The cost of aquatic plant control using grass carp in small water-bodies is less
than $250 per hectare with benefits extending more than seven years (Shireman, Colle,
and Canfield 1985). Because using grass carp to control aquatic vegetation is inexpensive
and longer term compared to other control techniques, there is widespread interest
in expanding its use throughout the United States.
The diploid grass carp has been used for biological control of aquatic plants from
the 1970s until the 1980s. Numerous studies have been conducted to evaluate its
potential for reproduction, feeding preferences, stocking rates, and impacts on
other aquatic resources (Smith and Shireman 1983). Based on a five-year study at
Lake Conway, Florida, practical guidelines for the use of diploid grass carp to
manage aquatic plants in ponds and lakes were developed (Miller and Decell 1984).
However, the potential of the diploid grass carp to naturally reproduce caused considerable
controversy over its use as a biological control agent. This eventually led to the
production or sterile, triploid grass carp which most states allow to be used for
the control of aquatic plants, at least for experimental purposes.
This article reviews the development and biology of the triploid grass carp and
provides recommendations for its use as a biological control of nuisance aquatic
vegetation. Triploid and diploid grass carp are morphologically identical and, reproduction
notwithstanding, are assumed ecologically similar. Therefore, most data obtained
from studies of diploid fish should be applicable to triploid fish (Sutton 1985).
Development of the Triploaid Grass Carp
In 1963, the grass carp was first introduced at Stuttgart, Arkansas; subsequent
introductions and dispersal rapidly expanded its range to at least 35 states (Guillory
and Gasaway 1978, Clugston and Shireman 1987). Successful spawnings of grass carp
in United States waters were predicted (Stanley, Miller, and Sutton 1978) and the
potential for negative impacts on aquatic ecosystems influenced many states to prohibit
or restrict the use of grass carp (Figure). Consequently, there has been a concerted
effort to produce a low-risk control agent, that is, a non-reproductive grass carp.
Grass carp, like most fishes, are diploid. They possess two sets of chromosomes
(one from each parent) and are capable of sexual reproduction. Triploid fish, however,
have three sets of chromosomes and are incapable of normal sexual reproduction and
the production of viable offspring. The use of triploid grass carp would severely
limit the likelihood and magnitude of any negative impacts, but still allow benefits
of its use, that is, long-term, low-cost plant control and the production of harvestable
fish (Sutton 1965, Sutton and Vandiver 1986).
Triploid fish were first produced in the United States in 1979 as inter-specific
crosses between female grass carp and male bighead carp Aristichthys nobilis (Molone
1982). These fish contained an extra set of chromosomes and were incapable of reproduction,
but were hybrids of the two species and not true grass carp; successful plant control
was limited. Later, triploid grass carp were produced in-transpecifically, by physically
shocking fertilized eggs with heat, cold, or hydrostatic pressure; this stimulated
the retention of a set of chromosomes that would normally be expelled during cell
division (Clugston and Shireman 1987, Cassani and Caton 1986, and Allen and Wattendorf
1967).
Physical shock techniques induce polyploidy with apparent yields of 100 percent
triploids (Cassani and Caton 1980). Because of this high success rate, this technique
is being used to produce large numbers of fish. Concern exists, however, that induced
polyploidy could result in the production of some reproductive grass carp (i.e.,
diploid or reproductive triploid fish). Consequently, triploidy in each must be
verified prior to stocking. Ploidy is most often determined by a Coulter Counter,
which electronically measures the volume of a red blood cell after the cell membrane
has been chemically removed (Allen and Wattendorf 1987, Clugston and Shireman 1987).
Since triploids have larger red blood cells (and nuclei) than diploids, cell size
differences are used to confirm triploidy (Allen and Wattendorf 1987).
Stocking of triploid grass carp will increase with greater availability of fish
and greater confidence in their use. Six hatcheries in the United States are commercially
producing triploid grass carp, and the number is likely to increase. As production
techniques are perfected, cost ($4.00 per 20 cm fish) should decrease (Clugston
and Shireman 1987). On December 2, 1965, in response to a request from South Carolina,
the US Fish and Wildlife Service (FWS) issued a "Biological Opinion." This opinion
stated that female triploid grass carp are functionally sterile and that the sperm
of male triploid grass carp is probably nonfunctional. This document, however, was
not intended to promote the use of grass carp but was forwarded to all FWS regional
offices for states that wish to implement studies on grass carp using federal funds.
Size at Stocking
Large grass carp provide more effective plant control than smaller individuals (Sutton
1985). First of all, small fish suffer high mortality due to predation by birds,
snakes, and other fish. In addition, although feeding rates of small fish are high,
quantities of plants eaten by smaller fish are not necessarily higher than those
eaten by larger individuals. The difference in consumption rates between small and
large fish is also a consideration. Fish weighing less than 3 kg (approximately
600 mm in length) eat 100 percent of their body weight daily, but substantially
larger fish (3 to 6 kg) eat 75 percent or their body weight. Because of their larger
body weight, use of larger fish can result in larger absolute quantities of plant
eaten per fish (Clugston and Shireman 1987). Large triploid grass carp, however,
are difficult and more costly to obtain commercially, so this often necessitates
the stocking of smaller fish and the use of alternative controls. Considering these
reasons, the stocking size of triploid grass carp usually ranges from 200 to 300
mm.
Time of Stocking
Plant control strategy, water quality, and availability of commercial fish determine
the time of year to stock grass carp (Sutton and Vandiver 1986). If grass carp are
used in conjunction with herbicides or mechanical methods, they should be stocked
after the effects of these treatments have been achieved and before re-growth of
the plants. Otherwise, cooler months of the year are the best time for moving and
handling triploid grass carp because fish are less susceptible to injury and disease
(Sutton and Vandiver 1986, Thomas and others 1990). Spring is the most common time
that commercial hatcheries have adequate numbers of triploid grass carp available
for transport and stocking. Pine, Anderson, and Hung (1990) suggest that triploid
grass carp be stocked in spring in order to match fish consumption with increases
in plant production in canals. In summary, grass carp are usually stocked in spring
when the problem aquatic plants begin to emerge
Stocking Densities
To achieve effective control of problem plant species, triploid grass carp must
be stocked in sufficient number such that the rate of total plant consumption by
the fish is equal to or exceeds growth rates of those plants (Sutton and Vandiver
1986). Consumption and stocking rates depend on several factors, including the size
of the fish, the density of the plant or plants to be consumed, the size of the
water body, the water temperature during the growing season, and whether mechanical
or herbicide treatments were used prior to the stocking of the fish. Several simulation
models are available that consider these factors and help determine stocking rates.
One model was developed from pond studies in Illinois using diploid grass carp (Wiley
and others 1965) and another model developed from data collected at Lake Conway,
Florida, over a five-year period (Miller and Decell 1984). The latter model was
further modified for triploid grass carp (Boyd and Stewart 1990) (See AMUR model
section).
An additional consideration for stocking large water bodies such as reservoirs is
the movement of grass carp. If grass carp move away from target control areas, they
would be ineffective for aquatic plant management. Grass carp of stocking size may
stay near their release points, but as these fish grow and reach sexual maturity
(about 650 mm or 3.5 kg), they can undertake extensive migrations that lead to dispersal
of grass carp outside the target areas (Rain, Steeger, and Tangedal 1989). Therefore,
annual stockings of grass carp may be required in large water bodies because fish
stocked in previous years may disperse to distant areas. Poor water quality conditions
(such as low dissolved oxygen and high water temperature) in vegetated areas may
also influence grass carp movement, causing the fish to move away from dense plant
beds (Chappelear and others, in preparation).
Given the number of variables that must be evaluated, stocking rates are extremely
variable and range from 2 to 500 fish per vegetated hectare (Stocker and Hagstrom
1985, Bonar, Thomas, and Pauley 1987, and Rates and Webb 1986). However, the most
common rates are 25 to 60 fish per hectare (Allen and Wattendorf 1987) using fish
approximately 300 mm long (0.5 kg per fish). Stocking rates should be based on the
amount of vegetation (for example, number of fish per vegetated acre) rather than
using the size of the water body as a determining factor.
Feeding Habitat and Growth Rates
Grass carp preferentially feed on or control many species of aquatic plants. Feeding
habitats are influenced by vegetation composition, water temperature and the size
of the fish (Bowers, Gilbert, and Thornton 1987, Clugston and Shireman 1987). Fiber
content, which varies among plant parts and plant species, can significantly influence
palatability of the plant to grass carp. Low-fiber plants are generally preferred,
but fibrous plants may be controlled by stocking in the spring when the fish can
feed upon the new shoots and buds (Prowse 1971).
Temperature affects the rate of feeding by grass carp: feeding stops when the water
temperature reaches 11 degrees Celsius, declines at temperatures above 30 degrees
Celsius, and is optimum at temperatures from 20 to 30 degrees Celsius (Young, Monaghan,
and Heidinger 1983, Wiley and Wike 1986). The daily consumption rates of grass carp
are relatively high compared to other herbivorous fishes. Small grass carp (< 400
mm) can consume up to two times their body weight under optimal conditions, a rate
which decreases to 80 percent of their body weight as the fish grow (Miller and
Decell 1984, Clugston and Shireman 1987).
Growth rates for the triploid grass can vary, but rates are generally lower than
those of diploids, possibly because triploids exhibit lower feeding rates than diploids
(Osborne 1982, Young, Monaghan, and Heidinger 1983). Although the triploid is a
very close energetic match to the diploid in assimilation efficiency and metabolic
rate, it exhibits a 10 percent lower rate of ingestion which is manifested as a
16 percent decrease in growth (Wiley and Wike 1986, Wiley and Garden 1984). Growth
of triploid grass carp ranges from 0.19 to 3.36 g per day compared to a range from
0.64 to 14.02 g per day for diploid grass carp (Wiley and Wike 1986).
Future Use of Triploid Grass Carp
Triploid grass carp are currently used for vegetation control in many states throughout
the country, although restrictions vary among the states. Alabama and Arkansas,
the initial importers of the fish, along with 12 other states, allow triploid and
diploid grass carp to be stocked in their waters. Fifteen states issue special permits
that allow the use of triploid grass carp for vegetation control (Allen and Wattendorf
1987). Four states permit experiments with triploids, and 19 states still technically
prohibit importation, but several of these may issue limited experimental-use permits.
The most important factor in the selection of triploid grass carp as a control agent
for aquatic macrophytes is its functional sterility. Absolute guarantees that a
triploid will never reproduce are tenuous, given longevity (10+ years), variability
in size (four orders of magnitude), and the absence of long-term studies (Wiley
and Garden 1984). The chances of reproduction among triploid individuals are certainly
negligible. Managers, however, must balance the risk with the beneficial results
of biological control of nuisance plants. With the technology available today, triploid
grass carp may be monitored and controlled and a decision for using them possibly
reversed (Allen and Wattendorf 1987).
Grass Carp Feeding Preferences
|
Plants Consumed Preferentially or Controlled
|
Plants Sometimes Preferred or Controlled
|
Plants not Preferred and Somtimes not Controlled
|
|
Cabomba caroliniana,fanwort
|
Azolla caroliniana, azolla or water fern
|
Najas flexilis, stonewart
|
|
Chara spp., muskgrass
|
Bacopa spp., water hyssop
|
Ceratophyllum demersum, coontail
|
|
Egeria densa, Brazilian elodea
|
Eleocharis, slender spikerush
|
Typha spp., cattail
|
|
Elodea canadensis, elodea
|
Nasturtium officinale, watercress
|
Nuphar spp., spatterdock
|
|
Hydrilla verticillata, hydrilla
|
Potamogeton spp., pondweeds
|
nymphaea spp.,water lily
|
|
Lemna spp. and Spirodela spp., duckweeds
|
Spirogyra spp., algae
|
Myriophyllum spicatum, Eurasian watermilfoil
|
|
Najas quadalupensis, souther naiad
|
|
Vallisneria americana, tapegrass or eel-grass
|
|
Sagittaria graminea, coastal arrowhead
|
|
Myriophyllum aquaticum, parrot-feather
|
|
Utricularia gibba, eastern bladderwort
|
|
Eichhomia crassipes, water hyacinth
|
|
Wolffia spp., watermeal
|
|
Hydrocharis morsus-ranae, frogbit
|
|
|
|
Carex pseudo-cyperus, sedge
|
|
|
|
Scirpus spp., bulrush
|
|
|
|
Ludwidgia repens, water primrose
|
|
|
|
Myriophyllum heterophyllum, variable leaf milfoil
|
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