AMES, Iowa — In 2021, Iowa licensed nearly 270,000 anglers, whose pastime is estimated to represent more than $500 million annually in economic activity.
Michael Weber, associate professor in the Department of Natural Resource Ecology and Management, studies a variety of issues that impact fisheries and success for anglers, with help from dedicated graduate students and state biologists. One component of their research seeks to better understand the factors that influence survival rates of stocked fish.
Among the sought-after game fish that attract anglers to the state’s reservoirs and lakes are Walleye and Muskellunge. As these species do not naturally reproduce in Iowa reservoirs, their existence here depends entirely on the success of stocking.
Nearly 100 million Walleye fry, only fractions of an inch long, are stocked annually throughout Iowa in the spring. Walleye fingerlings are given more time to grow in hatcheries and approximately 200,000 are stocked in the fall after they reach 7 to 10 inches in length. Muskies are grown in hatcheries for one year and stocked at 12 inches.
“Even really small increases in survival of stocked fish could have huge implications in the number of adults in Iowa lakes and reservoirs,” Weber said. “We have had several linked 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 transportation from the hatchery, predation, high water temperatures or other causes can be high. Weber’s research has found, hauling fish from the hatchery to their long-term home in reservoirs was not a major cause of loss. However, these fish can have trouble transitioning from eating pellets in a hatchery environment to finding their own food after being stocked. They are also vulnerable to being eaten by predators including other fish, birds and snakes.
For stocked fish that survive, Walleye and Muskie often live four to eight years in Iowa reservoirs, though some live up to 15 years. Sustaining the fish at desired sizes and population levels is challenging, and when catch rates are low, anglers notice. In the past, reasons for fish mortality and the extent of losses have been largely based on estimated guesses. An important goal of Weber’s research is to better quantify and understand the fate of stocked game fish, as well as to experiment with ways to prevent “escapement,” when fish leave reservoirs over spillways or other outlets.
One of the projects has been looking 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 2016-2020, Weber and a former master’s student, Robert Weber, studied Walleye and Muskie mortality, harvest and escapement. Their methods included intensive catching-and-counting and attaching microchips (similar to those implanted into cats and dogs) to fish to track their survival, movements and escapement over reservoir spillways via solar-powered telemetry antennas.
The findings, some of which were reported recently in Fisheries Research and the North American Journal of Fisheries Management, include a host of insights on the mysteries of fish behavior and fate. Natural mortality was similar between the Big Creek and Brushy Creek systems, ranging from 15 to 18% annually, and, as expected, increased with water temperature.
Among the surprises, researchers found escapement can have a larger effect than harvest or natural mortality on reservoir Walleye populations in a system without barriers designed to prevent fish from leaving. No fish escapement was observed past the barrier at Big Creek whereas 20-47% of adult Walleye were escaping Brushy Creek when water levels were high. Small fish were less prone to leave but Walleye at any size were more likely to escape in the spring, especially in April during the spawning period. Annual escapement of Muskies was 18-54%, and they were also more likely to escape in spring but continued to leave into the summer after high rainfall events. For both species and at both lakes, the majority of escapement occurred during the night and early morning.
The second phase of the project is underway, with help from graduate students Tom Miles and Madeline Lewis. They are assessing the effectiveness of barriers installed at the outlets of the reservoirs to prevent fish from swimming away. “The challenge is designing barriers that work, yet don’t cause water to back up from becoming clogged with debris, like aquatic vegetation and wood, and don’t require too much maintenance,” Weber said.
The primary barrier type being studied now is a modified chain link fence with round galvanized steel crossbars that have two-inch gaps between. Other, nonphysical types of barriers include electrical barriers and those that use sound, light and/or bubbles to deter fish. According to Weber, these may have advantages in some situations, but they are more complex and expensive.
“Our early results show the physical barriers installed to prevent escapement are really making a difference. That’s good, because they are relatively simple and inexpensive, easy to maintain, and they allow water to flow through 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 best use of funding and staff resources.
Support for the research has come primarily from the U.S. Army Corps of Engineers and the Iowa Department of Natural Resources.