Iowa monitors ‘Great Lakes’ gizzard shad after increases in population

Spirit Lake, Iowa — The Iowa DNR has documented an increase in gizzard shad across the so-called Iowa Great Lakes, including Spirit Lake and the Okoboji chain during recent annual shoreline seining work.

Native to the Little Sioux River drainage to the south, gizzard shad were unknown in the watershed until small numbers were detected over the past few years. August sampling represents an uptick in population numbers, highlighted by the collection of 1 adult, 23 subadults, and 1,846 young-of-the-year fish on the north end of East Okoboji Lake in a single 500 feet seine haul, confirming successful natural reproduction within the system.

“After detecting higher numbers of gizzard shad across Spirit Lake and the Okoboji chain, we are closely monitoring the local population to understand how this species may fit into the food web,” said Andrew Carlson, Iowa DNR fisheries biologist. “It’s too early to draw conclusions about how these fish will interact with the local aquatic ecosystem.”

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The exact ecological impacts of gizzard shad in the Iowa Great Lakes remain unknown. In some systems, young-of-the-year shad provide an additional food source for popular game fish like walleye and yellow perch, while in others they can potentially compete for zooplankton or affect water clarity via their bottom-feeding habits.

“Because the Iowa Great Lakes sit at the northern edge of their range, harsh winter conditions often naturally limit gizzard shad survival,” Carlson said. “Continued monitoring is vital to evaluate if these fish can regularly overwinter and establish a longterm, self-sustaining population.”

Research on gizzard shad at the northern edge of their distribution shows that low winter temperatures frequently trigger high winter mortality. Sustained water temperatures below 36 degrees or ice cover exceeding 100 days often result in significant winter kills of juvenile and adult shad, preventing long-term population growth.

Iowa DNR Fisheries teams will continue regular fisheries sampling across the Iowa Great Lakes to track the species.

For more information regarding fisheries management, local lake monitoring, or invasive species prevention, visit www.iowadnr.gov/fishing or consult the 2026 Iowa Fishing Regulations booklet.