Battling Invasive Species in our Iowa Lakes

Battling Invasive Species in our Iowa Lakes

By Steve Weisman

Early in the fall of 2022, a shocking discovery was made on the Okoboji chain of lakes. Eurasian watermilfoil, an aggressive invasive plant that can negatively impact a body of water by outcompeting the native plants and becoming the dominant species was found. The scary part is that Eurasian watermilfoil can become thick enough that it interferes with boating and recreation and negatively affects the fishery. In our region, the plant does not seed but spreads through fragmentation. If the plant is broken apart, each fragment can potentially grow another plant.

According to Mike Hawkins, district Fisheries Biologist at the Spirit Lake Hatchery, the discovery was made in late August. Here on the Iowa Great Lakes system, a detailed water surveillance system occurs twice each summer. A total of 153 sites are checked through the use of rake grabs, and all species are identified. The June surveillance found nothing out of the ordinary. Then came August and Eurasian watermilfoil was discovered. The largest infested area was around the boat ramp on Upper Gar. That was certainly problematic with all the boats entering and leaving. So, that area was treated immediately. Furthermore, testing found that Eurasian watermilfoil had shown up in areas multiple areas in East Okoboji, Minnewashta, Lower Gar and, of course, Upper Gar. The question at that point was this: what do we do now?

Eurasian watermilfoil
Hawkins says, “Eurasian watermilfoil is a plant we’ve been working to stop in Iowa since the mid 1990s. In fact, as part of the first Aquatic Invasive Species Plan in the nation, Iowa broke ground with its proactive approach to invasive species management. That original plan centered most of its attention on Eurasian watermilfoil prevention and treatments. It was the biggest invasive threat at that time. Of course, since that time we’ve seen other invaders like zebra mussels and invasive carp take center stage.”

Across the state, the Iowa DNR has a zero tolerance for Eurasian watermilfoil. Hawkins adds, “Since 1995, Eurasian watermilfoil has been found in over 50 Iowa lakes and ponds and has been successfully eliminated in most cases. This approach also reduces the chance of the plant spreading to other lakes. Unfortunately, other states have not had the same aggressive approach and the number of infested water bodies near Iowa’s borders has grown substantially.”

Right after the initial discovery, people went into action. A task force of local leaders from the various lake associations, county, cities, Chamber of Commerce, drinking water utilities, and the DNR met to discuss the situation and possible treatment options. After much discussion, the group decided the answer was not in spot treatments. The group felt the entire system needed to be treated. Hawkins notes, “What followed was an extraordinary effort to put together a plan (unique in its scale and timing) to find $300,000 to pull off this monumental task. The Okoboji Protective Association, East Okoboji Lakes Improvement Corporation, Dickinson County Supervisors, and Dickinson County Taxpayers Association matched the Iowa DNR’s contribution in a matter of weeks. Other organizations and cities were also standing by if more funds were needed. Meanwhile, the DNR’s biologists were working with consultants and product manufacturers to develop this very aggressive plan.”It was determined that Sonar A. S. was the best choice for the treatment. It has very little impact on native aquatic plants, is extremely easy to apply and has the potential to eradicate Eurasian water milfoil. According to Hawkins, “Sonar A.S. and the active ingredient fluridone has an excellent safety profile. The EPA has labeled this herbicide for even the most sensitive areas and in lakes used as drinking water sources. On September 21, the Iowa DNR made the first application of a targeted herbicide treatment. For success, the herbicide must stay in contact with the plant for a long period of time, preventing it from making food from sunlight, eventually starving it. The concentration of the herbicide must be maintained into the spring of 2023 to accomplish this. Water samples are pulled from each lake segment every couple of weeks and additional doses are calculated and administered to each lake section as needed. The herbicide and the application method is designed to have minimal impacts on native plants. Because of the timing of the treatment, another invasive plant, curlyleaf pondweed, will also be mostly eliminated in the treated areas. However, because this plant forms a kind of seed bed, the treatment will likely only have one year of control.

There are no restrictions on fishing, swimming, or irrigation during or after the application. In fact, the EPA label does not have applicator personal protective equipment requirements.” The chemical has been successfully used to eradicate Eurasian watermilfoil on smaller lakes and ponds in Iowa since the 1990s. Hawkins says, “However, this effort on 2,240 acres is one of the largest and most logistically challenging of its kind ever performed in the nation.”

Asian Carp
This was not the first time that the Iowa DNR and Iowa residents have rallied to fight invasive species. Back in August of 2011, while fisheries biologists were collecting data on the fish in the Okoboji chain, they discovered two bighead carp that were between 14 and 16 inches in length. The next spring a commercial fishing crew netted 82 bighead carp and 55 silver carp in that same area. These fish were able to fight their way up the Little Sioux River during high water events. A major concern right away if these fish can reproduce in the lakes? Thank goodness the answer is most likely not. Asian carp eggs need the flow of a river as they develop. The eggs are neutrally buoyant, meaning they neither float nor sink. If they don’t have that flow, the eggs will settle out to the bottom and get covered up by sediment.

The Iowa DNR determined that an electric fish barrier would be the best bet to stop this type of migration into the Okobojis. According to Hawkins, “As water levels come up, the electric barrier is designed to step its electrical field strength up and create a field that fish are not able to pass.” Now in 2023, the electric barrier is still doing its job.

Yellow Bass
In many lakes, yellow bass is an invasive species and as a result have no limit restrictions. Since the early 2000s, the Iowa DNR has battled infestations in several lakes in southern Iowa. The fisheries in these lakes were overrun with slow growing high density yellow bass populations. The only answer for these lakes was a total fish renovation and restocking.

In 2005, yellow bass were found in East Okoboji and are now prevalent throughout the Okoboji chain of lakes. Yellow bass were most likely introduced by an angler. Unfortunately, as an invasive species, the yellow bass have the ability to upset the balance in a lake as the compete for food and resources. If a fishery and lake is healthy, it may be resilient enough to absorb the species without causing too much harm. Biologists believe that is what has happened in the Iowa Great Lakes. Yellow bass are present, but their number seem to be in check.

Other natural lakes in the region haven’t been so lucky, with populations swelling to the point that other gamefish populations are being harmed. Yellow bass populations are being monitored in several northwest and north central Iowa lakes, and the DNR is actively looking for solutions to this manmade problem.

Zebra Mussels
These D-shaped clams have alternating light and dark bands or stripes. Most are less than one inch long. They are filter feeders that can form dense clusters as they attach to hard underwater surfaces. In the case of large infestations, they may interfere with aquatic food chains, kill native mussels, clog water intakes, increase algae blooms, and cover beaches with dead shells. Young zebra mussels are microscopic and can be unintentionally transported with water in bilges, live wells or bait buckets. Adult zebra mussels can attach to boats, trailers and aquatic vegetation.

Unfortunately, there is no known way to eliminate this species. According to Hawkins, “We have found ways to address the Eurasian watermilfoil and the Asian carp, but zebra mussels are an invader that we have no solution for at this time.” The impacts of zebra mussels on the Iowa Great Lakes are still evolving, and it may be a decade or more before all the impacts are truly known.

Gizzard Shad
Gizzard shad were found in East Okoboji on August 11, 2017, by Iowa DNR fisheries biologists during one of their routine fish samplings. A total of 26 gizzard shad 3-5 inches in length were found. This was the first time gizzard shad had been found in the Iowa Great Lakes. At the time, Mike Hawkins, fisheries biologist for the Iowa DNR at Spirit Lake, said, “The Iowa Great Lakes are at the northern edge of this species’ range. They do not tolerate long winters and are usually found further to the south. Mild winters the past few years may have allowed them to survive further to the north.” In southern Iowa, gizzard shad have caused huge problems with the fisheries because they multiply rapidly and out-compete desirable species for food and space.

Future Efforts
It is a wakeup call for all of us in the state of Iowa. Our mobility means we might be on a lake in Minnesota or Wisconsin one day and be down here the next day. It is our responsibility to leave any aquatic invasive hitchhikers behind. Do not spread the contamination! It’s really quite simple…clean, drain and dry! If we all do that, the problem is solved, and our lakes are safe.
I urge you to protect and preserve all our waters in the state of Iowa from any invasive aquatic hitchhikers! If we don’t, I guarantee you in the future we will be saying, “Why didn’t we…I wish we would have!”