A Fish for Biological
Management of Hydrilla and Other
Aquatic Weeds in Florida
David L. Sutton and Vernon V. Vandiver, Jr.
UNIVERSITY OF o ,-
Agricultural Experiment Station "
Institute of Food and Agricultural Sciences r
Cover: A young triploid grass carp. Photograph courtesy of Mr. Paul
Shafland, Florida Game and Fresh Water Fish Commission.
Stocking rates or suggestions for use of the grass carp fish represent the
opinion of the authors. The information given herein is supplied with the
understanding that no discrimination is intended and no endorsement by the
University of Florida and its Cooperative Extension Service or Florida
Agricultural Experiment Stations is implied. Successful control of aquatic
weeds depends on many factors; therefore, this publication is for
informational purposes only and is not intended to provide an endorsement
of a product or method to the exclusion of other products or methods
available for management of aquatic weeds.
A Fish for Biological
Management of Hydrilla and Other
Aquatic Weeds in Florida
David L. Sutton and Vernon V. Vandiver, Jr.
Dr. David L. Sutton is Professor (Aquatic Plants) and Dr. Vernon V. Vandiver is Associate
Professor (Aquatic Weeds Specialist) at the Fort Lauderdale Research and Education Center,
Fort Lauderdale, Florida.
Aquatic weeds cause serious problems in many ponds, lakes, rivers, and
irrigation and drainage ditches throughout Florida. In some situations, native
aquatic plants become weeds, and many exotic plants introduced from areas
outside the state flourish under the favorable growing conditions found in
Florida. Long-term economical solutions to Florida's aquatic weed problems
have been elusive and there is a need for new management techniques to
alleviate aquatic weed problems.
This bulletin provides information on a biological method for
management of some of Florida's aquatic weed problems. Emphasis is
placed on use of grass carp (Ctenopharyngodon idella Val.) fish to control
hydrilla and other aquatic weeds.
Since the grass carp is a living organism in contrast to either
herbicides or mechanical devices used for aquatic weed management a
somewhat different approach is required for using this fish. Knowledge of
life cycles of the aquatic plants that become weeds and of feeding behavior
of the grass carp are important for understanding the use of this fish to
manage aquatic weed problems. Grass carp will not be useful for all aquatic
weed problems; however, this fish does offer the potential to control, both
effectively and economically, certain weed species.
Man has been the single most important factor in the spread of hydrilla
(Figure 1) and other aquatic plants. The demand for aquatic plants for use
in aquaria and aquascaping has resulted in their sale and distribution
throughout the country.
Exotic aquatic plants have been introduced into natural waters by
individuals planting them for harvest at a later date or aquaria owners
dumping unwanted plants. Once established, plants such as hydrilla are
often spread by boaters traveling from one body of water to another.
Hopefully, public recognition of the problems that some of these plants
cause will lead to regulations and public cooperation to cease their
introduction and transportation, whether purposeful or inadvertent.
Herbicides are frequently used to control hydrilla, but biological
organisms offer unique advantages for management of hydrilla and other
aquatic weeds. Advantages of using effective, safe organisms to manage
biologically aquatic weeds include 1) longevity of the method once it has
become established; 2) constant feeding activity on the growing weed; 3)
low long-term costs; 4) high effectiveness on some plants; and 5) in the case
of fish, the potential for conversion of the weed to useful fish protein.
One of the most promising biological agents for many submersed plant
problems and some other aquatic weeds is the grass carp (Figure 2). This
fish is also sometimes called the white amur.
Description of Grass Carp
Grass carp are indigenous to those rivers in the eastern parts of Russia
and China that flow into the Pacific Ocean between latitudes 500 to 23.
Grass carp have been introduced into more than 50 countries throughout
the world for aquatic weed control and aquaculture purposes. In some
countries, grass carp are an integral part of fish culture and form an
important source of fish protein for human consumption.
Grass carp were considered for introduction into the United States
primarily for their plant eating ability, which was thought to have great
potential for control of aquatic weeds. In 1963, the U.S. Bureau of Sport
Fisheries and Wildlife Fish Farming Experiment Station, Stuttgart, Arkansas,
in cooperation with Auburn University, imported grass carp into the U.S. for
experimental purposes. In 1970, the United States Department of
Agriculture (USDA) and University of Florida transported grass carp from
Auburn University to Fort Lauderdale, Florida to evaluate the ability of
grass carp to control hydrilla.
Wide-scale use of grass carp in Florida and in many other states during
1970 to 1984 was limited and closely regulated due to fears about its
possible reproduction and potential negative impact on sport fishes. Since
the potential for grass carp to cause such problems was evident, early
research focused on developing a fish that would be non-reproductive, but
would retain the herbivorous diet of the grass carp.
Figure 1. Hydrilla may completely dominate a body of water by
excluding growth of native species and interfering with water uses
such as the irrigation pump system shown here.
Figure 2. A large grass carp grown primarily on hydrilla.
Grass carp are diploid with a chromosome number of 48 (2N). Sterile
fish are developed by shocking fertilized grass carp eggs with hot or cold
water, or with pressure. The shock causes the egg to retain an extra set of
chromosomes, resulting in a grass carp with a chromosome number of 72
(3N). The extra chromosomes make these fish sterile; however, the aquatic
weed control capabilities of the sterile triploid grass carp appear to be
essentially the same as the diploid grass carp.
The cell nuclei of triploid grass carp are larger than those of diploid
fish. Instruments such as a Coulter Counter can be used to measure the
nuclei diameter of grass carp blood cells. Therefore, it is possible to screen
fish considered for introduction into aquatic weed problem areas to assure
that each stocked fish is triploid.
Grass carp tolerate cold water and also flourish and grow at rapid rates
in warm waters such as those found in Florida. These herbivorous fish may
grow at a rate of 2 pounds (0.91 kg) or more per month when sufficient
vegetation is available. The young fish grow at a much faster rate than
older, mature fish, and females grow faster than males. In Florida, some
fish have grown to 90 pounds (41 kg) with a life span of 10 or more years.
The grass carp is primarily a grazerr"; it tends to feed near the surface
and in shallow water. At times, it can be seen feeding with its back and tail
extending above the surface.
The grass carp prefers submersed plants and the soft tips of young
tender plants (Table 1). Small grass carp prefer musk-grass over hydrilla
when both plants are present, but large fish will consume hydrilla when both
plants are present. Even though young fish will feed on various species of
filamentous algae such as Cladophora and Spirogyra, the grass carp is not
normally considered an effective method to control many algae problems.
Feeding rates of large, mature grass carp on algae are not well known.
When a preferred grass carp food is not available, this fish will feed on
other types of aquatic vegetation, and will even feed on terrestrial vegetation
hanging over the surface of the water. In fact, the name "grass carp" comes
from its unique ability to consume terrestrial grasses. Other terrestrial plants
eaten by the grass carp range from banana leaves (Musa spp.) to various
dried grasses, including clippings from golf courses or similar turf areas.
Caution should be exercised when feeding grass clippings to grass carp.
Some pesticides commonly used on turf areas are quite toxic to fish.
The ability of grass carp to consume and utilize aquatic plants depends
on the size of both plants and fish. Additional factors which influence the
feeding behavior of grass carp include size, age, gender, and population
density, as well as the species, abundance, and location of plants within a
body of water.
Table 1. A few common Florida aquatic plants eaten by grass carp in the approximate
order of preference.
Order of Common name Scientific name
East Indian Hygrophila
Hydrilla verticillata [L.f.] Royle
Egeria densa Planch
(Roxb.) T. Anderson
Lemna spp. and Spirodela spp.
Ceratophyllum demersum L.
Panicum repens L.
Stratioes aloides L.
Nasturtium oficinale R. Br.
Myriophyllum spicatum L.
Vallisneria americana Michx.
Pistia stratiotes L.
Nuphar luteum (L.)
Sibth. & Sm.
Birds, snakes, and other species of fish prey on small grass carp.
Based on measurements of mouth width and total length for largemouth bass
Figure 3A. Hydrilla growth prior to stocking of grass carp.
Figure 3B. The same pond after the hydrilla was controlled with
these herbivorous fish.
Figure 4A. An agricultural Figure 4B. A similar waterway
waterway filled with hydrilla. after control of hydrilla with
herbicides and grass carp.
*i~~ tduk?; -J
Figure 5. A barrier of evenly spaced PVC pipe to prevent escape
of grass carp in an agricultural area's irrigation ditch.
must be greater than 18 inches (450 mm) in length to eliminate all predation
from largemouth bass. When largemouth bass are present, grass carp
considered for stocking purposes should be at least 12 inches (0.3 m) in
length or 1 pound (0.45 kg) in weight minimize loss of stock fish due to
largemouth predation. Mortality due to largemouth bass predation increases
sharply with small grass carp. Other fish such as gar (Lepisosteus spp.) and
bowfin (Amia calva) may also eat small grass carp.
Small grass carp have a better chance of survival if they are stocked
when weeds are dense enough to provide protective cover. When the weed
biomass is low, such as after a herbicide treatment, use of mechanical
methods, or in a new body of water, fish in good condition and weighing 5
pounds (2.3 kg) or more will survive better than small fish.
To remove hydrilla and other aquatic weeds with grass carp alone, fish
must be stocked in sufficient numbers so that their consumption rate exceeds
the growth rate of the plants. With so many factors influencing the feeding
rate of the grass carp and the growth rate of the plants, it is impossible to
give one specific stocking rate that will apply for every situation. Rates of
20 to 255 grass carp per acre (50 to 638 fish per hectare) have been found
to provide effective control of hydrilla.
Removal of hydrilla with 40 fish per acre (100 per ha) is shown in
Figure 3. A dense growth of any weed will obviously require more fish
than a sparse amount of the plant. One approach is to stock 20 to 30 fish
per acre (50 to 75 grass carp per hectare). Then in 1 to 112 years, add more
fish if the desired level of control has not yet been achieved.
In a body of water, the area infested with weeds in relation to the total
area may be taken into consideration when determining the number of fish
to stock. Transect lines, recording fathometer tracings, biomass samples,
aerial surveys, or visual inspections are methods that can be used to help
estimate the amount of weeds present. In this way, the fish can be stocked
according to the amount of vegetation, rather than the total area of the body
of water under consideration.
Low oxygen levels less than 5 ppm can be encountered during the early
morning or on cloudy days in bodies of water containing dense aquatic plant
growth. In this case, it is better to use herbicides or to mechanically remove
the hydrilla before stocking grass carp. It would also be helpful to wait for
cooler water temperatures before stocking. Measurements of dissolved
oxygen made just prior to sunrise, particularly during the summer, will give
a good indication of whether sufficient dissolved oxygen is present for good
fish survival. Dissolved oxygen levels tend to decrease with increasing
water temperature. Dissolved oxygen readings of at least 3 ppm will help
ensure good fish survival. Feeding activity is also reduced during periods
of low dissolved oxygen, further reducing the ability of the grass carp to
control aquatic weeds.
Use of herbicides to remove a majority of hydrilla biomass before
stocking with grass carp will reduce the number of fish required, since the
fish need to consume only the newly-emerging hydrilla growth. This
integrated approach is an efficient and cost-effective way to manage hydrilla.
When herbicides are used, sufficient time must be allowed for the
indirect effects of the herbicide to diminish prior to stocking grass carp.
Although most aquatic herbicides are not toxic to fish, the decaying
vegetation may reduce dissolved oxygen to levels unsuitable for good fish
survival. Oxygen measurements should be taken to determine whether the
water is suitable for stocking grass carp.
In small bodies of water, five fish per acre (13 grass carp per hectare)
have controlled hydrilla regrowth following initial control with herbicides.
Rates lower than this may be possible, since five fish per acre in some
situations will eliminate all submersed plant growth. Therefore, to prevent
regrowth of hydrilla, but at the same time allow for growth of desirable
plants, rates of one to three fish per acre (three to eight fish per hectare)
may be used, with an occasional spot treatment with herbicide to control any
new growth of hydrilla which becomes more abundant than the fish can
Algal problems may occur once all the hydrilla has been removed from
a body of water. Also, other weed species may replace the hydrilla. These
two potential problems need to be considered when using grass carp or any
other method for management of hydrilla and other aquatic weed problems.
One area of interest is the use of grass carp to manage hydrilla
concomitant with establishment of desirable, native aquatic plants. In order
to accomplish this, low numbers of grass carp or the use of plants low on
the list of foods preferred by the fish are important considerations. With
grass carp, it may be possible to eliminate hydrilla and promote growth of
desirable aquatic plants to enhance water quality. Additional research is
needed for this potential use of the grass carp.
The grass carp has also provided control of hydrilla in some flowing
water situations. Figure 4 shows an agricultural canal before and after use
of herbicides and stocking of grass carp.
Limited work has been done on the use of grass carp in conjunction
with mechanical control methods prior to fish stocking. However, it is
likely that the number of fish required following mechanical removal of
hydrilla would be higher than the number required after application of
herbicides, but considerably lower than the number required if fish alone are
Grass carp should be stocked after any negative effects of mechanical
harvesting have diminished. For example, a temporary reduction in
dissolved oxygen may occur during harvesting, due to sediment disturbance
and suspension of decaying material into the water.
Mechanical methods can be especially useful for removing portions of
dense mats of weeds. This creates weed-free refuge areas where the fish
would not be subjected to the low oxygen levels that can occur in the thick
mat of plants.
The time of year to stock grass carp depends primarily on availability
of fish and on water quality. When the fish are used in conjunction with
herbicides or mechanical methods, stocking needs to be done prior to
regrowth of hydrilla, but after any negative effects of these treatments on
water quality have dissipated.
Grass carp can be transported and handled more safely during cooler
months than during the hot summer months. Injured fish are less susceptible
to diseases when stocked into cool water rather than warm water. Also, as
discussed earlier, cool water usually contains more dissolved oxygen than
In bodies of water with culverts or canals leading to other areas,
screens or gates must be installed to prevent escape of the grass carp.
Barriers need to be constructed so the fish cannot jump over them or go
through them. Rigid barriers (Figure 5) have proven effective in preventing
movement of grass carp while allowing unrestricted movement of water.
This type of barrier must have spaces between the bars narrower than the
body width of the stocked grass carp.
Grass carp are extremely difficult to remove from a body of water once
introduced. Draining the water or using a fish toxicant such as Rotenone are
two ways to remove them, but these methods will also remove other fish
Permit Requirements for Grass Carp
In Florida, a permit is required by law for use or possession of grass
carp. Only grass carp certified as triploid can be used in an aquatic weed
management program. Individuals interested in using grass carp for aquatic
weed management may contact the University of Florida Cooperative
Extension Service or the Florida Game and Fresh Water Fish Commission
for assistance in stocking rates, suppliers of certified triploid grass carp, and
procedures for obtaining a permit. Application for a permit may be obtained
by writing the Florida Game and Fresh Water Fish Commission, Bureau of
Fisheries Management, 620 South Meridan Street, Tallahassee, FL 32301.
Management of hydrilla and other aquatic weeds in many bodies of
water is possible with grass carp. An integrated management program
consisting of low stocking rates of fish combined with herbicide applications
or mechanical control methods may be the best way to quickly reduce
nuisance growth of weeds and maintain control. This approach may also
encourage growth of desirable native aquatic plants. Converting unwanted
weeds to valuable fish protein is an additional benefit of using grass carp.
Cross, D. G. 1969. Aquatic weed control using grass carp. J. Fish
Shireman, J. V. and C. R. Smith. 1983. Synopsis of Biological Data on the
Grass Carp Ctenopharyngodon idella (Cuvier and Valenciennes, 1844).
FAO Fisheries Synopsis No. 135. 86 pp.
Sutton, D. L. 1974. Utilization of hydrilla by the white amur. Hyacinth
Contr. J. 12:66-70.
Swingle, H. S. 1957. Control of pond weeds by use of herbivorous fishes.
Proc. South. Weed Conf. 10:11-17.
Van Dyke, J. M., A. J. Leslie, Jr., and L. E. Nall. 1984. The effects of the
grass carp on the aquatic macrophytes of four Florida lakes. J. Aquat.
Plant Manage. 22:87-95.
The authors would like to thank W. B. Ennis, Jr., J. C. Joyce, C. L.
Phillippy, J. V. Shireman, J. G. Stanley, J. M. Van Dyke, and F. J. Ware for
their critical review of this bulletin. We would also like to thank K. A.
Langeland and C. E. Cichra for their suggestions and review of the 1995
revision of this bulletin.
All programs and related activities sponsored for, or assisted by, the Florida
Agricultural Experiment Station are open to all persons regardless of race, color,
age, sex, handicap or national origin. Information from the publication is available
in alternate formats. Contact the Educational Media and Services Unit, University
of Florida, PO Box 110810, Gainesville, FL 32611-0810. This information was
published December 1986 as Bulletin 867; reprinted September 1988 and
November.1989; revised April 1995. ISSN 0096-607X