Iowa State Gamefish Research Provides Better Picture of Fish Fate and Escapement


AMES, Iowa — In 2021, Iowa licensed nearly 270,000 anglers, whose hobby is estimated to generate more than $500 million annually in economic activity.

Michael Weber, an associate professor in the Department of Ecology and Natural Resource Management, studies a variety of issues that impact fishing and the success of anglers, with the help of dedicated graduate students and state biologists. . One aspect of their research aims to better understand the factors that influence the survival rates of stocked fish.

Among the popular game fish that attract anglers to the state’s reservoirs and lakes are walleye and muskellunge. Since these species do not breed naturally in Iowa reservoirs, their existence here is entirely dependent on successful restocking.

Nearly 100 million walleye fry, measuring only fractions of an inch long, are stocked throughout Iowa each spring. Walleye fingerlings have more time to grow in hatcheries, and about 200,000 are stocked in the fall after reaching 7 to 10 inches in length. Muskies are raised in hatcheries for a year and stocked at 12 inches.

“Even very small increases in stocked fish survival could have huge implications for adult numbers in Iowa lakes and reservoirs,” Weber said. “We’ve had several related research projects asking related questions and looking at stocking success from different angles with the goal of increasing fish survival and ultimately angler satisfaction.”

Losses of stocked fish due to transport from the hatchery, predation, high water temperatures or other causes can be high. Weber’s research showed that transporting fish from the hatchery to their long-term homes in tanks was not a major cause of loss. However, these fish may struggle to transition from eating pellets in a hatchery environment to foraging for their own food after being stocked. They are also susceptible to being eaten by predators including other fish, birds, and snakes.

For stocked fish that survive, walleye and muskellunge often live four to eight years in Iowa tanks, although some live up to 15 years. Maintaining fish at desired sizes and population levels is a challenge, and when catch rates are low, anglers take notice. In the past, the reasons for fish kills and the extent of losses were largely based on estimates. An important goal of Weber’s research is to better quantify and understand the fate of stored game fish, as well as to experiment with ways to prevent “escape”, when fish leave reservoirs through weirs or other outlets.

One of the projects looked at fish populations in two popular fishing reservoirs in central Iowa – Big Creek Lake near Des Moines and Brushy Creek Lake near Webster City. In the first phase of the research, from 2016 to 2020, Weber and a former master’s student, Robert Weber, studied walleye and muskellunge mortality, harvest, and escapement. Their methods included intensive catching and counting and attaching microchips (similar to those implanted in cats and dogs) to fish to track their survival, movement and escape on reservoir outfalls via energy telemetry antennae. solar.

The findings, some of which were recently reported in Fisheries Research and the North American Journal of Fisheries Management, include a host of insights into the mysteries of fish behavior and fate. Natural mortality was similar between the Big Creek and Brushy Creek systems, ranging from 15-18% per year and, as expected, increased with water temperature.

Among the surprises, the researchers found that escapement can have a greater effect than harvest or natural mortality on reservoir walleye populations in a system without barriers designed to keep fish from leaving. No fish escapements were observed beyond the barrier at Big Creek while 20-47% of adult walleye escaped from Brushy Creek when water levels were high. Small fish were less likely to leave, but walleyes of all sizes were more likely to escape in the spring, especially in April during the spawning season. Annual muskellunge escapement was 18–54%, and they were also more likely to escape in the spring, but continued to leave through the summer after heavy rainfall. For both species and in both lakes, the majority of escapements occurred during the night and early morning.

The second phase of the project is underway, with the help of graduate students Tom Miles and Madeline Lewis. They assess the effectiveness of the barriers installed at the outlet of the tanks to prevent fish from moving away. “The challenge is to design barriers that work, but don’t back up water from debris, like aquatic vegetation and wood, and don’t require too much maintenance,” Weber said.

The main type of fence currently under consideration is a modified chain-link fence with round galvanized steel crossbars separated by two-inch gaps. Other types of non-physical barriers include electric barriers and those that use sound, light and/or bubbles to deter fish. According to Weber, these can have advantages in certain situations, but they are more complex and expensive.

“Our initial results show that the physical barriers installed to prevent leaks really do make a difference. That’s good because they’re relatively simple and inexpensive, easy to maintain, and they allow water to drain easily,” Weber said.

Weber and his team continue to study the state’s fisheries and hope their data can help natural resource managers make the most of financial and human resources.

Research support came primarily from the US Army Corps of Engineers and the Iowa Department of Natural Resources.


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