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J. Aquat. Plant Manage.
47: 2009. 1
J. Aquat. Plant Manage.
Ecological Impact of Grass Carp:
A Review of the Available Data
ERIC D. DIBBLE AND KATYA KOVALENKO
The exotic grass carp (
) has been
used for almost a half a century in the United States as a bi-
ological agent to control and manage aquatic plants. This
long-lived generalist herbivore consumes large amounts of
vegetation and can considerably alter habitat and impact
aquatic communities. We conducted a literature review to
determine whether previous studies adequately addressed
ecological impacts of grass carp and their underlying mech-
anisms. Our goal was to identify strengths and limitations of
ecological assessment in the literature and suggest a trajec-
tory of future research. The review yielded 1,924 citations
on grass carp; however, data on ecological interactions were
limited, and most research emphasized the biology of grass
carp or eradication of aquatic plants rather than ecological
mechanisms responsible for ecosystem-wide impacts. Very
few studies addressed effects on habitat complexity or com-
munity-structuring processes. We provide a comprehensive
tabulated overview of feeding preferences and environmen-
tal impacts of grass carp. We argue that ecology is para-
mount to evaluating grass carp impacts and thorough
understanding of these impacts is essential for the appro-
priate management of aquatic communities. Current
knowledge is not sufﬁcient to accurately predict long-term
effects of grass carp on freshwater ecosystems. We advise a
more cautious approach to developing guidelines for grass
aquatic plant management,
, habitat alteration, literature review.
During the last 45 years, a considerable amount of infor-
mation has been published on grass carp,
(Val.). Reports of grass carp use in management started
to proliferate in the mid 1970s, approximately 10 years after
the ﬁrst grass carp were stocked into Arkansas reservoirs with
the hope that this large exotic herbivore would control
growth of noxious aquatic plants (Pierce 1983). These ﬁrst
reports emphasized the biology and physiology of the ﬁsh
and its feasibility as a tool for weed control (Sills 1970, Stott
and Robson 1970, Shireman and Maceina 1981, Shireman
1984, Wiley et al. 1984, 1987, Leslie et al. 1987). Much of this
literature was stimulated by the apparent success and efﬁ-
ciency grass carp demonstrated in reducing and even eradi-
cating entire macrophyte communities. Encouraged by the
enthusiasm of ﬁnding an effective management tool for an
ever-increasing problem of invasive plant infestations, this
emphasis prevailed in the literature for more than a decade.
By the mid-to-late 1980s, the interest shifted toward address-
ing questions of the direct impacts these herbivores had on
environmental parameters other than targeted aquatic
Concurrently, literature was developing in the disci-
pline of aquatic ecology that addressed indirect effects on
the structuring of aquatic communities. Much of this liter-
ature emphasized effects of exotic introductions and dom-
inant organisms on community structure (Pimm 1987,
Ross 1991, Simberloff 1990, Soule 1990, Vitousek 1990)
and provided evidence that a keystone species can alter
environmental conditions and mediate community pro-
cesses (Paine 1966), along with other indirect mechanisms
responsible for altering community structure (Kerfoot
and Sih 1987). Many of these mechanisms involve multi-
species interactions, such as competition, predator-prey
interactions, and aquatic plants as ecologically important
components within aquatic ecosystems (Jeppesen et al.
1997). Submerged macrophytes are important for water
quality, nutrient dynamics, and invertebrate-ﬁsh interac-
tions (Jeppesen et al. 1997 and references therein). Spa-
tial heterogeneity across a landscape is important for
ecological interactions (Palmer and Poff 1997, Schaffer
1981). Like a landscape, a waterscape exhibits similar het-
erogeneity due to the differences in spatial complexity
and structural habitat provided by aquatic plants. Habitat
heterogeneity in aquatic systems mediates mechanisms re-
sponsible for determining trophic interactions, animal
distribution (Werner and Hall 1979, Savino and Stein
1982, Mittelbach 1988, Diehl 1993), and community com-
position, such as species diversity and abundance (Gilin-
sky 1984, Adams and DeAngelis 1987). It was previously
thought that macrophytes provide few functions other
than physical structure, but now it is clear they are grazed
by a variety of animals (Lodge et al. 1997) and also indi-
rectly contribute to lake primary productivity by support-
ing large biomass of periphyton (Wetzel 2001). Vegetated
habitats provide abundant food reserves for mammals, wa-
terfowl, amphibians, reptiles, ﬁsh, and invertebrates
(Swanson and Meyer 1973, Pardue and Nielsen 1979, Gi-
linsky 1984, Keast 1984, Eldridge 1990, Fredrickson and
Laubhan 1996). Grass carp can cause major reduction or
complete elimination of aquatic plants when introduced
into aquatic systems (Van Dyke et al. 1984, Wiley et al.
1987, Klussman et al. 1988). Diversity of aquatic plant
communities can be reduced considerably, even at carp
Department of Wildlife and Fisheries, P.O. Box 9690, Mississippi State
Univ., Mississippi State, MS 39762. Received for publication September 2,
2007 and in revised form September 9, 2008.
J. Aquat. Plant Manage.
densities typically required to control a target plant spe-
cies (Stott and Robson 1970, Stott 1977, Fowler and Rob-
son 1978, Shireman and Maceina 1981). Because such
changes in a plant community signiﬁcantly alter the habi-
tat and interactions of many aquatic organisms, it is pru-
dent to ﬁrst develop a thorough understanding of how an
exotic herbivorous ﬁsh can inﬂuence the aquatic commu-
nity before assessing its feasibility as a management tool.
Previous reviews have discussed mostly biology and feed-
ing preferences of grass carp, management applications such
as stocking rates, and effects on water quality (e.g., Bain
1993, Petr 2000, Cudmore and Mandrak 2004, Pipalova
2006). None of the previous reviews, however, emphasized
ecological interactions or attempted to address potential
mechanisms responsible for grass carp impacts. The goals of
this review are to evaluate the availability of data in previous
research on grass carp that attempt to validate community
impacts, identify strengths and limitations of this research,
and determine potential design and trajectory of future re-
search, and protocol for use of grass carp as a biocontrol
We used mainly BIOSIS previews and Cambridge Scientif-
ic Abstracts to search the literature and yielded 1,924 undu-
plicated citations on grass carp, 166 of which were pertinent
to this review. Research topics and the type of response vari-
ables quantiﬁed in the research were summarized, and rela-
tive prevalence of each topic was determined by calculating
the frequency of its occurrence. Publications were evaluated
on the basis of whether they attempted to: (1) quantify im-
pacts on community structure and process; (2) measure re-
sponse variables at habitat, population and/or individual
levels; and (3) investigate direct and indirect impacts on the
aquatic community. A direct impact was deﬁned as a single-
level interaction such as immediate alteration of plant abun-
dance due to grass carp feeding activities; indirect impacts
were deﬁned as multi-level interactions, such as changes in
zooplankton composition as a result of the increase in phy-
toplankton abundance due to accelerated nutrient loading
facilitated by grass carp foraging activities (Miller and Ker-
RESULTS AND DISCUSSION
Our review of the literature suggested that data are limit-
ed for a thorough assessment of grass carp impacts on the
aquatic community. We found that most research is related
to the eradication of aquatic plants rather than ecological
mechanisms potentially responsible for the impacts. Limited
data are available to validate long-term consequences and in-
direct impacts of system-wide introductions of grass carp.
Fewer than 50 citations addressing grass carp impacts re-
ferred to the term “ecology,” and of these, only a few (<10)
inferred ecological processes, yet did not validate causal
mechanisms (Table 1). The two most common response vari-
ables measured in studies investigating potential impacts by
the carp on aquatic communities were direct impact on
aquatic plants and indirect alteration of water quality. Less
than 2% of all studies investigated structural alteration of
habitat or the dynamic processes responsible for structuring
the aquatic community.
Aquatic plant abundance
Changes in plant abundance and community composition
occur due to immediate foraging activities as well as alter-
ation of water transparency, disturbance of the sediment,
and deposition of fecal matter by grass carp. Grass carp intro-
ductions may lead to unexpected and undesirable changes
in the plant community. Hanlon et al. (2000) reported sever-
al cases of hydrilla (
[L.f.] Royle) over-
growth after carp introduction. A shift in plant community
composition to less palatable/grazing-resistant species has al-
so been observed (Vinogradov and Zolotova 1974, Hanlon et
al. 2000, Pipalova 2002). Several studies reported that grass
carp eliminated native plants, leaving invasive vegetation in-
tact (Van Dyke 1994, McKnight and Hepp 1995). Limited da-
ta are available to quantify the direct impact on non-target
plants since many studies focused only on speciﬁc plant spe-
cies targeted for control and did not assess other plants in
the community. In addition, the impact on small and less
conspicuous plant species was likely unnoticed because stud-
ies frequently used aerial photography to estimate changes
in plant abundance.
Grass carp can be considered “selective generalists” in
their foraging behavior because they eliminate species in or-
der of decreasing palatability (Colle et al. 1978, Van Dyke et
al. 1984, Leslie et al. 1987). More than 50 genera of food
items, including aquatic macrophytes, algae, invertebrates
and vertebrates, were reported to be eaten by grass carp (Ta-
ble 2). Feeding preference is one of the best-studied aspects
of grass carp biology; however, contradictory evidence exists
for several plant species this ﬁsh consumes (Tables 2 and 3;
also see Bonar et al. 1990, Cooke et al. 2005). Hydrilla and
) were the most common food
items reported to be eaten by grass carp, and species most
commonly avoided were in the genera
. For the same set of
macrophyte species, the order of preference changes under
different environmental conditions, most likely determined
by ease of mastication (Wiley et al. 1986, Pine et. al 1989, Bo-
nar et al. 1990). Cellulose, silica, and iron content were
shown to be the best predictors of unpalatability, whereas cal-
cium and lignin were positively correlated with consumption
rates (Bonar et al. 1990). Predictions of the order of plant
elimination in the ﬁeld still have a large degree of uncertain-
ty; the situation may be improved by a series of ﬁeld studies
across a range of environmental conditions.
Aquatic plants provide habitat heterogeneity important at
both system (i.e., among plant beds) and micro (i.e., within
plant beds) scales. The level of this heterogeneity is deter-
mined by the magnitude of available spatial “patchiness”
constituted by unique plant attributes (i.e., stem-leaf mor-
phology and architecture) found in the habitat (Dibble et al.
2006). Variability and juxtaposition among these different
habitat attributes may collectively deﬁne the ecological value
of habitat in a community or ecosystem as a whole (Wiens
1976, Stensen 1980, Holt 1984, Horne and Schneider 1995,
J. Aquat. Plant Manage.
47: 2009. 3
Moloney and Levin 1996). Species diversity is positively cor-
related with habitat complexity, and increasing heterogene-
ity among habitats at a landscape scale is a common objective
in ecosystem management (Bookhout 1996). Direct impacts
by grass carp on habitat heterogeneity, even though ecologi-
cally important, were not adequately assessed in the research
Similarly, the heterogeneity at a smaller scale within aquat-
ic plants is just as important for habitat structure (Dionne
and Folt 1991, Lillie and Budd 1992, Budd et al. 1995), yet
grass carp impacts on structure have gone unmeasured. This
level of habitat heterogeneity can be determined as spatial
complexity (e.g., plant stem density or frequency of interstic-
es; Lynch and Johnson 1989, Dibble et al. 1996) and is equal-
ly important to individual organisms (Anderson 1984, Diehl
1988, Dibble and Harrel 1997, Pedlow et al. 2006) and col-
lectively to populations (Persson and Eklov 1995). Spatial
complexity in aquatic habitats is important for predator
avoidance and ontogenetic niche shifts (Werner et al. 1977,
Lodge et al. 1988, Persson and Crowder 1997) and, at least
theoretically, greater macrophyte densities allow higher equi-
librium densities of both ﬁsh and macroinvertebrates (Diehl
and Kornijów 1997). A few investigations on grass carp hy-
pothesized mechanisms of trophic interactions and popula-
tion dynamics (Bettoli et al. 1991, 1992, 1993), yet no
attempt was made to measure heterogeneity at this level or
validates the mechanism that explained the effects. Even
without complete vegetation removal, preferential feeding
may signiﬁcantly affect habitat heterogeneity.
The problem of reaching the desired habitat heterogene-
ity is complicated by the inability to accurately predict effec-
tive stocking densities. Despite a relatively high number of
studies published on this topic, speciﬁc stocking densities
that would result in complete elimination of a speciﬁcally tar-
geted plant cannot accurately be predicted across a range of
different plants and environmental conditions. Partial con-
trol of vegetation is rarely achieved with grass carp (Petr
2000, Bonar et al. 2002. After studying 38 lakes in Florida,
Hanlon et al. (2000) concluded that the relationship be-
tween stocking density and plant abundance was unpredict-
able except for maximum stocking density that consistently
resulted in complete eradication of macrophytes. The prob-
Topical emphasis References
Effects on Community Structure
Algae biomass/phytoplankton 8, 11, 24, 30, 32, 41, 44, 48, 53, 55, 56, 61, 63, 65, 75, 110, 114, 123, 124, 137, 161
Crayﬁsh 27, 41, 123
Detritus 61, 113
Fish 3, 4, 8, 9, 10, 11, 16, 19, 22, 26, 42, 46, 50, 52, 53, 54, 55, 56, 58, 61, 62, 63, 67, 73, 78, 80, 85, 86, 92, 94, 99, 100,
104, 105, 108, 113, 121, 123, 141, 143, 151, 157
Habitat alteration 3, 10, 11, 16, 43, 50, 112, 114, 119, 139
Grass shrimp 45, 143
Macroinvertebrates 6, 8, 10, 27, 32, 41, 53, 55, 63, 65, 73, 96, 97, 105, 108, 112
Native plants 30
Periphytic algae 30, 32, 115
Phytoplankton diversity 44, 61, 160
Plant abundance 8, 11, 12, 13, 19, 59, 58, 64, 65, 81, 91, 112, 127, 128, 135, 160
Plant biomass 19, 60, 61, 66, 70, 74, 81, 83, 90, 94, 97, 100, 104, 110, 121, 134, 160, 161
Plant composition 104, 127, 150, 161
Plant diversity 11, 12, 13, 19, 48, 81, 91, 97, 100, 161
Plants, percent coverage 55, 58, 60, 63, 64, 97, 108, 112, 135, 137, 155, 158, 160, 161
Standing crop 84
Seasonal effects on plants 19, 71, 72, 76
Zoobenthos 123, 160
Zooplankton 32, 44, 54, 55, 63, 65, 96, 97, 98, 101, 105, 108, 110, 115
Water quality 4, 7, 8, 11, 17, 18, 19, 30, 31, 32, 44, 47, 48, 53, 54, 55, 58, 61, 63, 65, 66, 67, 74, 75, 79, 94, 95, 96, 98, 100, 101,
104, 105, 106, 108, 110, 113, 114, 115, 121, 123, 124, 137, 143, 150, 160
Effects on Community Process
Carbon ﬂow 8, 19
Eutrophication 96, 114
Food chains 8, 10, 62, 123
Lake productivity 8, 19, 49, 55
Nutrient loading 18, 24, 86, 113, 114, 124
Photosynthesis 8, 53
Primary production 97, 98, 101, 104, 106, 107, 108, 121, 124
Reduction of buffer capacity 18
Shoreline erosion 33
Weevill activity 45
Reference numbers correspond to the number preceding citations in the bibliography listed in the appendix.
J. Aquat. Plant Manage.
35, 123, 131
31, 107, 108, 112, 155
15, 123, 132
123, 126, 142
4, 10, 19, 35, 48, 55, 60, 64, 79, 93, 95, 98, 101, 104, 107, 121, 123, 126, 127, 131, 132, 134, 150, 155
15, 35, 48, 66, 72, 75, 81, 95, 99, 123, 126, 128, 130, 142, 143, 146, 160, 164
131, 160, 161
84, 91, 95, 134
26, 35, 45, 112, 131
40, 84, 87, 112, 142, 143, 161
15, 19, 71, 81,93, 95, 99, 104, 107, 121, 123, 126, 131, 143, 163
2, 4, 7, 10, 13, 17, 18, 20, 31, 33, 35, 44, 48, 49, 54, 55, 58, 62, 63, 64, 70, 78, 79, 91, 95, 96, 98, 105, 108, 120, 123, 133, 135, 155, 158
15, 35, 74, 107, 123, 126, 131, 132
4, 10, 15, 18, 35, 48, 55, 64, 66, 71, 75, 93, 95, 96, 99, 101, 107, 123, 126, 130, 132, 134, 142, 155
11, 19, 35, 60, 104, 107, 121, 123, 131, 142, 143, 155, 164
15, 84, 123
110, 123, 126
99, 142, 143
11, 12, 15, 18, 19, 54, 66, 71, 72, 74, 75, 83, 84, 93, 95, 96, 99, 104, 107, 121, 123, 126, 127, 128, 130, 131, 132, 143, 150, 161, 164, 166
99, 123, 126, 132
87, 107, 123
Reference numbers correspond to the number preceding citations in the bibliography listed in the appendix.
Non-plant food item.
J. Aquat. Plant Manage.
47: 2009. 5
lem of appropriate vegetation control is exacerbated by the
long life span of this ﬁsh; grass carp live longer than 10 years,
and they have been shown to persist in a lake for at least 2
years after complete removal of the invasive plant it was
stocked to control (Kirk et al. 2000, Kirk and Socha 2003).
Long persistence in the environment may also hinder future
native macrophyte revegetation efforts if the policy or stake-
holder interests change.
Early life stages
Most literature discussed only direct impacts relevant to
adult grass carp and neglected potentially important direct
impact of early life stages of carp on other ﬁshes. Grass carp
stocked for plant control is usually >200 mm in length to re-
duce mortality due to predation (Pierce 1983, Cassani 1996);
however, mounting evidence suggests that naturalized popu-
lations of grass carp are reproducing in the United States
(Connor et al. 1980, Pﬂieger and Grace 1987, Brown and
Coon 1991, Raibley et al. 1995). In fact, one recent study re-
ported juvenile grass carp in two rivers in Oklahoma previ-
ously deemed unsuitable for grass carp reproduction
(Hargrave and Gido 2004).
Vegetated habitats provide important nursery grounds for
many native ﬁshes (Gregory and Powles 1985, Chubb and
Liston 1986, Dibble et al. 1996b), and competition for avail-
able food and habitat among and between early life stages of
these ﬁshes can be intense and critical to growth and survival
(Werner and Hall 1979, Mittelbach 1981, Diehl 1993). In-
creases in the number of young non-native ﬁsh may exacer-
bate competition and niche overlap. Larval and juvenile
grass carp are not herbivorous and have been shown to feed
on zooplankton, insect larvae, chironomids, cladocerans,
and copepods (Chilton and Muoneke 1992, Cudmore and
Mandrak 2004). Young grass carp may therefore alter trophic
dynamics within communities by directly competing for food
with native ﬁshes and their larvae.
Secondary effects from a primary disturbance may be
stronger in aquatic communities than the more easily mea-
sured direct impacts (Kerfoot and Sih 1987). There is indis-
putable evidence for presence of trophic cascades in aquatic
systems (Brett and Goldman 1996), and some of the stron-
gest cascades are observed in benthic habitats of lakes (Shu-
rin et al. 2002). Presence of novel phytophagous ﬁsh
exceeding gape size of predator has a potential to involve
novel energy pathways (e.g., with more energy ﬂowing to-
wards detrital food chain), which may strongly affect rates of
nutrient cycling. Increase in nutrient cycling leads to re-
duced resilience of an ecosystem (Wetzel 2001), often mani-
fested in population explosions and subsequent crashes of
dominant taxa. Because macrophytes play an important role
in benthic-pelagic coupling by subsidizing pelagic ﬁsh popu-
lations and affecting nutrient loading (Schindler and
Scheuerell 2002), it is natural to expect that effects of grass
carp would propagate beyond the littoral zone and change
the role of benthic subsidies to the pelagic habitat of lakes.
Mitzner (1978) demonstrated that introduction of grass carp
resulted in reduced primary production, possibly due to de-
creased littoral-pelagic coupling (Lodge et al. 1988).
Grass carp may be viewed as a novel keystone species that
reduces spatial heterogeneity, and the resulting loss of com-
plex habitat can potentially inﬂuence exploitative competition
and other interactions among resident species. This is an ex-
ample of how the indirect effect of a large herbivore may be
transmitted through a resource base, consequently altering
the community structure. Other behavioral alterations due to
the presence of large ﬁsh, whether they are direct predators or
not, can impact populations (Sih 1980, Sih et al. 1985, Miller
and Kerfoot 1987) and potentially alter ecosystem processes.
Secondary changes in aquatic vertebrate and invertebrate
communities have been documented after grass carp intro-
ductions (see next section). Some observed effects are not
consistent across different studies, which may be an indication
of our limited understanding of the underlying mechanisms.
Aquatic plants form a base for macrophyte-periphyton-
grazer complex (Carpenter and Lodge 1986) and support
high densities of macroinvertebrates by providing them with
food, habitat, and refuge from predation (reviewed in Diehl
and Kornijów 1997). Changes in vegetation due to grass carp
Plant Genera Reference
40, 161, 164
54, 81, 123, 131, 161, 164
31, 81, 108, 109
31, 93, 107, 108, 109, 123
Polygonum 107, 123
Potamogeton 35, 40, 107, 166
Ranunculus 109, 165
Sagittaria 107, 131
Typha 31, 107, 108
aReference numbers correspond to the number preceding citations in the
bibliography listed in the appendix.
6J. Aquat. Plant Manage. 47: 2009.
foraging lead to decreased macroinvertebrate diversity and
abundance (Vinogradov and Zolotova 1974). Due to de-
crease in attachment substrate typically provided by vegetat-
ed habitat, richness and abundance of epiphytic
macroinvertebrates can decrease, whereas the number of
benthic invertebrate species can increase (Martin and Shire-
man 1976, Leslie and Kobylinski 1985, Klussman et al. 1988,
Kirkagac and Demir 2004). However, Fedorenko and Frazer
(1978) reported that benthic invertebrates also decreased af-
ter vegetation removal by grass carp, possibly due to direct
competition, low food availability, and increased predation
due to the loss of refuge.
In general, when vegetated habitats become too spatially
complex, ﬁsh growth is negatively impacted due to reduced
food availability and decreased feeding efﬁciency (Crowder
and Cooper 1982, Savino and Stein 1982, Anderson 1984).
Moderate (20%) removal of overly dense vegetation resulted
in improved growth of some age classes of bluegill (Olson et
al. 1998); however, further reduction in aquatic plant density
and the corresponding decrease in spatial complexity results
in increased competition for limited food sources and refugia
(Mittelbach 1988). Thus, plant levels at either extreme of the
density spectrum may decrease ﬁsh growth and survival and,
ultimately, alter population dynamics within the community
(Adams and DeAngelis 1987, Savino et al. 1992, Diehl 1993).
As a general tendency, a decrease in the abundance of ﬁsh
species are dependent on aquatic plants, such as largemouth
bass (Micropterus salmoides) and other sunﬁshes (Lepomis spp.),
whereas non-phytophyllic species, such as gizzard shad (Dor-
osoma cepedianum) and silversides (Labidesthes spp.), tend to in-
crease in abundance (Ware and Gasaway 1978, Klussman et
al. 1988, Maceina et al. 1991, Bettoli et al. 1992,). Bettoli et al.
(1993) observed a radical change in ﬁsh community follow-
ing grass carp introduction, with a decline in the total num-
ber of species over a 7-year period (Table 4). Most notable
declines were observed for Lepomis spp., crappie (Pomoxis
spp.), brook silversides (Labidesthes sicculus), and juvenile
largemouth bass. Colle and Shireman (1994) reported that
after complete elimination of vegetation by grass carp, ﬁsh
harvest declined, and several species had disappeared.
In contrast, Killgore et al. (1998) observed no change in
the number of littoral species and an increase in the total
ﬁsh catch after hydrilla control by grass carp, although the
mean total length of the largemouth bass declined (Table 4).
This study was unusually successful in achieving intermediate
levels of vegetation control, a result that may not be easily ex-
trapolated to many other situations.
Other aquatic vertebrates
Previous work has evaluated indirect impacts of grass carp
on waterfowl, suggesting that abundance of bird species can
be reduced in the presence of grass carp due to shifts in
plant community and competition for preferred food plants
(Gasaway and Drda 1976, Johnson and Montalbano 1984,
1987, Leslie et al. 1987, McKnight and Hepp 1995). Howev-
er, no study speciﬁcally addressed grass carp impact on other
vertebrates (e.g., aquatic mammals, reptiles, and amphibi-
ans). Many of these animals are highly dependent on vege-
tated habitats for food and protection from predators, and
macrophytes are critical to their survival. Salamanders feed
on small prey species living in aquatic plants, and larval stag-
es of amphibians rely on these habitats to avoid predation
(Brophy 1980, Zaret 1980).
Muskrats (Ondatra zybethicus), beaver (Castor canadensis),
and nutria (Myocastor coypus) rely heavily on aquatic plants
(Chabreck 1988, Fredrickson and Laubhan 1996), and more
research is needed to quantify how changes in aquatic plants
alter habitat and distribution of these animals. A better un-
derstanding of how grass carp impact animal distribution will
improve the management of these ecosystems; this is espe-
cially important for controlling exotic species such as nutria.
Indirect effects operate at different temporal and spatial
scales. Parameters that validate those mechanisms are not
easily measurable and usually require a period of time before
response is detected (Miller and Kerfoot 1987), which may
explain why these effects are frequently ignored or inade-
quately tested. Nevertheless, they need to be incorporated
into the designs of future experiments attempting to predict
and explain how grass carp impact aquatic communities.
Aquatic plants decrease sediment resuspension, play an
important role in nutrient cycling (Carpenter and Lodge
1986, Barko and James 1997), and create microclimates in
the littoral zone (Lodge et al. 1988). Submerged macro-
phytes are responsible for several positive feedback mecha-
nisms promoting retention of the vegetated state (Scheffer
and Jeppesen 1997, Van Donk 1997), particularly important
under the conditions of increasing eutrophication. Shallow
TABLE 4. IMPACT OF GRASS CARP ON OTHER FISHES.
Topical emphasis References a
Populations 4, 8, 9, 10, 11, 16, 46, 50, 52, 53, 54, 78, 92, 116,
Density/abundance 53, 55, 56, 58, 62, 67, 85, 92, 108, 123, 141, 151,
Biomass 8, 50, 55, 58, 61, 78, 81, 92, 100
Diversity 50, 55, 56, 62, 100, 157
Production (kg/ha) 53, 80, 123
Total stand. crop (kg/ha) 58, 62, 63, 73, 78, 108, 123
Weight 92, 105
Relative weight 58
Length 92, 105, 157
Distribution 85, 151
Vulnerability to predation 85
Diets 3, 10, 26, 85, 92, 110, 113
Growth 11, 22, 26, 42, 67, 80, 85, 86, 99
Condition 50, 54, 78, 85, 92, 105, 143
Recruitment 42, 67, 78, 92, 99
Reproductive success 62, 73, 92, 123
Male/female ratio 92
Spawning impacts 16, 81, 123
Angler success 19, 50, 62, 94, 99, 104, 121
aReference numbers correspond to the number preceding citations in the
bibliography listed in the appendix.
J. Aquat. Plant Manage. 47: 2009. 7
lakes provide one of the most dramatic examples of a cata-
strophic shift when they suddenly change from clear-water
vegetated state to an alternative stable state, turbid and un-
vegetated, in response to gradual changes in conditions,
such as nutrient loading or macrophyte removal (Scheffer et
Detrimental changes in water quality parameters (in-
crease in nitrite, nitrate, phosphate concentrations) follow-
ing vegetation control by grass carp were reported in most
studies that evaluated water quality (Table 5; Shireman and
Smith 1983, Kirkagac and Demir 2004). These changes re-
sult from sediment resuspension during feeding and fecal
matter deposition by carp as well as collapse of mechanisms
responsible for maintenance of the vegetated state due to
removal of macrophytes. The rate at which aquatic plants
are eliminated determines the magnitude of impact (Hest-
land and Carter 1978, Lembi et al. 1978, Leslie et al. 1983,
1987, Richard et al. 1984, 1985). These changes in water
quality are often followed by algal blooms (Vinogradov and
Zolotova 1974, Canﬁeld et al. 1983, Klussman et al. 1988,
Shireman and Smith 1983), which in most lakes signal a
shift to an alternative stable state (Scheffer and Jeppesen
1997). Increased rates of nutrient cycling after resuspen-
sion of sediments lead to decreased ecosystem stability
(Wetzel 2001). When weighing pros and cons of complete
macrophyte eradication, note that these water quality
changes are often irreversible on relatively long time scales,
even after herbivorous ﬁsh are removed (Scheffer et al.
Many natural freshwater lakes were isolated for some peri-
od of time and may harbor genetically distinct and/or locally
rare populations of plants and animals. While it is not clear
whether other methods of vegetation control or the invasion
itself may have an effect just as detrimental as vegetation con-
trol by grass carp, the issue of potential impact on such popu-
lations needs to be addressed in the future if carp is to be
used in systems other than ponds.
According to the invasional meltdown hypothesis, as more
non-native species are introduced into the system, popula-
tions of resident native species are increasingly disrupted,
and the community becomes more susceptible to future inva-
sions (Simberloff and Von Holle 1999, Ricciardi 2001). As
the history of biological introductions has shown, these as-
pects are very difﬁcult to forecast and assess (Simberloff and
Stiling 1996). Until proven otherwise, such possibility exists
for grass carp introductions; for instance, frequent distur-
bance of sediment and vegetation may in fact promote fur-
ther plant invasions. This may be especially true when grass
carp is used without simultaneous mitigation of factors caus-
ing nuisance species overgrowth (e.g., eutrophication or fre-
Current stocking practices
Grass carp are now recorded in 45 states (Mitchell and
Kelly 2006). In some states, only triploid carp can be stocked
TABLE 5. PHYSICAL PARAMETERS MEASURED TO DETERMINE IMPACTS BY GRASS CARP ON WATER QUALITY.
Parameter References a
Alkalinity 7, 17, 19, 47, 53, 54, 55, 58, 63, 66, 75, 94, 95, 104, 114, 121, 143
Ammonia nitrogen 55, 75, 108, 113, 114, 123, 137
Calcium 17, 47, 48, 58
Carbonates 11, 44, 55, 58, 63, 66, 75, 95, 115
Chlorophyll 4, 17, 18, 30, 31, 32, 44, 54, 58, 65, 67, 114, 137, 143, 150, 160
Conductivity 7, 17, 54, 55, 58, 63, 66, 95, 108, 114, 115
Dissolved oxygen 7, 44, 47, 18, 54, 66, 95, 96, 100, 101, 105, 110, 114, 116
Dissolved organic matter 8, 79, 98, 106, 115
Iron 115, 123
Kjeldahl nitrogen 18, 54, 66, 95, 96, 115
Light compensation point 105
Magnesium 17, 47, 48, 58, 115, 123
Nitrates/Nitrites 19, 31, 44, 47, 48, 54, 55, 65, 75, 104, 113, 114, 137, 160
Oxygen demand 19, 47, 53, 104
pH 7, 17, 44, 47, 48, 53, 54, 63, 66, 95, 96, 104, 105, 108, 113, 114, 143
Phosphate/Phosphorous 11, 17, 18, 19, 31, 44, 47, 48, 54, 55, 58, 61, 63, 65, 67, 75, 79, 98, 100, 104, 106, 108, 113, 114, 115, 123, 124, 137, 160
Potassium 17, 48, 58, 75, 105, 113, 115, 123
Sulfate 47, 55, 105, 115
Sulfur bacteria 48
Tannin-Lignin levels 31, 47, 108
Temperature 54, 113
Total nitrogen 5, 17, 18, 48, 58, 63, 100, 108
Total photosynthetic pigments 53
Turbidity 18, 19, 47, 48, 53, 54, 66, 95, 104, 108, 110, 123, 143
Water clarity/Secchi 55, 58, 98, 114, 137
aReference numbers correspond to the number preceding citations in the bibliography listed in the appendix.
8J. Aquat. Plant Manage. 47: 2009.
for vegetation control, and the procedure for ensuring infer-
tility is tightly regulated by the U.S. Fish and Wildlife Service.
Only certiﬁed triploid grass carp can be possessed west of the
Continental Divide and in the Rio Grande Basin (California
Laws: Fish and Game Code, Section 6440-6460). Alaska,
Maine, Maryland, Massachusetts, Michigan, Minnesota, Mon-
tana, Oregon, North Dakota, Rhode Island, Vermont, and
Wisconsin banned both diploid and triploid grass carp (re-
viewed in Mitchell and Kelly 2006), while some states allow
possession of diploid grass carp for aquaculture purposes,
production of triploids, or even vegetation control.
In states where grass carp possession is regulated, a special
permit (from the departments of natural resources) is re-
quired for stocking grass carp. Often, however, a free permit
can be obtained by anyone who demonstrated a need to con-
trol aquatic vegetation. Satisfactory guidelines have been de-
veloped from the standpoint of limited knowledge about
ecological effects of this introduction (Table 6). These guide-
lines, if followed closely, may help ensure that, if deleterious
effects occur after grass carp introduction, such effects are at
least contained to small ponds not connected to any natural
aquatic systems (also see Cooke et al. 2005, p. 443). The rec-
ommended maximum stocking density is sufﬁciently low to
prevent, in most cases, complete vegetation removal, and ad-
equate precautions are taken with respect to rare and endan-
gered species. Also, this publication provides the pond
owner with sufﬁcient information on the importance of
aquatic vegetation and possible changes in water quality after
vegetation control by grass carp. Due to its generalist habits,
grass carp is not well-suited for selective control of invasive
aquatic plants when some level of native plants is desired.
Other Asian carps, such as black (Mylopharyngodon piceus
Richardson), silver (Hypophthalmichthys molitrix Valenci-
ennes), and bighead (H. nobilis Richardson) have become in-
vasive species of special concern (reviewed in Schoﬁeld et al.
2005), and there is a high likelihood that asian carp may be-
come invasive as well. Some populations have already natu-
ralized, and it is now critical to study their effects on
ecosystems as well as make it unlawful to continue stocking
diploid grass carp. The importance of changing regulations
in states that still allow possession and stocking of diploids
cannot be overemphasized (also see Cooke et al. 2005, p.
Recommendation for future research
More research is needed to investigate both direct and in-
direct impacts of grass carp, with greater emphasis on indi-
rect impacts. Future investigations need to better quantify
these impacts within and across vegetated habitats for a bet-
ter understanding of community processes. Improved mea-
surement of habitat heterogeneity using new descriptors
(e.g., complexity indices, fractal dimension) is essential for
an accurate assessment of habitat changes and their effect on
community. Additional information is also required to deter-
mine how grass carp impact multi-species interactions (e.g.,
interspeciﬁc competition and trophic/predator-prey interac-
tions) and individual behavior (e.g., foraging efﬁciency,
predator avoidance, and habitat use). Future investigations
need to address impacts of all life stages of grass carp, espe-
cially in systems where reproduction of naturalized popula-
tions has been conﬁrmed. It is important to study ecosystem
effects of grass carp, such as changes in rates of nutrient cy-
cling, nutrient compartmentalization, and habitat coupling.
Data are needed to determine appropriate levels of abun-
dance and diversity at which aquatic plants should be main-
tained to prevent a shift to turbid state and negative effects on
animal communities. This review shows that aquatic macro-
phytes are important for habitat heterogeneity and ecological
stability; however, it is difﬁcult to achieve optimal habitat het-
erogeneity and/or only target invasive plants with this non-se-
lective biocontrol agent. Further study of how grass carp
feeding preferences across a range of environmental condi-
tions inﬂuence spatial attributes of aquatic habitats will help
predict community responses to changes in habitat heteroge-
neity. Meanwhile, innovative research is needed to ﬁnd suc-
TABLE 6. INFORMATION FROM “GRASS CARP IN NEW YORK PONDS”, PUBLICATION OF NEW YORK STATE DEPARTMENT OF ENVIRONMENTAL CONSERVATION, DIVISION
OF FISH, WILDLIFE & MARINE RESOURCES.
“. . . approve and issue permits for stocking of up to 15 United States Fish and Wildlife Service certiﬁed triploid grass carp [from approved suppliers
possessing a valid permit to import and sell triploid grass carp in New York State] per surface acre for aquatic plant management purposes in ponds ﬁve (5)
acres or less in size which lie wholly within the boundaries of lands privately owned or leased by the individual making or authorizing such treatments if:
•aquatic plants targeted for control signiﬁcantly impair the intended use(s) of the pond;
•the pond harbors no species of wildlife, ﬁsh, shellﬁsh or crustacea identiﬁed by the Department as being endangered,
threatened or special concern; or any species of plant identiﬁed as being endangered, threatened or rare;
•the pond is not contiguous to or part of a New York State regulated freshwater wetland.
•is not an impoundment or natural pond on a permanent stream or a source of a permanent stream.
•at least two years have elapsed since the last stocking of triploid grass carp
“Triploid grass carp are extremely potent plant consumers. If overstocked, they are capable of eradicating all plants from a pond for periods exceeding 10
years. Besides the obvious impact such complete plant removal will have on vegetation-dependent ﬁsh and wildlife, total devegetation of a pond can also
result in the development of severe algae blooms, foul smells and an overall decline in water clarity. To minimize or prevent such adverse impacts, plant
populations should be maintained at approximately 20-30% of the pond’s surface area.
Due to various factors that impact triploid grass carp feeding, it is impossible to precisely predict the exact number of ﬁsh to stock to achieve the 20-30%
plant coverage target. The only way to prevent excessive plant control is through use of an incremental approach. This approach involves the stocking of
triploid grass carp at the stocking rates suggested below, followed by a two-year waiting period for the ﬁsh to achieve maximal control. Then, if needed,
more ﬁsh are added in small increments at two-year intervals until plant populations are reduced to the 20-30% threshold.”
J. Aquat. Plant Manage. 47: 2009. 9
cessful methods to provide more accurate control for different
levels of plant growth, abundance, and composition. Detailed
evaluation of community responses to differential stocking
rates is needed, as are the techniques to control current grass
carp populations already released into natural systems.
Understanding ecological impacts is paramount to appro-
priate management of aquatic communities. Current data
are not sufﬁcient to adequately answer important questions
about use of grass carp as a biocontrol agent for aquatic
plants. The problem of assessing all possible impacts is not
unique to grass carp: it has been shown for most introduced
biocontrol agents (reviewed in Simberloff and Stiling 1996).
The extent to which these approaches are used in future re-
search will determine the amount of knowledge gained rela-
tive to understanding the ecological impacts of grass carp on
aquatic communities. Until these data are collected, a more
conservative approach should be used when developing
guidelines for grass carp use.
We appreciate the help provided by J. Cruickshank, E.
Forrester, R. Kimbrough, B. LaValley, and J. O’Keefe for
their assistance in the retrieval and initial analysis of litera-
ture, and S. Harrel and P. Smiley for editorial comments pro-
vided on an earlier draft of this manuscript. This manuscript
was signiﬁcantly improved by the suggestions of three anony-
mous reviewers. The Aquatic Ecosystem Restoration Founda-
tion funded an earlier review of the literature that was
incorporated in this manuscript. We thank J. Madsen and the
GeoResources Institute at Mississippi State University for sup-
port and the U.S.G.S. Invasive Species Science Program for
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A Triploid Grass Carp
Risk Analysis Speciﬁc to Florida
PAUL W. ZAJICEK1, T. WEIER2, S. HARDIN3, J. R. CASSANI4 AND V. MUDRAK5
Three hydrilla (Hydrilla verticillata) biotypes are resistant
to ﬂuridone, the principal herbicide used in Florida. Be-
cause of an anticipated demand for triploid grass carp
(Ctenopharyngodon idella), a risk analysis was conducted to ex-
amine the use of triploid grass carp to control hydrilla in
large (>200 ha), open systems in Florida. An expert panel
utilized the Generic Nonindigenous Aquatic Organisms Risk
Analysis Review Process developed by the Aquatic Nuisance
Species Task Force to assess ﬁve hydrilla management op-
tions. Speciﬁcally, the panel assessed the risk of (1) eliminat-
ing all vegetation for three years or more; or (2) vegetation
coverage exceeding 50% for ﬁve consecutive years. Herbi-
cide application, followed by stocking low levels of triploid
grass carp and subsequent herbicide treatment, was consid-
ered to be a lower risk option to achieve hydrilla manage-
ment objectives. The expert panel emphasized the necessity
of implementing well-supported system management and
monitoring and determining stocking rates on a lake-by-lake
Key words: aquatic macrophytes, biological control,
Ctenopharyngodon idella, fluridone resistance, Hydrilla verticil-
lata, risk analysis.
Florida has an abundance of shallow (<5m) natural lakes
with diverse emergent and submersed plant communities.
1Division of Aquaculture, Florida Department of Agriculture and Con-
sumer Services, 1203 Governor’s Square Blvd, Tallahassee, FL 32301, zaji-
2888 North Almeda Street, Apartment 409E, Los Angeles, CA 90012.
3Florida Fish and Wildlife Conservation Commission, 620 South Merid-
ian Street, Tallahassee, FL 32399-1600.
4Lee County Hyacinth Control District, P.O. Box 60005, Fort Myers, FL
5U.S. Fish and Wildlife Service, Warm Springs Technology Center, 5308
Spring Street, Warm Springs, GA 31830. Received for publication May 1,
2008 and in revised form November 30, 2008.
J. Aquat. Plant Manage. 47: 15-20