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August 2005
BASS Times
Vegetation Matters
Habitat management with chemicals
By Mike Pehanich
To kill or not to kill? That is the question fisheries managers
ask every year about the surface mats of Eurasian water milfoil
that choke fisheries, impede navigation and, ultimately, stunt
fish growth rates on prime bass waters across the nation.
Indeed, it had been the question for a number of years at
Spring Lake, a fishery located in Tazewell
County south of Pekin, Ill.
Fed by cold spring waters, the fishery hosts a strong bass
population. North Spring Lake also has provided brood stock
for a successful muskellunge stocking program, despite having
an average depth of only 2.9 feet.
Prior to 1916, Spring Lake was a natural backwater marsh created
by the Illinois River. But that year, levees impounded the lake
and stabilized water levels as part of a 12,000-acre drainage
district project. In 1978, the lake was divided into north and
south sections.
The lake seemed to win a battle with rough fish after rotenone
treatment in 1981. Seventeen species of aquatic vegetation rebounded
at the 1,200-acre lake, and gamefish populations rebounded,
too.
Once Eurasian water milfoil (Myriophyllum spicatum) entered
the mix, however, things began to change. The milfoil spread
quickly and choked out many native plants. This plant coverage,
in turn, severely restricted access to fishermen, resulting
in a short window of weedless fishing opportunities between
mid-March and mid-May.
National nuisance
A combination of federal dollars and state matching funds derived
from the Aquatic Nuisance Species fund brought the $10,000 needed
to take on the milfoil in 2004. The Illinois Department of Natural
Resources saw an opportunity to test the effectiveness of two
different chemicals on the northern and southern portions of
the lake.
"Spring Lake was the guinea pig," admitted Wayne
Herndon, a district biologist with the Illinois Department of
Natural Resources (DNR). "We studied the literature. We
worked with biologists in Minnesota and Wisconsin to determine
the best methods of treatment. Everyone is interested in getting
an answer to this problem."
Spring Lake became the ideal laboratory for the fisheries biologist.
With distinctly defined north and south sections, the DNR could
compare results of side-by-side treatment within relatively
equivalent aquatic environments.
Herndon and colleagues rejected biological and mechanical options
(see related story on facing page) and opted for chemical treatments.
They used 2, 4-D L.V. 6 Ester (commonly called 2-4D)
in the south lake and Fluridone
(also referred to as Sonar) in North Spring.
"We wanted to see which chemical delivered the best control
and economics," explained Herndon.
Hardiness and resistance to cold gives Eurasian milfoil an
advantage over other aquatic plants. Stolons, roots and lower
stems will grow even under the ice. Only a total freeze out
of a lake or a drawdown under freezing conditions will kill
it.
Moreover, milfoil's ability to store carbohydrates gives the
plant a head start over other vegetation in spring. It photosynthesizes
and grows rapidly, spreading a thick canopy that prevents sunlight
from reaching other plants.
Chemical treatment is most effective shortly after ice-out
when new shoots are beginning to grow.
Defining the plant
For the sake of clarity, some species of milfoil are native
to the U.S. and live harmoniously with other native aquatic
plants. On the other hand, Eurasian water milfoil is a hardy
exotic endemic to Europe, Asia and north Africa.
Bass and other sportfish thrive amid healthy vegetative growth.
But too much milfoil can open up Pandora's Box.
"First, it interferes with human use: swimming, boating
and most forms of fishing," explained John Madsen, assistant
professor of research and extension at Mississippi State University
and an acknowledged milfoil authority.
"Ecologically, it reduces the transfer of oxygen, which
reduces the natural re-aeration of the water. Oxygen levels
under the milfoil canopies are very low. It also interferes
with light penetration and shades out the native plants, reducing
the diversity of plant and animal life."
Anglers often spread milfoil inadvertently via boats, motors,
trailers, bait buckets and livewells. It also spreads by natural
downstream transmission.
But the biggest problem with Eurasian milfoil is that it doesn't
know when to stop. Once it takes root, it spreads like wildfire,
driving anglers and waterfront owners nuts.
Bass grow to hate it, too. Or don't grow at all. And therein
lies the problem.
"Milfoil alters the predator-prey balance that can be
maintained more readily with native plants," said Madsen.
"Bass need edges to be really effective predators,"
added Herndon. "The prior year [at Spring Lake], we could
see the thick milfoil was hurting the bass. They were thin,
not the rotund fish they should have been. The heads were too
big for the bodies."
Thin and long, the muskies had been working too hard for their
supper as well.
"The vegetation was tying up too much forage," explained
Herndon. "Now the individual condition of the fish is much
better. They are thick bodied. Reproduction is better too."
Sweet success
Both Fluridone and 2-4D treatments proved effective at Spring
Lake in 2004, but the 2-4D was significantly less expensive.
Herndon noted another advantage to 2-4D: "Native
plants began to rebound faster with the 2-4D," he
said. "Within 90 days — by July — native coontail
had made a comeback."
The successful milfoil control effort bodes well for other
Illinois lakes strangled with milfoil, particularly other Illinois
River backwaters experiencing a renaissance of late. (See the
February 2005 issue of BASS Times.)
Already this year, Herndon, DNR microbiologist Steve Shults,
and Gary Sullivan, a biologist for The Wetlands Initiative,
replicated the Spring Lake 2-4D treatment on another pair of
backwater fisheries — Hennepin and Hopper lakes. Both
had experienced a similarly rapid Eurasian milfoil invasion
over the last two years.
As for Spring Lake, a local bass tournament held there earlier
this year found 68 anglers entering 425 pounds of legal bass,
including 16 limits and a "big fish" winner of 6.75
pounds.
"And the muskie have benefited too," concluded Herndon.
"Like the bass, they are in super condition & thick-bodied
and beautiful."
Weed control options
John Madsen, assistant professor of research and extension
at Mississippi State University, calls Eurasian water milfoil
"possibly the most widespread non-native aquatic plant
in North America."
An acknowledged expert in the field, he evaluates the most
common methods of Eurasian milfoil control as follows:
Systemic herbicides
These chemicals are spread over large expanses of water at
low concentrations, killing the stems and shoots of undesirable
plants like Eurasian water milfoil.
The most common chemical in this category is 2-4D, which has
a long track record and offers several distinct advantages over
other systemic herbicides:
1.) At $20 to $30 per gallon and effective at low concentrations,
it's both reliable and economical;
2.) It works selectively on broadleaf vegetation like milfoil
and will control native and Eurasian milfoil, and at higher
concentrations, it can also affect coontail; and
3.) Used at recommended concentrations, it is safe for fish,
invertebrates and other plants. Safety warnings suggest, however,
a minimum 21-day waiting period before using water treated
with 2-4D for swimming, drinking, fish consumption or irrigation.
Native plant regeneration may commence within 90 days of treatment.
Fluridone usage entails none of
the restrictions of 2-4D and can be just as effective. However,
it is far more expensive at $600
per quart versus the $20- to $30-per-gallon cost of 2-4D. Furthermore,
it is more difficult to fine-tune effective dosages. Fluridone
must maintain contact with the plant for 60 to 90 days to be
effective and, in some cases, may require repeated application.
A rapid water exchange rate or extensive deep water acreage
may compromise its effectiveness.
Triclopyr acts similarly to 2-4D and has fewer usage restrictions;
however, it is also more expensive.
Contact herbicides
Contact herbicides are used to treat small areas of milfoil
growth. Two of the most popular chemicals are Endothall and
Diquat. Both may work effectively even at small dosage levels,
but several applications per growing season may be required.
Mechanical
This simple and direct method is of dubious value as a milfoil
control measure. Though the effects of mechanical cutting equipment
are immediate, milfoil grows back quickly and will likely spread
more rapidly with the migration of severed stems.
Drawdowns
Eurasian milfoil remains remarkably hardy in cold water, but
portions of the plant will die after being entrapped in winter
ice for extended periods. Fisheries managers may draw down water
levels, in combination with using chemical treatment, to freeze
exposed and shallow milfoil.
Biological
Chinese grass carp are plant eaters. They were introduced to
North America to control excessive aquatic weed growth. One
big problem associated with this control measure, according
to Madsen: Grass carp don't like to eat Eurasian water milfoil.
They are much more effective with hydrilla infestations.
Three types of non-native insects — species of aquatic
caterpillar, midge and weevil — can help control milfoil
growth and are gaining favorable attention in some parts of
the country. But they are effective only under very narrow sets
of circumstances. However, high concentrations of weevils may
reduce milfoil density in lakes with low sunfish populations.
The naturalized fungus known as MT (Mycoleptodiscus terrestris)
has shown some ability to reduce milfoil density, but further
study seems necessary.
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