Invasive Weeds and Herbicides Are Threatening Fish in Flint Lake

Tue, 06 May 2025 17:00:10 +0000

A closer look at how Eurasian milfoil and chemical treatments are harming aquatic life

Flint Lake on Valparaiso’s north side is in trouble. Invasive aquatic weeds, especially Eurasian milfoil, have become so overgrown that they’re even interfering with fish population surveys. During a recent Department of Natural Resources (DNR) survey, biologists had to stop two or three times just to clean the weeds off their boat’s propellers.

Herbicide Treatments: A Short-Term Fix with Long-Term Consequences

While treatments for Eurasian milfoil might seem like a good solution, they often come with serious environmental trade-offs. Herbicides do not actually remove the plants—they simply kill them where they grow. This leaves large amounts of decaying plant matter in the water, which can significantly reduce oxygen levels. As the plants rot, they consume oxygen that fish and other aquatic life depend on, potentially creating dead zones that are uninhabitable.

To make matters worse, the milfoil in Flint Lake is becoming more resistant to the most commonly used chemical treatments. And if the herbicide isn’t specific to milfoil, it can endanger beneficial native plants as well.

Native Plants: Critical for Young Fish and Water Quality

These native aquatic plants are more than just vegetation—they’re lifelines for young fish. “Anytime you damage these plants, you’re killing hundreds of thousands of babies,” DNR specialist said. Smaller fish use these plants to hide from predators and as feeding grounds. Destroying them reduces fish survival rates and upsets the balance of the ecosystem.

Mechanical Removal Isn’t a Silver Bullet Either

Even mechanical methods have their risks. Fragmenting Eurasian milfoil—chopping it up—can actually help it spread. Each small piece of the plant has the potential to take root and grow elsewhere in the lake.

A Call for Smarter Management