Report on aquatic plants, Eurasian water milfoil and
fish by
Laura Stremick-Thompson, Fisheries Biologist-Horicon DNR
Aquatic plants form the foundation of a healthy and flourishing lake ecosystem.
They not only protect water quality, they also produce life-giving oxygen. Aquatic
plants are a lake's own filtering system, helping purify and clarify the water
by absorbing nutrients like phosphorus and nitrogen that could stimulate undesirable
algal blooms. Plant beds also stabilize soft lake and river bottoms and limit
shoreline erosion by dampening the effect of waves and current.
It makes sense that the best fishing spots are typically near aquatic plant
beds. Aquatic plants supply critical habitat to fish, in the form of cover from
predators, spawning habitat, and juvenile rearing areas. In addition aquatic
plants serve as habitat for invertebrates and zooplankton that are primary foods
of most small and some adult fish such as crappie and bluegill. While
aquatic plants form the base of a healthy lake ecosystem, no one plant species
has the ability to supply all of the habitat and food to all fish species throughout
their various life stages. In addition, native plant species have adapted
to grow with other native plant, fish and invertebrate species, in a delicate
ecosystem balance or dynamic. Therefore, native aquatic plant diversity
is also important in maintaining ecosystem health and supporting a healthy fishery.
For example, emergent plants (those with leaves that extend above the water
surface) such as bulrushes, create prime spawning habitat for predator fish
such as northern pike. Cattails, another emergent plant, may not serve
as great spawning habitat for pike, but they support invertebrates for food
for small pike, and also help to filter water, thus helping to maintain better
water quality that supports all species of fish. Floating leaved plants
such as lily pads often harbor bluegill attempting to hide from largemouth bass
patrolling the perimeter looking for prey. Another important benefit to
maintaining healthy native aquatic plant communities is that they discourage
the spread of nuisance causing exotic plants like Eurasian water milfoil.
Unlike many other plants, Eurasian water milfoil does not rely on seed for reproduction,
and its seeds germinate poorly under natural conditions. It reproduces vegetatively
by fragmentation, allowing it to disperse over long distances. The plant produces
fragments after fruiting once or twice during the summer, and the shoots may
then be carried downstream by water currents or inadvertently picked up by boaters.
Milfoil is readily dispersed by boats, motors, trailers, bilges, live wells,
or bait buckets, and can stay alive for weeks if kept moist.
As an opportunistic species, Eurasian water milfoil is adapted for rapid growth
early in spring. Stolons, lower stems, and roots persist over winter and store
the carbohydrates that help milfoil claim the water column early in spring,
photosynthesize, divide, and form a dense leaf canopy that shades out native
aquatic plants. Once established in an aquatic community, its ability
to spread rapidly by fragmentation and effectively block out sunlight needed
for native plant growth often results in monotypic (only one type) stands. Monotypic
stands of Eurasian milfoil provide only a single habitat, and threaten the integrity
of aquatic communities in a number of ways; for example, dense stands disrupt
predator-prey relationships by fencing out larger fish, and reducing the number
of nutrient-rich native plants available for waterfowl. Dense stands of
Eurasian water milfoil also inhibit recreational uses like swimming, boating,
and fishing. The visual impact that greets the lake user on milfoil-dominated
lakes is the flat yellow-green of matted vegetation, often prompting the perception
that the lake is polluted "infested" or "dead". Cycling
of nutrients from sediments to the water column by Eurasian water milfoil may
lead to deteriorating water quality and algae blooms of infested lakes.
The next time you’re fishing and concentrating on working the vegetation
to get a bite, think about the vegetation types you’re seeing in the lake.
While you may be able to pull a fish out of a stand of Eurasian water
milfoil, it can be an indicator of declining native aquatic plant species, disrupted
fishery dynamics, and ultimately poor water quality for your favorite fishing
lake. Think about the “big-picture” and remember that all
things in an ecosystem are connected in some fashion, and when the balance is
disrupted, the effects may be direct or indirect, and may not even be perceptible
to us. Do your part to halt the spread of all exotics, by remembering
to inspect all boats, motors, trailers, bilges, live wells, and bait buckets
when leaving and entering a body of water.
Laura Stremick-Thompson
Fisheries Biologist-Horicon DNR