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.



Natural Lakes Investigations

122 252nd Ave, Spirit Lake, IA 51360, 712-336-1840
Jonathan Meerbeek ; Daniel Vogeler; Vaughn Wassink

The Natural Lakes Research team provides high quality and relevant research information to fisheries management and hatchery programs to enhance fisheries resources 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, 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 on 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.

Shallow lakes in Iowa are known as “tweener” lakes because they are too shallow to always be good lakes and too deep to always be good marshes. Shallow lakes in Iowa can be in one of two conditions; turbid or clear water state. Lakes in the turbid water state have very dirty water, little to no aquatic vegetation, limited emergent vegetation, a sparse fishery dominated by carp and bullheads, and limited waterfowl production. Many of these lakes can also be in a clear water state with clear water, lots of aquatic vegetation, shallow bays covered with emergent vegetation, a desirable fishery dominated by sport fish species, and good waterfowl production.

In the past, many “tweener” lakes needed long drought periods to remove common carp and let emergent vegetation reestablish to shift back to the clear water state. The lack of large drought periods has caused many “tweener” lakes to stay in the turbid state for several decades. During this period, some shallow lakes were managed by applying pesticides and restocking with desired sport fish; little attention was given to aquatic macrophyte growth or long-term rough fish management and eventually these lakes shifted back to the turbid water state. 

Carp caught during a fish sampling survey.



A renovation project at Lost Island Lake started in 2008 that did not include lake drawdowns or chemicals to eliminate the fish. Commercial harvest of carp, fish barriers at connecting wetlands, increased predator stocking, and targeted watershed improvements were used to renovate the fishery and improve water quality. Since 2008, about 800,000 pounds of carp were removed from Lost Island Lake and carp population estimates dropped from 136,718 fish in 2008 to 4,387 fish in 2017. Substantial improvements in water quality and fishing were observed in Lost Island Lake after renovation. Due to the success of this project, in 2017, additional shallow natural lakes were identified that could benefit from a similar carp and/or bigmouth buffalo reduction strategy. Center Lake, Five Island Lake, Silver Lake (Dickinson County), Storm Lake, Blue Lake, North Twin Lake and South Twin Lake were selected and pre-renovation carp population estimates were conducted at each lake in 2018 and 2019. Pre-renovation carp population estimates are important to establish population benchmarksthat may guide a commercial fishing incentive program.

Continued monitoring of carp population and incentive-based commercial carp harvest will be important tools to guide future management and research in all of these lakes. In addition, carp barriers, stocking regimes, and watershed projects will be identified that may help improve the fishery. The findings from the suite of shallow lakes studied will be compiled and management tools that can be used to shift and maintain shallow lakes in a clear water state will be identified.

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 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.

Iowa’s walleye culture program has focused on developing production techniques that produce the largest walleye fingerlings. Many natural lakes are stocked in the fall with 9-10 inch walleye. Current research is evaluating large walleye fingerlings survival; early findings suggest that large fingerlings can provide valuable additions to walleye populations. Since walleye fry are the most important driving force for strong walleye year classes, research has also been done to evaluate the survival of fry stocked near shore directly from distribution tanks and those stocked away from shore via bags. Recaptures of marked age-0 walleye in the fall has shown that in some years, fry stocked offshore contribute better to the fall age-0 walleye population than those stocked near shore.

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 adult walleye numbers, walleye size structure and growth rates, as well as walleye harvest and release rates before and after the regulation change, mostly because of a very large 2001 walleye year class that became vulnerable to harvest with the regulation change. At the same time, adult walleye broodstock numbers increased greatly in these lakes due to reduced competition among adult walleye and better growth rates. By 2013, walleye harvest rates had dropped a lot, but adult densities remained high, thus reducing walleye stocking survival through predation. Walleye populations have varied widely in Clear and Storm lakes, but were more a result of variable stocking success than angler harvest rates.

Further research monitoring broodstock densities, life history features of the broodstock populations, and 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 strategies changes. Findings from this research will guide walleye management decisions and strategies for Iowa’s natural lakes.  

Yellow Bass are not a native fish species to Iowa’s natural lakes. Yellow bass were accidentally stocked in Clear Lake in the 1920s during Mississippi River fish rescue operations and were first reported in Clear Lake in 1932. Two other natural lakes in Iowa were later found to have non-native yellow bass populations in the 1950s and 1980s. Historic yellow bass  densities in natural lakes have often resulted in stunted populations. Stunted yellow bass populations can cause cascading negative effects to the entire fishery that become difficult to overcome with traditional management practices. A net full of yellow bass that fisheries staff caught during sampling efforts.

Recently, new yellow bass populations have been discovered in several of Iowa’s natural lakes. In 2005, yellow bass were found in East Okoboji Lake and currently are one of the most caught and harvested fish species. Yellow bass were discovered in Lost Island Lake in 2008 and now are the most abundant fish caught by anglers and by DNR sampling. The cause for the recent expansion of yellow bass in natural lakes is unknown, but it is suspected that a combination of consistently higher water levels/extreme flow events and the popularity of the fish by anglers have increased their expansion into natural lakes. Angler popularity for yellow bass has mainly increased due to their ease of capture and their size potential in newly established natural lakes. Since 2013, new yellow bass populations have been discovered in five more Iowa natural lakes.  Concurrently, the Minnesota Department of Natural Resources has identified new populations of yellow bass in two of southern Minnesota’s shallow lakes within the last five years.  Yellow bass expansion is occurring at an alarming rate in Iowa and there is a need to understand, evaluate, and communicate the potential impacts of their introductions. 

Although Yellow Bass have occurred in natural lakes in Iowa since the 1930s, relatively little is known about their impacts to existing fish communities in shallow natural lakes. Work completed in 2017 for this study focused on evaluating the distribution of yellow bass in shallow natural lakes. Of the 32 lakes sampled, 12 had either established or recent populations of yellow bass. In 2018 and 2019, comprehensive fisheries surveys were conducted in several lakes identified as having either established or recent yellow bass populations. Fish were collected monthly or bimonthly to determine diet overlap among yellow bass and native fish species. Relative abundance was determined by sampling yellow bass at night during the fall with electrofishing gear. Length, weight, and aging structures were taken from a subsample of fish for age and growth analysis.  Yellow Bass growth in newly established natural lakes was extremely fast compared to established lakes.

This study will seek to thoroughly examine changes in fish community structure both before and after yellow bass introductions by examining catch rates and diet overlap of popular sport fish and yellow bass. Understanding effects of introduced predators on existing fish communities is crucial for determining management strategies that will ultimately improve fishing.



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 PIT tagged before stocking since 2011 to identify factors such as total length or condition that may contribute to increased/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/decreased survival rates.

Results from two years of tracking in Spirit Lake found that yearling muskellunge survival to 100 days was 50-65% and was related to size at time of stocking, with larger fish surviving at a higher rate. We also found that stocking technique did not influence 100 day survival rates. Using this information, yearling muskellunge stocked in 2018 and 2019 were required to be at least 13 inches before stocking in May. Those that were not 13 inches or larger were held at the Spirit Lake hatchery for a final 30 day grow-out period to try to improve yearling 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 both 2018 and 2019, survival rates of grow-out yearling muskellunge was ≥75.0%. Based off survival models, the grow-out period improved yearling muskellunge survival by at least 30% each year. 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 (i.e., 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 both 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. Recent muskellunge population estimates are the lowest they have been since the mid-1990s, despite increased stocking rates. Spring broodstock gillnetting data found that 8.6-42.5% of the adult muskellunge population moved each year from Spirit Lake to the Okoboji chain from 2014-2017.  Historically, walleye movement in these systems was not as dramatic, except in 2018, when 33% of recaptured walleye caught below the Spirit Lake spillway (e.g., East Okoboji Lake) were found to be originally tagged in Spirit Lake.

This study will focus on evaluating the effectiveness of a low-pulse electric barrier to reduce both walleye and muskellunge loss into Milford Creek. Recaptures of tagged walleye and muskellunge will provide movement information that will be critical to determe the long-term effectiveness of such a barrier.

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. However, 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, bass populations in 16 natural lakes were sampled capturing a total of 1,251 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. Results from this analysis will be used to identify population characteristics and determine if management options can be implemented to improve bass populations in Iowa’s natural lakes.

Completed Projects:

Large River Investigations

24143 Hwy 52, Bellevue, IA 52031, 563-872-4976
Ryan Hupfeld; 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.


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.

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.

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.

Completed Projects:

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.


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.

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.

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.

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.

Man-made Lakes Investigations

57744 Lewis Road, Lewis, IA 51544-5103, 712-769-2587 Lewis Bruce; John Lorenzen

Man-made lakes research provides fishery managers the tools they need to manage small public lakes for anglers.


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.

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. Current 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.

A platter of cooked fish.

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 also 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.

Largemouth Bass otolith marked with OTC.

The known age fish project is in its 7th year of collecting data. Over 300 age structures have been collected from bluegill, channel catfish, largemouth bass, and redear sunfish. These species were marked with Oxytetracycline (OTC) before they were stocked into four small lakes. Each year after the first stocking, a number of fish were collected and age structures from these fish were read under a microscope. The OTC leaves a fluorescent ring in the age structure of a fish so we know how old the fish was when it was stocked, we consider these marked and known age fish.

Over the years, we have cataloged these structures by species, structure type and age for training purposes in the laboratory. Throughout the study we have found other interesting pieces of information beyond just knowing the age of these fish.

Each year of the study made bluegill and redear sunfish collection more difficult. Bluegill spawn throughout spring and summer, while most other species spawn only in the spring. After a few years into the study, finding our marked fish was like looking for a needle in a haystack. This is evident in our data where the maximum age collected was three years old. This is one reason why we stock predator fish such as largemouth bass in lakes with Bluegill. Largemouth bass feed on bluegill and keep the population in check so they grow to an acceptable size for anglers to catch.

Largemouth bass spawn in the spring, and because they are higher up on the food chain than bluegill, there are fewer fish in a lake. This lets us effectively capture marked fish for a longer period of time. One of the interesting things we are seeing is a difference in growth between populations in each lake. Every study lake was stocked with the same number of fish and on the same day 7 years ago. Largemouth bass sampled in one pond this year were 18 inches long, while another pond had a maximum length of 15 inches. This shows how environmental factors can influence fish growth. One of these lakes is steep sided and the other is shallow. The shallower lake has the larger fish. Shallow water lets sunlight reach a large area of the lake bottom allowing aquatic vegetation to grow. Steeper sided lakes have less vegetation than shallow lakes, and food is not as abundant because invertebrates have less vegetation to use for cover. This could explain why fish growth in the shallower vegetated lake is better than the steep sided lake with limited vegetative growth.

All of Iowa’s significant public owned lakes (SPOLs) were surveyed to measure lake depth at a known elevation or lake level. These surveys were used to create contour maps for the many user groups working and recreating on Iowa lakes. Engineers, management biologists, and anglers are some of the groups that request the available depth data found on the Iowa DNR website.

We can now analyze accumulated sediment from these mapping surveys. Surveys completed in 2012 can be compared with new surveys to calculate accumulated sediment in lake basins. Accumulated sediment can negatively affect water quality, change fish composition and size structure, and increase vegetation growth that have negative effects on recreational activities.

Accumulated sediment map for Easter Lake.

During the last two years, 14 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 annually 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.

Much like the rest of the country, angling participation in Iowa has steadily decreased for decades. One reason for this decline is thought to be related to an increasingly urban population and the resulting disconnect between people and natural resources. Recently, there has been a growing effort to promote urban fisheries and connect these residents with local fishing opportunities. 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.

Two men fishing at a Community Fishing Lake.


Iowa’s Community Fishing Program started in 2016 to increase angling participation and opportunities within Iowa’s larger cities. This program focuses on 38 communities in 17 cities, whose populations represent 35% of Iowa’s total population. These communities, ranging from long-established cities to new suburbs, are among the fastest growing population in the country. There are currently 214 ponds ranging in size from 0.5 to 232 acres within these communities. Much like the communities in which they are located, these ponds vary from aging park ponds to newly constructed water retention basins. Many ponds and retention basins are being built with little guidance as to amenities, basin morphology, fish stocking strategy, or watershed size. Managing these systems for fishing requires a different set of tools than traditional methods used to manage rural ponds.

Assessing angler demographics, angling pressure and species preference, fishery composition, fish species abundance, water quality and aquatic plant composition will allow us to make informed decisions and guide communities to provide angling opportunities for local people. During the next four years, the Small Impoundment research team will collect data to answer these questions.


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.


Five lakes were surveyed in the fall of 2020 to collect bluegill. Age structures were collected from about 400 fish between 3 inches and 10 inches. Bluegill collected were weighed, measured for length, and sex was determined on mature individuals. Similar to trees, fish structures provide biologists with annual growth rings to determine age and growth from one year to the next. Additional lakes throughout Iowa will be surveyed during the next three to four years to give Iowa DNR managers up-to-date information that will help them to make science-based decisions for managing bluegill management Iowa.


Bluegill are the most fished for species 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 many 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 first year-class following a renovation from getting overabundant before the first year-class of largemouth bass can be established. If this first bluegill stocking is supplemented from another source (e.g., escapement 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 sampling season, genetic samples were collected from individual bluegill using fin clips. These fish were collected from three lakes representing three different bluegill populations. Information from this study will be used to guide future bluegill restocking efforts after lake renovations. If bluegill fisheries become overabundant after a renovation as a result of non-stocking related contributions, the Iowa DNR can take steps to limit those contributions or adjust the initial re-stocking rate. If bluegill genetics are contributing to fish growth, Iowa DNR hatcheries can obtain broodstock from desirable populations.

If this study determines that bluegill genetics are not a contributing factor to create an angler desirable fishery, then fisheries managers can rule this out during future assessments. The ultimate goal for bluegill fisheries after renovation is to provide anglers with desirable size fish as soon as possible.


Walleye is the most stocked species of fish in Iowa and is tied with bass as the most fished for species. To maintain quality populations across the state, three sizes of walleye are stocked into lakes: fry, fingerling, and advanced fingerling. Advanced fingerling fish are the largest size, 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 into lakes it is important them to survive and grow so anglers have an opportunity to catch them.

In recent years aquatic invasive species (AIS) have started to spread across Iowa. These species can be plants, fish, mollusks, or other types of animals and can cause significant damage to native ecosystems and infrastructure. Zebra mussels have caused problems in Iowa hatcheries. 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 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 2019, three different advanced fingerling walleye products were stocked into lakes in October and then sampled in 2020 to evaluate how each product survived after stocking. Results from the first year of this study found walleye produced in a flow through system had better survival than walleye grown in a RAS system. This was the first year of a three year project to determine if RAS can be used to provide anglers in Iowa with a high quality product.


Completed Projects:







Interior Rivers and Streams Investigations

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

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.


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 two 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.  Channel Catfish numbers also increased at sites above the former dam.

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 6,600 fish of 18 species were marked downstream of the dam to monitor fish movement over the new structures.  Sampling in 2015 and 2016 found 234 marked fish representing 9 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 are often popular fishing spots. A common concern is that dam removal or modification projects will negatively impact angling, particularly below the dam. The impact of dam removal or modification 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 is 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.  Anglers were surveyed in April-October for three years before, and will be surveyed for three years after construction.  During 2012-2014, total angler participation ranged from 4,232 to 6,797 angler hours per year. Smallmouth Bass, Common Carp, Walleye, Bluegill, Crappie, Suckers, and Channel Catfish were caught most often during this period. The second year of post-project monitoring began in April 2016.  In 2016, total angler participation was 3,770 angler hours per year, and Smallmouth Bass, Walleye, Bluegill, and Suckers were caught most often.

Measuring the impacts of a dam modification or removal project 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.

Evaluation of Interior River Fingerling Walleye Stocking Strategies

man holding large walleye

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.

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.


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.

Completed Projects:

Reservoir Investigations

24570 US Hwy 34, Chariton, IA 50049, 641-774-2958
Rebecca Krogman; Mark Richardson ; Savannah Muhlbauer

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.


Walleye are a very popular game fish in Iowa, despite a low rate of natural reproduction in most waters. Natural reproduction is especially low in artificial lakes (reservoirs), so Iowa DNR hatcheries supplement walleye populations with yearly stockings. Our management goal is usually a target population density of walleye in each lake.

Smarter Stocking Past research at Rathbun Lake showed that stocking both fingerlings and fry produced more consistent year-classes of adult walleye and met our fishery management goals better than stocking only fry. Study results have been inconsistent in identifying which size of fingerling or stocking location is best. Fingerling walleye cost more to produce than fry and limited availability may cause small or irregular fingerling stockings in some reservoirs. Better ways to share the stocking of advanced (>6 inches) fingerlings, 2-inch fingerlings, and fry are being studied to maximize the number of reservoirs that meet our walleye management goals.

A fingerling walleye with a circle brand to indentify which year and size it was stocked.

This research project started in 2011 at Big Creek Lake, when the Iowa DNR stocked 3,000 walleye fry/acre and five 8-inch fingerlings/acre into the reservoir. The fingerlings received a unique brand (like a tattoo) on their side to identify which year-class they were from. Their survival from year to year was tracked by electrofishing each fall to identify the age and origin (fry versus fingerling) of each fish. In addition to this aggressive stocking method, a physical fish barrier was installed on the Big Creek Lake spillway in 2012. This helped to bring the Walleye population up to almost six adult walleye/acre. Stocking has been reduced to let the existing walleye grow. We learned that fry stocking was 17 times more cost effective than stocking advanced fingerlings. However, advanced fingerling Walleye have much higher survival rates and make up a large part of the Big Creek Lake population, so it is important to stock them when and where fry do not survive.

Big Creek Lake’s walleye stocking success and popularity led to the expansion of this study to six other reservoirs in 2014: Lake Manawa, Lake Macbride, Lake Icaria, Little River Lake, Pleasant Creek Lake, and Twelve Mile Lake. These waters will be stocked with fry and fingerlings to better determine when each stocking size should be used. The study is also working to develop an effective sampling method to estimate fry stocking success before fingerling stocking; waters where fry did not “take” could then be prioritized in fingerling stockings.

Reducing Escapement Keeping strong walleye populations in reservoirs is a challenge because fish can escape downstream through spillways during spring flooding. After passing over the dam, escaped walleyes cannot move back into the reservoir. This has been seen at Big Creek Lake and Rathbun Lake, when tagged fish stocked into the reservoir were caught by anglers in the river below, and likely happens at many reservoirs across Iowa. Fish escaping reduces the success of stocking and drains a reservoir’s walleye population over time.

The Iowa DNR is testing a variety of methods to monitor and prevent more fish from escaping. For example, we can track fish tagged with Passive Integrated Transponders (or “PIT tags”) when they pass by or through a tag reader installed on a dam spillway. The Iowa DNR is working with the Army Corps of Engineers and Iowa State University to test the effectiveness of a physical barrier installed at Big Creek Lake in 2012 and an electric barrier at Rathbun Lake which will be installed soon. An effective barrier that reduces fish escaping will help the Iowa DNR strengthen current walleye populations and improve the success of stocking.

DNR fisheries employee Randy Schultz holds a hybrid striped bass caught during electrofishing.

Hybrid striped bass are becoming more popular in Iowa's large reservoirs and urban ponds, but little is known about how to best manage these fish in Iowa. Ongoing culture research may make it cost-effective to raise hybrid striped bass in Iowa hatcheries; previous stocking efforts have been supplied by out-of-state hatcheries. The first hybrid striped bass cross, the "palmetto bass," was the strain of choice for many years. Palmetto bass are produced by fertilizing striped bass eggs with white bass milt. Recently, the reciprocal cross, or "sunshine bass," was stocked in several Iowa reservoirs. Sunshine bass are produced by fertilizing white bass eggs with striped bass milt. Although they look alike as adults, palmetto and sunshine bass may provide different fisheries to Iowa anglers.

The Iowa DNR is evaluating the success of palmetto and sunshine bass in five reservoirs: Easter Lake, Lake Icaria, Badger Creek Lake, West Lake Osceola, and Lake Wapello. Both crosses were stocked at fingerling size (usually 2-4 inches) at all lakes in 2012 and again from 2014-2017 (except for Easter Lake, which is undergoing renovation). Every reservoir is sampled each year to compare each cross's population size and survival rate. So far, more palmetto bass have been collected during sampling (implying better survival), but no difference in size has been seen between the two crosses. Several other factors, in addition to genetic cross, can affect the success of establishing hybrid striped bass fisheries. For example, the size at stocking (fry versus fingerling) can affect survival and expense. Ideal stocking numbers are also unknown at this time.

In 2014, fish from Lake Icaria were also sampled for stable isotope analysis, which can show dietary differences between the two crosses and possible competition or predation with other species. Several other factors, in addition to genetic cross, can affect the success of establishing hybrid striped bass fisheries. For example, the size at stocking (fry versus fingerling) can affect survival and expense. Ideal stocking numbers are also unknown at this time.

A variety of sampling methods for hybrid striped bass, including fall gill netting and daytime and nighttime electrofishing, were also evaluated. Different sampling tools can be more or less effective at catching fish and may catch different sizes of fish; more importantly, some are less stressful for the fish and less likely to cause accidental sampling injury. Population dynamics can be measured without affecting the fish population when the best tool is used. Each of the focus reservoirs was sampled with all three sampling methods from 2013-2015, resulting in the selection of fall experimental gill netting as the best sampling method. This is now the statewide standard sampling method for hybrid striped bass in lakes and reservoirs.

The Iowa DNR can more effectively manage hybrid striped bass fisheries with better stocking strategies and population assessments. Our goal is to create high-quality trophy hybrid striped bass fisheries for Iowa anglers.

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 restortation 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.

The Iowa DNR Fisheries Bureau manages fisheries across the state to conserve and improve Iowa’s aquatic resources, including small streams, large rivers, natural lakes, ponds, and reservoirs. Fisheries biologists regularly sample many aquatic systems to identify and monitor fish communities, fish health and growth, and fishery quality. The diversity of aquatic habitats requires a diversity of sampling tools and methods. The Iowa DNR regularly uses electrofishing, gill nets, fyke nets, hoop nets, trawls, traps, and seines to collect fish; each of these have biases such as fish size and species. The design of each tool and method of its use may lead to different data. Effective fisheries management and research rely on the understanding that different sampling methods may result in different data. This is a challenge for fisheries biologists across the country.

A custom-build modified fyke net used for sampling crappies and other panfish.

Iowa DNR developed standard operating procedures for fisheries sampling in 2012. However, these do not entirely align with newly created North American standards. This can limit collaboration with other agencies and data sharing. Sampling instructions or design may not be adequately detailed, despite evidence that minor differences within a single tool may affect results. For example, Iowa fisheries biologists routinely use modified fyke nets to catch structure-oriented fish species in deep lakes, reservoirs, and large rivers. Differences in design such as trap dimensions, mesh size, funnel size, and number of hoops can change the fish captured and/or retained. Similar issues may affect other sampling tools, but are unknown due to a lack of research. An examination of current sampling methods and possible revisions is needed to ensure data quality and long-term dataset viability.

The modified fyke net was the first tool studied. A survey of modified fyke net dimensions was done in all fifty states during the fall of 2014 and yielded a wide variety of descriptions, all referred to as a “modified fyke net.” Four of these net designs were tested in Lake Ahquabi, Williamson Pond, and West Lake Osceola for their ability to capture and keep fish overnight. Extra catch information was also taken from lakes and reservoirs across Iowa to help determine if catch rates were different. One net design, similiar to the photo above, yielded higher and more reliable catch rates for target fish species and was recommended as the statewide standard for modified fyke nets. This work was published in the scientific literature and is available by subscription at:

Another net studied is the experimental gill net, a long net with a variety of mesh sizes to capture different sizes of fish. These nets are effective on Walleye and Hybrid Striped Bass, but an additional special portion with extra-large mesh sizes can improve the catch of trophy-size fish for population monitoring. The special portion was tested in 2018 and 2019 at Lake Macbride and Red Rock Reservoir, showing that large-mesh special panels were likely needed to capture the full size range of fish that can grow in Iowa reservoirs. Additional netting studies may be conducted for unbaited hoop nets, which could improve fisheries monitoring in shallow natural lakes.

In addition to netting, electrofishing is an important sampling tool Iowa DNR fisheries biologists use. However, a 2017 review of the DNR’s electrofishing boat fleet revealed many differences in boat configurations. A common difference was boom shape and size, which affect the size of the electrical field created in the water. This could lead to big differences in fish catch rates between boats. Continued review and standardization of electrofishing boats will improve the consistency and safety of electrofishing as a sampling gear.

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.

An acoustic receiver and a tag, which is surgically inserted into the fish’s body cavity.

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.

Tailwater of Red Rock Dam

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.

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