Iowa DNR monitors increased gizzard shad numbers across Iowa Great Lakes
SPIRIT LAKE - The Iowa Department of Natural Resources (DNR) documented an increase in gizzard shad across the Iowa Great Lakes, including Spirit Lake and the Okoboji chain during recent annual shoreline seining activities.
Native to the Little Sioux River drainage to the south, gizzard shad were unknown this far north 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.”
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 through 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,” said Carlson. “Continued monitoring is vital to evaluate if these fish can regularly overwinter and establish a long-term, self-sustaining population.”
Research on gizzard shad at the northern edge of their distribution shows that low winter temperatures frequently trigger high winter mortality. In northern glacial lakes, sustained water temperatures below 36 degrees or ice cover exceeding 100 days often result in significant winter kills of both 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 dynamics of forage fish, game fish, and other species.
How anglers and boaters can help
Anglers and outdoor enthusiasts play a vital role in keeping Iowa's waters healthy. To protect local ecosystems, the DNR reminds the public to:
- Never transfer live fish or bait between different water bodies.
- Dispose of unwanted live bait in the trash, never in the water.
- Clean, Drain, and Dry all boats, trailers, and gear before entering a new body of water.
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.
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