and
et al.
(1985) provide the following case history which demon-
strates the comparative low cost-high effectiveness of grass carp.
At Lake Baldwin, an 80-ha lake in Orlando, FL, about 80 percent of the
area was infested with
About 0,000 was spent over 3 years to
control the plants with HYDOUT (an endothall salt), at an average annual cost
Treatments provided control but had to be
of ,333 for 64 ha (0
In 1975 and 1979, grass carp were added at a total
repeated annually.
stocking rate of 35 fish per vegetated hectare and a cost of ,499
There was no appreciable growth of aquatic plants between 1980
(6
When costs are amortized over the expected duration of effective-
and 1985.
ness (10 years or more*), they fall to ha
(1975 dollars) or
in
Over that same lo-year interval, had herbicide treatments con-
1987 dollars.
tinued with no inflation of chemical and labor costs, about 3,333 would
have been spent.
Environmental Impacts
The positive aspects of the use of grass carp to control aquatic plants
are easily identified.
These include elimination of the target plants (and
often all plants); very low initial and long-term costs; low maintenance costs
(unless fish removal is required); greatly enhanced boating, water skiing, and
sometimes fishing; and long-term effectiveness.
These animals can approach
the "ideal" biocontrol agents when the system to be treated is large (making
other management procedures very costly) and when the nuisance plant is a
preferred species such as
The negative effects are less easily identified, in part because so
little is known about the long-term consequences of the elimination of all
macrophytes from a lake or reservoir.
Eradication of submersed and emergent
plants is not uncommon when stocking densities are high; it has so far proven
difficult to stock just enough carp to allow a certain desired percentage of
plant cover to remain, but stocking models (e.g., Wiley, Tazik, and Sobaski
1987) may correct this problem.
With techniques such as harvesting or
herbicides,
far more precise control of plant densities is possible.
In many
cases, distinct water quality changes accompany macrophyte control or erad-
ication by grass carp.
These changes almost always include increased
Personal Communication, 1987,
Van Dyke, Florida Department of Natural
Resources, Tallahassee, FL.
136
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