ICES mar. Sei. Symp., 199: 175-182. 1995
Review of effects of variation in crayfish abundance on
macrophyte and macroinvertebrate communities of lakes
John Mark Hanson and Patricia A. Chambers
Hanson, J. M., and Chambers, P. A. 1995. Review of effects of variation in crayfish
abundance on macrophyte and macroinvertebrate communities of lakes. - ICES mar.
Sei. Symp., 199: 175-182.
This study examined the effects of variation in crayfish abundance on the structure of
the plant and animal communities in lakes. Field and laboratory studies indicated that
relatively low biomasses of crayfish can have marked direct and indirect effects on the
littoral zone habitat. In small pools, male crayfish (Orconectes virilis) stocked at
biomasses of 5 to 10 g m-2 significantly reduced the abundance of Myriophyllum
exalbescens, Nuphar variegatum, Potamogeton richardsonii, and Sparganium eurycarpum. In the laboratory, male and female crayfish showed a strong preference for
Chara sp. and Lemna trisulca, ate smaller amounts of M. exalbescens and Utricularia
vulgaris, and consumed very small amounts of S. eurycarpum, N. variegatum, Hippuris vulgaris, P. richardsonii, Ceratophyllum demersum, and P. vaginalus. These feeding
preferences were not correlated with measures of plant fiber content or crude alkaloid
extract and were negatively correlated with phosphorus levels, available nitrogen
levels, and organic content. In small pools, the abundance of snails (Physa gyrina and
Stagnicola elodes) was greatly reduced by biomasses of male and female crayfish of 5 to
10 g m~2. Indirect effects of crayfish foraging on macroinvertebrate communities could
be much stronger than direct effects if crayfish foraging results in changes in the species
composition of aquatic weedbeds. Replacement of Chara by rooted plants in lakes
could result in reduced total biomass of macroinvertebrates, reduced abundance of
large organisms (e.g., Anisoptera and Gastropoda), and marked changes in the
taxonomic composition of the macroinvertebrate community, independent from di­
rect effects of crayfish predation. The consequences of these crayfish mediated
changes in the littoral zone on the populations of fish and waterfowl dependent on this
zone for food and cover are unknown.
John Mark Hanson: Marine and Anadromous Fish Division, G ulf Region, Fisheries
andOceans, PO Box5030, Moncton, New Brunswick, Canada E l C 9B6 [tel: (+1)506
851 2047, fax: (+1) 506 851 2387], Patricia A . Chambers: National Hydrology
Research Institute, Environment Canada, 11 Innovation Boulevard, Saskatoon, Sas­
katchewan, Canada S7N 3H5.
Introduction
Crayfish are one of the key components of the littoral
zone community of lakes (Momot et al., 1978; Lorman
and Magnuson, 1978; Lodge etal., 1985) yet their role in
structuring this community is poorly understood. Most
research on crayfish has focused on the harvest of wild
crayfish for food or bait (e.g., Momot and Gowing,
1977; Nielson and Orth, 1988), the establishment of
efficient aquaculture operations (Eversole and
Pomeroy, 1989; Huner, 1989; Romaire and Lutz, 1989),
or assessing the impact of introduced species on native
crayfishes (Unestam, 1975; Butler and Stein, 1985;
Lodge et al. , 1986). Moreover, there is surprisingly little
information on the consequences of introducing crayfish
into waterbodies previously devoid of them, even
though this is a widespread practice (Lowery and
Mendes, 1977; Hepworth and Duffield, 1987; Hobbs et
al., 1989). Similarly, little is known of the effects of
variation in crayfish abundance, or the loss of crayfish,
on the community structure of waterbodies currently
supporting them. For example, crayfish of the genus
Orconectes appear to be particularly sensitive to acidifi­
cation of watersheds; their populations collapse when
pH drops below 5.5 (Berrill et al., 1985; France, 1987;
Davies, 1989). When pH levels improve in acidified
watersheds (e.g., Kelso et al., 1990), the littoral zone
community may be greatly different from that which
176
J. M. Hanson and P. A. Chambers
ICES mar. Sei. Symp., 199 (1995)
Table 1 Selected studies that report consumption of submersed macrophytes by crayfish in lakes and ponds. The lakes and Donds
surveyed were not used for commercial culture of crayfish.
Crayfish species
1.
Astacus astacus
2.
Orconectes causeyi
3. O. nais
4. O. rusticus
5. Pacifastacus leniusculus
6. Paranephrops spp.
7. Procambarus clarkii
Plant species
Source
Chara sp.,
Potamogeton sp.,
Myriophyllum sp.,
Ranunculus sp.
Ceratophyllum demersum,
P. pusillus, P. natans,
P. folios us, Chara sp.,
R. aquatilis,
M. exalbescens,
Elodea canadensis
P. foliosus, Chara sp.
Vallisneria americana,
Megalodonta beckii,
P. robinsii
Chara sp.,
M. exalbescens
Chara sp.
P. pectinatus
preceded acidification if crayfish cannot recolonize these
watersheds. These are important questions because
crayfish, by altering the abundance and species compo­
sition of plant and invertebrate communities, can affect
the habitat and food resources of important animal
groups in rivers and lakes, i.e., fish and waterfowl.
In Alberta, crayfish (Orconectes virilis) are only
native to one small watershed (Aiken, 1968), yet they
have been reported recently from waterbodies up to
400 km from the nearest possible source. Consequently,
a series of field and laboratory studies was conducted to
examine possible direct and indirect effects of these
introductions on the macrophyte and macroinvertebrate
communities typically found in Alberta lakes - before
further introductions occurred (Chambers et al. , 1990;
Hanson et al. , 1990; Hanson, 1990; Chambers et al.,
1991). This article provides a synthesis of these studies
and incorporates their findings with the existing litera­
ture.
Effects of crayfish on aquatic plant
communities
It is generally accepted that adult crayfish feed on plants.
A number of crayfish species have been shown to de­
crease the biomass of a variety of plant species (Table 1).
However, feeding selectivity and the biomass of crayfish
needed to effect these changes are less well established.
Flint and Goldman (1975) found that crayfish (Pacifasta­
cus leniusculus) biomasses >69 g m '2 reduced the
density of Myriophyllum exalbescens in Lake Tahoe.
The rusty crayfish (Orconectes rusticus) appeared to
have a much greater impact on the abundance and
Abrahamsson (1966)
Dean (1969),
Saiki and Tash (1979)
Rickett (1974)
Lodge and Lorman (1987)
Abrahamsson and Goldman (1970)
Flint and Goldman (1975)
Coffey and Clayton (1988)
Feminella and Resh (1986)
species diversity of macrophyte communities after this
crayfish was introduced into three Wisconsin lakes
(Lodge and Lorman, 1987). In one experiment, crayfish
biomasses >19 g m 2 reduced the biomass of Potamoge­
ton robinsii and Megalodonta beckii, totally eliminating
these plant species at crayfish biomasses >140 g m“2. In
a second experiment, crayfish biomasses of 24 to
223 g m resulted in significantly reduced biomass of
Vallisneria americana. In three other experiments, how­
ever, different plant species (as well as V. americana)
were not affected by crayfish foraging, partly due to
problems related to duration of the experiments, the
inability to maintain the desired crayfish abundances in
the enclosures, and possible differences in palatability
among the plant species.
In order to minimize some of these problems,
Chambers et al. (1990) conducted field experiments in
small pools (4.7 m2 by 0.4 m deep) to examine the impact
of female and male crayfish (O. virilis) stocked at four
levels (0, 5, 10, and 18g m 2) on the biomass, density,
and shoot morphology of four commonly occurring
aquatic plant species in Alberta lakes (P. richardsonii,
M. exalbescens, Nuphar variegatum, and Sparganium
eurycarpum). Plant species, crayfish sex and activity,
and the abundance of alternate food affected the impact
of crayfish foraging on macrophyte growth. The effects
of foraging by female crayfish on macrophyte growth,
especially that of P. richardsonii and M. exalbescens,
were generally positive (Table 2). This positive effect
may have been due to reduction in the abundance of
snails (as a consequence of crayfish predation) or a
stimulatory effect similar to that reported by Flint and
Goldman (1975) for low biomasses (<60g m-2) of Paci­
fastacus leniusculus feeding on M. exalbescens. How­
ICES mar.
sd. Symp., 199 ( 1995)
Variation in crayfish abundance
177
Table 2. Summary of significant effects (MANOVA) of variations in crayfish (O. virilis) density on density and biomass of four
species of aquatic macrophytes. Separate trials were conducted with females and male crayfish.
Species
Females
M. exalbescens
N. variegatum
P. richardsonii
S. eurycarpum
Males
M. exalbescens
N. variegatum
P. richardsonii
S. eurycarpum
Shoot density
Biomass
Shoot length
Comments
n.s.
-ve*
+ve*
n.s.
+ve*
n.s.
+ve*
n.s.
+ve**
+ve**
variable
-
-v e*
n.s.
n.s.
-ve*
+ve***
-ve**
—ve*
n.s.
n.s.
-ve***
-ve***
eaten
clipped
clipped
clipped
* p < 0.05; ** p < 0.01; *** p < 0.001; n.s. not significant.
ever, in the study by Chambers et al. (1990), the female
crayfish were carrying eggs or hatchlings for most of the
5-week study and spent most of their time in hiding. In
contrast, male crayfish were very active, even during the
day, in the second trial of the same study. Here there
were significant reductions in the biomass of M. exalbes­
cens, N. variegatum, and P. richardsonii', the density of
M. exalbescens and S. eurycarpum; and the average
shoot length of S. eurycarpum and P. richardsonii as­
sociated with biomasses of male crayfish between 5 and
18 g m-2 (Table 2). The only plant species observed to
be eaten, however, was M. exalbescens, indicating that
male crayfish did not feed indiscriminately upon the four
plant species. These results (Lodge and Lorman, 1987;
Chambers et al., 1990) indicate that relatively low den­
sities of crayfish can reduce the abundance of aquatic
plants and affect the species composition of macrophyte
beds.
Crayfish feeding preferences for various aquatic mac­
rophyte species have been examined in only a few
studies. It was difficult to compare the results of these
studies because there were few plant species that were
common to any of them. In one of the earliest studies,
Seroll and Coler (1975) reported that O. immunis pre­
ferred Elodea sp. and Ceratophyllum sp. over Vallis­
neria sp. A study by Lodge and Lorman (1987) showed
that O. virilis and O. rusticus caused a greater reduction
in the abundance of Megalondonta beckii than of Potamogeton robinsii in one experiment and consumed more
Myriophyllum exalbescens than Vallisneria americana in
another. In a laboratory study designed to evaluate
consumption and digestibility of 14 species of sub­
merged and emergent macrophytes to O. virilis. Brown
et al. (1990) fed known amounts of each species separ­
ately to crayfish and measured the amounts consumed
and excreted as feces. The species tested were: Typha
sp., Equisetum sp., Scirpus sp., Sagittaria sp., Chara sp.,
Lemna sp., Nelumbo lutea, Zannechelliapalustris, Val­
lisneria sp., Ranunculus longirostris, Heteranthera
dubia, Potamogeton crispus, P. nodosus, and P. pectinatus. Although the goal of the study was not to test for
feeding preferences, and the palatability of the plants
themselves was likely altered owing to the fact that the
plants were frozen and thawed twice before being fed to
the crayfish, there were clear differences in the amounts
eaten between plant species. The crayfish ate more
Chara, Lemna, Equisetum, and P. pectinatus than other
species and did not feed on N. lutea (floating leaved),
Typha, Scirpus, or Sagittaria (three emergent species).
Plant morphology is thought to affect feeding prefer­
ences of crayfish. Olsen et al. (1991) fed equal amounts
of Ceratophyllum demersum (highly branched growth
form), Elodea canadensis (highly branched growth
form), Isoetes spp. (rosulate growth form), and Potamo­
geton richardsonii (single-stemmed growth form) to
three species of crayfish (Orconectes rusticus, O. virilis,
and O. propinquus) in a laboratory study designed to
test the prediction that O. rusticus preferred (relative to
congeners) single-stemmed plants to plants of highly
branched or rosulate form and the prediction that O.
rusticus consumed or destroyed more macrophyte bio­
mass than its congeners. The three crayfish species
showed the same selectivity: C. demersum = E. cana­
densis = P. richardsonii > Isoetes spp., which falsified
the hypothesis that crayfish preferred single-stemmed
species over plants of branched or rosulate growth form.
In addition, feeding rates were different between cray­
fish species. Although the crayfish were all of the same
size, O. rusticus and O. virilis consumed or destroyed
more macrophytes on a daily basis than O. propinquus.
The rate of macrophyte consumption by crayfish was low
during this study - the maximum consumption of any
plant species was only 30% after 6 days. These low
consumption rates were consistent with results of
Chambers et al. (1991), where C. demersum and P.
richardsonii were among the least preferred plant
species of O. virilis (Isoetes spp. and E. canadensis were
not tested in the latter study).
178
J. M. Hanson and P. A . Chambers
Chambers et al. (1991) conducted laboratory experi­
ments to quantify directly feeding selectivity of crayfish
(O . virilis) for 10 species of aquatic plants commonly
found in Alberta lakes and to correlate this selectivity
with morphological and chemical characteristics of the
plant species. The consumption of fresh macrophytes by
female crayfish did not differ from that of the males in
this study. The crayfish showed clear feeding prefer­
ences as follows: Chara > Lemna trisulca > Myriophyl­
lum exalbescens = Utricularia vulgaris > Sparganium
eurycarpum = Nuphar variegatum > Hippuris vulgaris
> Potamogeton richardsonii > Ceratophyllum demer­
sum > P. vaginatus. Isoetes sp. is present in Alberta
lakes but was not included in this study; however, the
study by Olsen et al. (1991) indicates that Isoetes would
be one of the least preferred plant species in Alberta
lakes. Chambers et al. (1991) also reported significant
differences in the amount of the various plant species
eaten by O. virilis. In a 48-h period, the crayfish con­
sumed 53 to 72% of the biomass of the first four plant
species tested in the study but never more than 24% of
the biomass of the last six species. These feeding prefer­
ences were not significantly correlated with plant fiber
content (p > 0.05) nor with crude “alkaloid” extract
(p > 0.05) and were negatively correlated with total
phosphorus concentration (p < 0.01), available nitrogen
concentration (p < 0.001), and plant organic content (p
< 0.025). Identifiable alkaloids were only found in N.
variegatum and C. demersum and they were not the least
preferred species. Although crayfish showed strong
feeding preferences among the 10 plant species tested,
the functional basis for this selection is not clear,
although Lodge (1991) has suggested that the presence
of phenolic compounds may affect plant feeding prefer­
ences for a variety of aquatic grazers. With regard to
plant morphology, Chambers et al. (1991) suggested that
the strong preference of crayfish for Chara and Lemna
trisulca over the other eight plant species examined was
due to the ease in handling small, short, bottom-dwell­
ing species as compared with large erect, roseate, or
floating leaved forms.
The strong feeding preference of crayfish for Chara
reported by Brown et al. (1990) and Chambers et al.
(1991) is consistent with field observations that Chara is
usually absent from areas of moderate to high crayfish
abundance (Abrahamsson and Goldman, 1970; Rickett,
1974; Coffey and Clayton, 1988). Because Chara is
abundant in many Alberta lakes, the establishment of
large populations of crayfish in these lakes could greatly
alter the abundance and species composition of aquatic
macrophyte beds and, in so doing, affect the compo­
sition of the macroinvertebrate community.
ICES mar. Sei. Symp., 199 (1995)
Direct effects of crayfish on
macroinvertebrate communities
Crayfish are generally considered to be carnivores as
juveniles, consuming more plant material and detritus as
adults (Abrahamsson, 1966; Lorman and Magnuson,
1978; Momot et al., 1978). Nevertheless, macroinverte­
brates can comprise a significant proportion of the diet
of adult crayfish (Abrahamsson, 1966; Covich, 1977;
Capelli, 1980), but the impact of this predation on mac­
roinvertebrate communities is not clear.
Laboratory studies show that adult crayfish will
readily consume snails and small bivalves (Covich, 1977;
Covich et al., 1981; Crowl and Covich, 1990). In one
field study. Lodge and Lorman (1987) observed differ­
ences in snail abundance between enclosures containing
crayfish (O. rusticus and O. virilis at biomasses of 24223 g m~2) and those without. However, they noted that
the differences could also have been due to snail mi­
gration into and out of the enclosures. Hanson et al.
(1990) conducted field experiments in small pools to
examine the impact of predation by female and male O.
virilis stocked at four densities (0,5,10, and 18 g m-2) on
the abundance of a variety of macroinvertebrates com­
monly found in Alberta lakes. The pools were stocked
with known numbers of amphipods, trichopteran larvae,
and adult snails. The abundance of snails (Physa gyrina
and Stagnicola elodes) was greatly reduced by biomasses
of crayfish between 5 and 10g m“2. This is much lower
than the biomass of crayfish associated with reduced
snail densities in Lodge and Lorman’s (1987) study.
Because snails are efficient grazers on epiphytes, one
consequence of reduced snail densities in lakes could be
to release these resources to other grazers, such as chironomids and oligochaetes (Cattaneo, 1983; Cuker, 1983;
Kairesalo and Koskimies, 1987) or crayfish themselves
(Abrahamsson, 1966; Flintand Goldman, 1975; Capelli,
1980). In addition, recent laboratory experiments indi­
cated that crayfish predation affected the life history
characteristics of the snail Physella virgata virgata by
increasing the size and age at first reproduction (Crowl
and Covich, 1990). Furthermore, a recent laboratory
study (Olsen et al., 1991) showed that O. rusticus con­
sumed more snails (Amnicola sp.) than either O. virilis
or O. propinquus, indicating that simple replacement of
one crayfish species by another in a waterbody could
have a strong adverse effect on existing snail popu­
lations. Clearly, variation in crayfish abundance can
strongly affect snail populations.
In contrast, the effects of crayfish predation on the
abundance of non-molluscan invertebrates are less well
known. Rickett (1974) found that experimental ponds
that had been invaded by crayfish (Orconectes nais) had
greatly reduced total macroinvertebrate biomass, more
oligochaetes, and fewer chironomids and ephemerop-
Variation in crayfish abundance
ICES mar. Sei. Symp., 199 (1995)
terans than ponds containing few or no crayfish. Hanson
et al. ( 1990) found that female and male crayfish stocked
in separate trials at densities of 0,5,10, or 20 g m-2 (each
treatment in triplicate) had different effects on nonmolluscan invertebrate abundances in their study con­
ducted in small pools. This was thought to be due to the
high densities of age-0 crayfish (6-116 g m-2) present in
all pools in the trial conducted with males. In the female
crayfish trial, age-0 crayfish were not present until the
last week of the trial and even then were too small to eat
macroinvertebrate prey. Thus the strong inverse corre­
lation of amphipod density with crayfish biomass (p <
0.001) and weak inverse correlations of oligochaete
density (p < 0.02) and total invertebrate density (p <
0.05) with crayfish biomass were attributable to direct
predation by female crayfish. In addition, there were
proportionately more large organisms (>8 mg) in pools
containing female crayfish than in pools devoid of them,
which indicates that female crayfish were consuming
small-sized invertebrates. The results were less clear in
the trial that used male crayfish. In that trial, the density
of amphipods and the total biomass of non-molluscan
invertebrates were lower in the 5 and 10g m -2 treat­
ments than in the 0 and 18 g m-2 treatments. Unfortu­
nately, unlike the trial with female crayfish, the age-0
crayfish were large enough to eat macroinvertebrates
(but not snails) in this trial, which may have masked any
direct predation effects by male crayfish. One interest­
ing finding was that the benthic fauna at the end of the
trial with male crayfish was dominated by oligochaetes.
This is similar to the findings of Rickett (1974) and
suggests that the more carnivorous age-0 crayfish were
responsible for the species composition of the non-mol­
luscan invertebrate community at the end of both
studies.
Indirect effects of crayfish predation on
macroinvertebrates
Indirect effects of crayfish on macroinvertebrate com­
munities have rarely been studied. Abrahamsson (1966)
reported that large populations of leeches and molluscs
appeared when Chara became abundant after extinction
of crayfish from small ponds in Sweden. Rickett (1974)
found higher abundances of snails, dipteran larvae, and
ephemeropteran larvae in ponds that contained few or
no crayfish but did contain abundant vegetation (Chara
and Potamogeton foliosus). Feminella and Resh (1986)
found that Procambarus clarkii reduced the density of
Potamogeton pectinatus in a marsh, which eliminated
the predation refuge for the mosquito Anopheles occidentalis, and mosquito densities were reduced almost
150-fold. These studies represent the extreme case
where plants were present or absent. However, less
179
dramatic changes in the species composition of aquatic
weedbeds caused by crayfish selective foraging could
also affect the structure of the macroinvertebrate com­
munity.
The macroalga Chara is very abundant in many
Alberta lakes and, should crayfish become established in
these lakes, it is expected that Chara would be replaced
by plant species less preferred by crayfish. This could
alter the macroinvertebrate community structure of the
littoral zone because different plant species appear to
support different abundances and species of inverte­
brates (Keast, 1984; Talbot and Ward, 1987; Cyr and
Downing, 1988). Because crayfish feed directly on mac­
roinvertebrates, the question of indirect effects of cray­
fish foraging on macroinvertebrate communities due to
selective foraging on plant species is best addressed
using a comparative approach in lakes devoid of cray­
fish.
Hanson ( 1990) compared the macroinvertebrate com­
munities of weedbeds dominated by Chara and those
dominated by rooted plants (various Potamogeton spp.,
Myriophyllum exalbescens, Nuphar variegatum, and
Isoetes sp.) in a small Alberta lake that is devoid of
crayfish. The two weedbed types were sampled at 2week intervals for 18 weeks and the total macroinverte­
brate biomass (excluding unionid clams), taxonomic
composition, and size structure (using biomass size spec­
tra, Schwinghamer, 1981; Strayer, 1986; Hanson et al.,
1989) of the two communities were compared. The
seasonal mean total biomass of the macroinvertebrate
community in the Chara beds was 3.5 times higher than
that of the rooted plant weedbeds (Table 3). Amphipods
Table 3. Comparison of biomass ( g m '2, fresh weight) and
taxonomic composition of macroinvertebrate community of
weedbeds dominated by Chara versus rooted plants in Narrow
Lake.
Taxon
Chara
%
Rooted plants
%
Total biomass
Amphipoda
Chironomidae
Anisoptera
Gastropoda
Sphaeriidae
26.4
0.69
2.32
1.04
1.64
2.05
_
9
29
13
20
26
7.61
2.64
0.82
0.15
0.26
0.05
_
64
20
4
6
1
and chironomids dominated the rooted plant com­
munity, whereas anisopterans, gastropods, sphaerid
clams, and chironomids dominated the Chara com­
munity. The distributions of amphipods and gastropods
appeared to represent true preferences of these taxa for
the particular weedbed type. The comparatively low
abundance of gastropods in the rooted plant community
is noteworthy because crayfish prey directly on snails,
especially Physa spp., when they are available (Rickett,
180
J. M. Hanson and P. A . Chambers
1974; Covich, 1977; Hanson et al., 1990) and the data in
Hanson (1990) suggest that crayfish can also affect snail
abundance indirectly by altering the species composition
of the weedbeds.
The average size structure of the macroinvertebrate
communities inhabiting the two plant communities de­
scribed by Hanson (1990) differed markedly. There
were no clear peaks in the biomass distribution of the
macroinvertebrate community associated with the
Chara beds but that of the rooted plant weedbeds
showed a strong peak in the 4 to 8 mg size class and a
weak peak in the 32 to 64 mg size class. Further, there
were proportionately more large (>64 mg) organisms in
the Chara beds (30% total biomass) than in the rooted
plant weedbeds (11% total biomass). In areal terms,
there were 8 g m -2 of large organisms in the Chara beds
compared with 0.8 g m -2 in the rooted plant weedbeds.
Because fish depend on prey of increasingly greater size
in order to grow (Kerr, 1971; Wankowski, 1979; Diana,
1987), this latter result suggests that crayfish induced
changes in the species composition of aquatic weedbeds
could affect not only the structure of the macroinvertebrate community but also the total biomass and growth
of the fish species dependent upon macroinvertebrate
prey by reducing total prey abundance or by reducing
the abundance of large prey needed for good fish
growth.
Effects of crayfish on fish
The preceding sections have indicated that relatively low
abundances of crayfish can greatly affect the structure of
the plant and invertebrate communities of the littoral
zones of lakes. In so doing, crayfish affect the food
resources and habitats of economically valuable fish.
Unfortunately, studies of ecological interactions be­
tween fish and crayfish are uncommon, despite the fact
that many fish species prey upon crayfish, and one
reason for introducing crayfish into waters previously
devoid of them has been to provide food for fish (e.g.,
Dean, 1969; Saiki and Tash, 1979; Hepworth and Duffield, 1987). The few studies available show that only
one fish species, largemouth bass (Micropterus salmoides), can control crayfish abundance in small water­
bodies (Saiki and Tash, 1979; Rach and Bills, 1989). For
some other fish species, growth has been found to
decline when the fish population could not effectively
exploit the introduced crayfish (e.g., Rickett, 1974;
Gowing and Momot, 1979; Hepworth and Duffield,
1987). The mechanisms responsible for these decreases
in fish growth are unknown.
Crayfish can have direct negative effects on fish popu­
lations by preying on eggs and young, interfering with
fish harvest in hatcheries, and by evicting forage fish
ICES mar. Sei. Symp., 199 (1995)
from shelters (Horns and Magnuson, 1981 ; Rahel and
Stein, 1988; Rach and Bills, 1989). Furthermore, many
fish species depend on aquatic macrophytes for cover
from predation (Werner et al., 1983; Keast, 1984; Rozas
and Odum, 1988) and, in extreme cases, introduced
crayfish can completely remove the macrophyte cover
needed by these fish (Lodge et al., 1985). In general,
indirect effects of variation in crayfish abundance on fish
populations are very difficult to predict. It is clear, how­
ever, that crayfish can reduce the abundance of such
invertebrate prey as snails either by direct predation or
by selective predation on plants preferred by snails.
Therefore, crayfish are potential competitors with snaileating fish (e.g., pumpkinseed Lepomis gibbosus).
Further, many fish species prey on macroinvertebrates
(Wissmar and Wetzel, 1978; Boisclair and Leggett,
1985; McQueen et al., 1986) and any changes in the
abundance or structure of the macroinvertebrate com­
munity are likely to affect these fish. Unfortunately, the
responses of fish to changes in the macroinvertebrate
prey community are poorly understood, in part because
studies that compare the availability and profitability of
prey to fish between weedbeds comprising different
plant species are lacking.
In summary, there is insufficient information cur­
rently available to predict how variation in crayfish
abundance will affect the structure of the littoral zone
plant and invertebrate communities. However, the
studies summarized here indicate that crayfish bio­
masses as low as 5 to 10 g m-2 could greatly affect the
structure of the plant and invertebrate communities of
lakes, with unknown consequences for the fish depen­
dent upon these resources. Thus, crayfish may be a
keystone species in freshwater waterbodies.
Acknowledgements
We thank T. W. Sephton, S. C. Courtenay, and D. P.
Swain for reading and commenting on an earlier version
of this article.
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