Fish Culture Research Team

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.