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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

April 1997

[photo of man & weeds]

Making the weed line work for your lake

When life gives you invasive weeds,
make habitat.

Karen Wilson and Steve Carpenter

Why fish & people need aquatic plants
How a cutback leads to growth
Nuisance vs. native
Milfoil forever?
A deep cut

Throughout Wisconsin, lakes heavily infested with Eurasian water milfoil often contain stunted populations of bluegill and largemouth bass. We tend to manage these lakes by cutting the watery "weeds" without appreciating the connection between the plants and the fish. Could we achieve more by viewing and managing these plant and animal communities together? The Department of Natural Resources, the University of Wisconsin's Center for Limnology, and the Dane County Public Works Lakes Management program took on a joint project to find out. We spent three years investigating if the way aquatic plants are harvested can grow bigger fish more quickly.

Many nutrient-rich lakes in Wisconsin are dominated by Eurasian water milfoil, an invasive aquatic plant that can grow from the surface to as deep as 16 feet, forming dense beds of leafy canopies over strong, spaghetti-like stems. Milfoil most often reproduces by breaking into fragments, which are then spread around by wind and boats within a lake. The plant is commonly moved from lake to lake on boat trailers and in live wells.

Though it is often viewed as an invasive pest, milfoil infestations aren't always bad. In lakes where there is little native vegetation, Eurasian milfoil can provide refuge and spawning habitat for fish and invertebrates. Even in heavily infested lakes, milfoil typically lasts only about 20 years, eventually dying down as weevils and other factors reduce its spread. Nevertheless, when swimmers get tangled and boats get mired in the dense weeds, milfoil becomes a problem.

Why fish (and people) need aquatic plants

Our project focused on milfoil-dominated lakes containing stunted populations of largemouth bass and bluegill – a predator-prey combination found in many Wisconsin lakes. The life histories of bluegill and bass are interconnected. When newly hatched, bluegill and bass compete for the same food items, usually while hiding in aquatic plant beds. As bass grow, they begin to eat small bluegill, often by the end of the bass's first summer. In their formative years, bluegill tend to grow chunkier bodies without getting much longer. It typically takes until their 5th or 6th year for bluegill to grow bodies that are high and wide enough that most bass can't swallow them. Once a bluegill is deep enough to have "size refuge" from largemouth bass, it can move freely around the lake.

Largemouth bass, bluegill and many other fish depend on aquatic plants at some stage in their lives for refuge and as a source of invertebrate and fish prey. Humans, however, view water plants like milfoil as nuisances because they grow to the surface, reduce access to docks and landings, make swimming uncomfortable, block water skiing and make boating difficult or impossible.

When county and city employees, lake associations and private lakeshore owners must weigh options to deal with dense aquatic plant growth, mowing is frequently selected as the remedy. Typically, floating weed harvesters are used to mow the plants about four feet under the surface. This provides short-term relief for boaters and swimmers from milfoil infestations – the same temporary relief in the water that mowing the grass provides for your lawn: most of the plant remains, plenty of light still reaches the plants, and they rapidly grow back to the surface. As a result, aquatic plant beds often must be cut several times each summer. Moreover, indiscriminate mowing can cut beneficial plants as well as nuisance species.

We were interested in testing if aquatic plants could be cut in such a way to improve fish growth while clearing areas that would remain open for a summer or longer.

We did our first experimental cuts by hand. Scuba divers cut the milfoil stems near the base, where they emerged from the lake bed sediments. This technique slowed the growth rate considerably. The channels hand-cut by divers in dense beds of milfoil were still visible from the air three years later.

We wanted to test if cutting the weed beds in different patterns or cutting narrower or wider channels might also affect fish growth. Fortunately for the divers, we conducted these tests on computer, synthesizing the known information on fish behavior, food availability and many other factors that change fish growth. We found that the more open edges we created between the lake and the plants, the more we affected bluegill and bass growth. Straight channels radiating from the shore to the middle of a lake were chosen to maximize the amount of edge cut. These channels were relatively easy to cut and were popular with home owners and lake users alike because they provided access to docks, swimming areas, boat launches and favorite fishing spots, as well as increasing the amount of "weedline" – a favorite structure for casting.

To test our theory, we chose 11 lakes with dense aquatic plant growth and stunted bluegill populations located throughout southeast and south-central Wisconsin. Many of the lakes were recommended by lake managers and lake associations as having a "weed problem" due to large beds of Eurasian milfoil. We selected lakes where both the lakeshore associations and area DNR lake managers wanted to work with the project team.

We cut channels in four of the 11 lakes, removing approximately 20 percent of the plants in each lake. A conventional weed harvester was outfitted with a "deep cutting" bar for the job. The bar, similar in design to a hedge trimmer, could be lowered as deep as 16 feet to cut the plants within a foot of the bottom. Channels ranged from six to 10 feet wide, and were cut from the lakeshore (six feet deep) out to the edge of the plant beds.

Aquatic plant communities were monitored throughout the experiment to be sure that plant communities remained similar between treatment and control lakes, and to track how long the open channels persisted in the cut lakes. After two years of regrowth, some lakes showed no visible evidence of channels, while over 60 percent of the channels in one lake with heavy milfoil growth remained at least partially visible from the air.

Fish growth from 1993 (a year before the channels were cut) and 1995 (a year after the cuts) was compared between cut and uncut lakes for both bluegill and largemouth bass. In all lakes where milfoil was cut, bluegill showed slightly faster growth, which we attributed to the channels. In their third and fourth years of life, bluegill in the uncut lakes grow approximately one inch per year, but in the cut lakes, three and four-year-old bluegill grew about an inch and a half per year.

The increases in growth during the bluegill's youth accumulate, just as long-term investments accumulate more interest in a bank account. The result is larger bluegill in the cut lakes.

Largemouth bass, on the other hand, showed little or no response to the cut channels, although they did not appear to grow slower than normal as a result of the channels.

How a cutback leads to growth

There are many possible explanations why cutting channels may increase bluegill growth. Based on the work of many researchers, we believe that increasing the amount of edge gives small bluegills greater access to prey – the snails, insects and other invertebrates associated with the plants. Channels may also give largemouth bass greater access to the smaller bluegill hiding in the plants, which reduces the number of bluegill competing for food in the plant beds. Fewer bluegill leave more food for remaining bluegill and young-of-the-year smallmoutth bass.

Bluegill.
Illustration by Virgil Beck, © 1997
[illustr. of a bluegill]

The results of this multi-lake experiment show that cutting channels into dense aquatic plant beds appears to increase bluegill growth without harming largemouth bass growth. The technique of creating channels by cutting plants deep down near the base can provide a temporary solution to managing nuisance plants such as Eurasian milfoil until they naturally go away. Still, the best way to return a lake to health is to slow nutrient runoff and protect the shoreline from the effects of development to give native aquatic plant communities a chance to recover.

Long, deep-cut channels can become one more tool in the lake manager's kit. Channels can help clear small lakes of the nuisance plants blocking access to docks, swimming beaches, and fishing areas. In lakes in which bluegill are stunted and there are dense beds of milfoil, we expect that channels will improve the growth of some age classes of bluegill, eventually resulting in larger fish. Coupled with size limits and protection of bluegill on their nesting grounds, channels can increase the size and population of fish in the lake. But, don't expect quick or dramatic increases in fish growth.

Nuisance versus native plants: what's the difference?

Not all aquatic plants are a nuisance or undesirable. In fact, native plants are, and should be, a prominent and important part of a healthy lake. One goal of aquatic plant management is to replace nuisance plants (such as Eurasian water milfoil) with native plants such as pondweeds and wild celery.

All aquatic plants, whether introduced or native, reduce the flow of water and muffle wave action. Reduced water flow settles out sediments and prevents re-suspension of sediments in the water. Without plants to play this important role, lakes can become turbid and lose any remaining plants for lack of light. A diverse complement of aquatic plant species provides food for waterfowl and tremendously important near-shore habitat for many fishes. Plants provide habitat for the invertebrates fish feed on. Fish such as northern pike and yellow perch lay their eggs on aquatic plants. Plants provide small fishes with a place to hide from predators.

Even though a mix of native plants is most desirable, it is important to realize that non-native plants such as Eurasian milfoil still play an important role in lakes. Non-native plants are better than no plants at all. If you are interested in clear water and good fishing, think twice before clear-cutting the plants in front of your cabin. Instead, spend your time working to protect the shoreline and reduce runoff in your community.

Milfoil does not last forever

Past experience in Wisconsin has shown that milfoil tends to invade and dominate lakes quickly, but subsides after about 10 to 20 years. The reasons for the plant's eventual decline are unknown. A build-up of organic matter around the roots may inhibit growth, or damage from a native water weevil may increase. Most beds of milfoil are clones (it reproduces when broken fragments take root), and it's possible the milfoil clone may simply lose vitality over the years. Interestingly, in Europe where Eurasian water milfoil is native, it is "just another" member of the plant community, and is not known to take over in dense beds. Elodea, a plant native to North America, is as aggressive in Europe as milfoil is in North America. The best defense is prevention. Keep milfoil infestations out of new lakes by removing milfoil fragments from live wells, boat hulls and boat trailers. Once milfoil has arrived in a lake, the only lasting way to "control" it may be to allow time to take its course.

A deep cut

Is a deep-bar weed harvester affordable for one community or lake district?

Maybe.

Purchasing a special weed harvester capable of cutting lake plants from six to16 feet deep is pretty costly. Manufactured mechanical deep-cutters cost $75,000 to $100,000. Further, these machines typically only cut the weeds, and must be followed by a conventional harvester to collect the weeds as they float to the surface. State statutes require that cut weeds be picked up unless a majority of the property owners decide to take personal responsibility to remove the weeds that wash up on shore, or all owners agree to let the weeds float freely in the water.

Weed rake. ©Robert Queen
Weed rake.

© Robert Queen

Contracting a crew for deep cuts isn't cheap: Few contractors own the equipment, it's very costly to mobilize the big rigs, contractors have to return every two to three years, a conventional harvester must follow the deep-cutter to scoop up weeds, and the plants eventually grow back.

About the only economical approach is to follow the example of the Dane County Public Works Lakes Management Program. Joe Yaeger and his work crew developed a deep-cutter bar that could be attached to the county's existing fleet of lake plant harvesters. Construction was fairly straightforward: a 20-foot aluminum I-beam was fitted with a six-foot-wide agricultural sickle bar that could be used under water. The I-beam is bolted to the harvester and is hoisted up to the vertical position by a hydraulic ram. The bar is raised and lowered by a hydraulically-driven winch. To avoid damage to the harvester and cutter, the deep-cutting attachment is designed with a break-away feature that swings the assembly away from the vertical position if the sickle bar hits an obstruction while the harvester is moving.

The cutter bar is designed as an attachment that can be removed when the harvester cuts weeds less than six feet down. Cost for design, materials and construction ran about $10,000, so a deep-cutter attachment might be economical for a lake district, community or county that already maintains a fleet of harvesters. Given basic design specifications, local metal fabricating machine shops should be able to construct the assembly.

Operating a harvester with a deep-cutter attachment does involve other costs. Each harvester must have a minimum two-person crew – one to operate the harvester and a second person to watch the depth finder to keep the cut at the right height and clear the bar. Unless a second harvester follows the first one, the operators will need to turn around and backtrack over the channel to pick up cut weeds.

Underwater obstructions like car bodies and cement anchor blocks can badly damage the sickle bar as the operator attempts to cut plants as close to the bottom as possible. Accurately measuring the depth to the bottom can be difficult as the harvester moves through dense weed beds. Like all harvesters, this one needs continual maintenance and repair.

Good cuts require a patient operator who goes slow, knows the lake contours and has a good feel for the equipment. Channels cut by experienced, patient operators last longer and do the intended job.

Shoreland residents often apply pressure on operators to clear all weeds around their docks. Communities should contact the Wisconsin Department of Natural Resources before adopting cutting practices. – by Paul Cunningham, DNR Fisheries Management

Karen Wilson and Steve Carpenter are lakes researchers with the University of Wisconsin Center for Limnology in Madison, Wis.