trout from Manchester Fish Hatchery that were grown to a catchable size (11 inches) and stocked into urban fisheries in Ottumwa and Davenport, Iowa.

This project will determine how many fish can be produced in a RAS during summer and winter production seasons, using species that grow better at summer and winter temperatures that reduce the need to heat or cool water out of season. In the end, a more efficient production system will be developed to enhance fisheries for Iowa anglers.

Large River 1 + Walleye and Sauger in Upper Mississippi River

Walleye and sauger support popular and important fisheries on the Upper Mississippi River bordering Iowa. Walleye continually rank high in angler catch and harvest in summer pool wide and winter tailwater angler surveys. Saugers are the majority of fish caught and harvested at tailwaters fisheries from October – March. This study was started because of concerns with high sauger death rates, highly variable reproduction in Walleye, and the desire to maintain and improve these important fisheries.

Concerns with deep water post-release hooking deaths of Sauger led to a study completed in 2012. Sauger were caught from the tailwaters of Guttenberg and Bellevue and held in a deep-water net pen to measure 72-hour death rates. Overall, hooking deaths was 18%, but rates increased with depth. Death rates were 7% for Sauger caught from depths of 20-29 feet, 17% from 30-39 feet, 25% from 40-49 feet, and 41% from 50 feet or greater. Sauger length was also found to be inversely proportional to depth. Larger Sauger were caught on average at shallower depths than small Sauger. Tailwater anglers can use this information to decide where they should fish. Fishing in deep water yields small Sauger; a large proportion of released fish will likely die. Fishing in shallower water yields larger fish and a greater proportion of released fish will survive.

Walleye reproduction on the Upper Mississippi River is highly variable with boom and bust years. Increasing and stabilizing reproduction would improve the consistency of the fishery. Seventy percent of Walleye eggs come from 20-27 inch fish. Protecting this size class to increase the number of eggs in the system may improve future reproduction. In 2004, a 20-27 inch release slot limit was started from Lock and Dam 11 in Dubuque to the Missouri border. Evaluation of the Walleye slot limit has shown an increase of 20-27 inch Walleyes in pools where the regulation is in effect (Pool 13) versus pools without the regulation (Pool 11). While proportional stock density (% fish > 10” that are > 15”) was high at both Pools 11 and 13 (78 and 90 respectively), the percent of Walleye over 20 inches was only 15 in Pool 11 compared to 44 in Pool 13.

A fisheries biologist holds two walleye collected during fall night electrofishing

Night electrofishing is done each October in the tailwaters of Pools 11 and 13 to measure Walleye and Sauger year class strength. High water levels have not allowed us to effectively sample the past few years. Surveys in 2016 showed weak Walleye and Sauger year classes at Bellevue (Pool 13) and Guttenberg (Pool 11); high water levels during sampling likely negatively affected catch rates. There are good numbers of 14-inch and larger Sauger available, so anglers should have good success in the tailwaters this winter and spring. The slot limit regulation appears to have been highly successful so far, yet future work on this project will continue to measure the effects of the slot limit on Walleye reproduction.

Large River 2 + Mississippi River Habitat Restoration Projects

A close-up of a radio tagged crappie.

Fish telemetry has provided proof that the availability of overwintering habitat is a limiting factor for Centrarchid (e.g., Bluegill, Crappie and Largemouth Bass) populations in the Upper Mississippi River (UMR). Centrarchids traveled long distances (> 3 miles) to reach suitable overwintering backwater areas with low current speeds, water depth > 1 m, water temperatures 1-3° C warmer than the main channel, and adequate dissolved oxygen levels. Lock and dam construction in the 1930’s greatly increased the total aquatic area of the UMR and provided deep backwater areas favorable to Centrarchid populations; but, sediment deposition in backwaters has reduced the quantity of deep-water lentic habitats. As the post-impoundment UMR ages and backwater sedimentation continues, abundance of Centrarchids will likely decline unless management actions are taken.

The Iowa Department of Natural Resources works collaboratively as part of the partnership of state and federal agencies that make up the Upper Mississippi River Restoration - Environmental Management Program. Many Habitat Rehabilitation and Enhancement Projects (HREP) are designed to increase sportfish populations important to Iowa anglers. Multiple HREPs have specifically focused on mitigating effects of backwater sedimentation through sediment dredging, restoration of aquatic connections between backwater and channel areas, and installing control structures that let oxygen-rich channel water enter into backwaters areas during periods of low dissolved oxygen. Research has documented positive effects of HREPs on Centrarchid populations in backwater habitat and identified habitat variables (e.g., dissolved oxygen, velocity, and depth) critical to Centrarchids.

Despite our understanding of the benefits of overwintering HREPs, many questions remain. For example, we do not know how many or at what interval overwintering areas are needed in a pool to keep Centrarchid populations healthy. Information is lacking on the best size of an overwintering backwater and the “sphere of influence” for an individual overwintering backwater (i.e., area of the pool inhabited by fish from an individual overwintering backwater during the rest of the year).

By better understanding overwintering requirements of Centrarchids, we can ensure enough winter refugia is available to sustain a viable sunfish and bass fishery. There is also much to learn from studying newly constructed HREPs to ensure project features function as designed. While backwater restoration has received much attention on many projects, other habitat restoration techniques such as island construction, side-channel restoration, and bankline stabilization present future study opportunities as well. Evaluation of different HREPs and restoration features will provide information on which techniques are most beneficial to riverine sport fish populations.

This study, started in 2014, will help us gain a greater understanding of these questions and lead to improved efficiency and success of future overwintering habitat project design and placement.

Large River 3 + Yellow Perch Habitat Use in the UMR

Yellow Perch provide important and popular sport fisheries across their range. Yellow Perch in South Dakota are listed as the second most popular sportfish in the state, with several lakes in North Dakota and South Dakota receiving national attention for their recreational Yellow Perch fishing.

A fisheries employee holding a tagged yellow perch.

Yellow Perch have been documented in creel surveys within the Upper Mississippi River, ranking anywhere from the 7th to 9th most popular sportfish. It was also reported that Yellow Perch were only a minor constituent of the ice fishing catch from several backwaters in Pools 9 and 10; but, based on the Long Term Resource Monitoring Element’s annual sampling from 1993-2018, Yellow Perch populations have increased for the past 10 years in Pools, 4, 9, and 13. Yellow Perch have become an important sportfish in some locations of the Upper Mississippi River, including within multiple completed Habitat Rehabilitation and Enhancement Projects (HREP). According to the most recent survey of Iowa anglers, Yellow Perch were the 3rd most popular panfish species, behind only Crappie and Bluegill; yet, only 8% of respondents said they had fished for Yellow Perch in Iowa in the past 12 months.

Historically, Yellow Perch fishing in the Upper Mississippi River in Iowa was only a minor component of those fisheries. Latent demand for Yellow Perch fishing in Iowa and recent population increases in the Upper Mississippi River may have created an opportunity for fisheries managers. But, despite understanding that this is a popular sportfish across its range and has the potential for increases in popularity on the Upper Mississippi River, there is limited information available on Yellow Perch in the Upper Mississippi River.

The objective of this project is to evaluate telemetry methodology and seasonal habitat use of Yellow Perch in the Upper Mississippi River. The findings from this research study could help with the development of more diverse HREPs, provide greater habitat diversity, and expand Yellow Perch fishing opportunities for anglers.

Natural Lakes 1 + Muskellunge in Iowa’s Natural Lakes

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.

Fisheries staff gillnetting muskellunge in the spring.

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 Lakes 3 + Walleyes in Iowa’s Large Natural Lakes

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.

DNR fisheries staff conducting a gillnetting survey on walleyes in Spirit Lake.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.

Natural Lakes 4 + Fall Electrofishing Guides Fingerling Walleye Stocking Protocols

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.

Fisheries staff conducting night electrofishing surveys.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.

Natural Lakes 5 + Stocked Muskellunge in Spirit and Okoboji Lakes

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.

Stocked muskellunge in Spirit Lake.

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.

Natural Lakes 6 + Electric Barrier Effectiveness in Natural Lakes

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.

DNR fisheries staff sampling for walleye and muskellunge in the Iowa Great Lakes.

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.

Natural Lakes 7 + Bass Populations in Iowa’s Natural Lakes

In a recent survey assessing Iowa anglers’ fishing preferences and behaviors, largemouth bass tied bluegill as the 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.

DNR fisheries staff sampling for bass in Iowa’s natural lakes.

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.

Natural Lakes 8 + Evaluating the Yellow Bass Biomass Reduction Approach at Lake Cornelia

Yellow bass are an invasive species to Iowa’s natural lakes. Once established, yellow bass are difficult to manage due to their high reproductive levels and ability to adapt to lake environments. In some lakes, yellow bass can become so abundant that their population “stunts” and fish never reach sizes acceptable to anglers. In addition to being unusable to anglers, stunted yellow bass populations often lead to other problems within the fishery that become difficult to overcome with traditional management practices.

Lake Cornelia, a small relatively deep natural lake in Wright County, has historically been a popular destination for panfish and walleye angling. Yellow bass were first discovered in Lake Cornelia in 2006. By 2017, adult yellow bass catch rates were among the highest observed for natural lakes in Iowa and their growth was among the poorest. Declines in walleye, yellow perch, and crappie age-0 catch were also observed following the explosion of yellow bass. The stunting of yellow bass caused devastating effects that cascaded to the entire Lake Cornelia fishery. The purpose of this study is to evaluate a yellow bass biomass reduction approach that includes: stocking adult predators, protecting adult predators with harvest regulations, and chemical or physical removal.

Unfortunately, yellow bass have expanded their range to include many natural lakes in Iowa, as well as lakes in southern Minnesota. Lakes of glacial origin are extremely sensitive to ecological invaders and it is relatively unknown how these fisheries will respond to introductions of yellow bass and potential stunting of these populations. Exploring practical management techniques that can be used in deeper, natural lakes to control unwanted species will be of increasing importance as these populations expand.

It has been well documented that the simple introduction of a predator species to control unwanted prey or sport fish has inconsistent results. Designing and evaluating a hybrid approach to control stunted yellow bass populations could substantially improve natural lake fisheries for Iowa anglers.