Treating lakes for Eurasian watermilfoil with herbicides can harm young fish

Wisconsin Sea Grant’s emerging contaminants scientist, Gavin Dehnert, earned his Ph.D. by studying the effects of commercial 2,4-D herbicide exposure on the development and behavior of freshwater fish at different life stages. Now, he’s taking his research out of the lab and into the natural environment, where 2,4-D herbicides are used to treat lakes for the invasive plant, Eurasian watermilfoil.

During his doctoral studies, Dehnert found that exposure to concentrations of 2,4-D similar to those allowed during application to lakes significantly decreased survival in fathead minnow larvae and also other young fish species such as walleye, yellow perch, largemouth bass, northern pike, white crappies and white suckers.

“We saw an increase in about 20 to 35% mortality of the young fish when exposed to 2,4-D,” Dehnert said. “But we kept getting this big question: We know what happens in the laboratory, but what happens in the real world?”

With funding from the Wisconsin Department of Natural Resources, Dehnert designed two sets of experiments this summer in lakes that were undergoing 2,4-D treatments. For the first, water was taken directly from a lake that had 2,4-D applied, then distributed to tanks where the fish were held.

One of two lakewater systems Dehnert uses. In this one, water is taken directly from a lake that had 2,4-D applied, then distributed to tanks where the fish are held. Image credit: Gavin Dehnert, Wisconsin Sea Grant

The second employed an in-lake exposure system. Young fish were put in the lake in two-liter buckets with holes in them covered in mesh, which allowed water and food to pass through, but not the fish.

Dehnert explained, “This allowed us to see what goes on during an actual herbicide treatment. It’s applied to the entire lake and we look at what goes on with the fish.”

He anticipates a possible higher mortality in the lake setting because there are more variables at play. “I would expect more like a 35 to 45% decrease in survivorship because there are more stressors on the fish – temperature changes, storms, nutrient runoff, etcetera. That’s why it’s important to do this experiment in a natural lake setting, so we can get those real-world scenarios,” Dehnert said.

Dehnert is just beginning to process the data from his lake experiments and expects to finish up next year (2022).

Wisconsin lake associations are interested in Dehnert’s work because they want to eradicate Eurasian watermilfoil. Besides the use of an herbicide, the invasive plant can be controlled by manually removing the plants or by introducing beetles that eat it.

“All of these lake associations want to make sure they’re causing the least amount of impact to the other organisms in the lake,” Dehnert said. “So, it’s really exciting to work with them to determine the risks of the different control methods. How do we get rid of this invasive species but keep intact what we already have in the lake?

“Let’s understand what could happen, so we can make an educated decision on whether the benefits outweigh the cons,” he said.