Freshwater Biology (2002) 47, 2337–2344
Effects of a filter-feeding fish [silver carp,
Hypophthalmichthys molitrix (Val.)]
on phyto- and zooplankton in a mesotrophic reservoir:
results from an enclosure experiment
R O B E R T J . R A D K E and U W E K A H L
Institute of Hydrobiology, Dresden University of Technology, Dresden, Germany
SUMMARY
1. Silver carp, Hypophthalmichthys molitrix (Val.), feeds on both phyto- and zooplankton
and has been used in lake biomanipulation studies to suppress algal biomass. Because
reports on the effects of silver carp on lake food webs have been contradictory, we
conducted an enclosure experiment to test how a moderate biomass of the fish (10 g wet
weight m)3) affects phytoplankton and crustacean zooplankton in a mesotrophic
temperate reservoir.
2. Phytoplankton biomass <30 lm and particulate organic carbon (POC) <30 lm were
significantly higher in enclosures with silver carp than in enclosures without fish, whereas
Secchi depth was lower. Total copepod biomass declined strongly in both treatments
during the experiment, but it was significantly higher in fish-free enclosures. Daphnid
biomass was also consistently higher in enclosures without fish, although this effect was
not significant. However, the presence of fish led to a fast and significant decrease in the
size at maturity of Daphnia galeata Sars. Thus, the moderate biomass of silver carp had a
stronger negative effect on cladoceran zooplankton than on phytoplankton.
3. Based on these results and those of previous studies, we conclude that silver carp
should be used for biomanipulation only if the primary aim is to reduce nuisance blooms
of large phytoplankton species (e.g. cyanobacteria) that cannot be effectively controlled by
large herbivorous zooplankton. Therefore, stocking of silver carp appears to be most
appropriate in tropical lakes that are highly productive and naturally lack large cladoceran
zooplankton.
Keywords: biomanipulation, enclosure experiment, Hypophthalmichthys molitrix (Val.), plankton, silver
carp
Introduction
The use of the filter-feeding silver carp, Hypophthalmichthys molitrix (Val.), as a biomanipulation tool to
reduce phytoplankton biomass in lakes and reservoirs
remains controversial (Costa-Pierce, 1992; Starling
et al., 1998; Domaizon & Dévaux, 1999). While Starling
et al. (1998) list 14 successful studies, others found no
Correspondence: Robert J. Radke, Institute of Hydrobiology,
Dresden University of Technology, Mommsenstr. 13, 01062
Dresden, Germany. E-mail: [email protected]
2002 Blackwell Science Ltd
positive effects of silver carp (Miura, 1990; Wu et al.,
1997; Domaizon & Dévaux, 1999; Fukushima et al.,
1999). Four main factors can probably explain the
inconsistent results: (1) the stocking level of silver
carp (Domaizon & Dévaux, 1999); (2) the initial
zooplankton species pool (Starling et al., 1998;
Fukushima et al., 1999); (3) the initial phytoplankton
species pool (Datta & Jana, 1998) and (4) temperature
conditions (Fukushima et al., 1999). All of the successful experiments have in common the fact that they
were performed under eutrophic or hypertrophic
conditions where nuisance algae blooms ought to be
2337
2338
R.J. Radke and U. Kahl
suppressed and large or colonial algae (mainly
cyanobacteria) were the predominant phytoplankton
forms. The stocking with silver carp was considered
successful in these cases, because the reduction of
cyanobacteria such as Microcystis spp. was the only
important goal or because an overall reduction of
phytoplankton biomass was achieved because small
algae, which cannot be effectively removed by silver
carp (Herodek et al., 1989; Dong & Li, 1994; Vörös
et al., 1997), were unable to build up high biomass
within the time frame of the studies, resulting in an
overall reduction of phytoplankton biomass.
Large herbivorous cladocerans, such as Daphnia
spp., are used more commonly than filter-feeding fish
to reduce phytoplankton biomass in eutrophic lakes,
because Daphnia have high community filtration rates
(Sterner, 1989). Success of biomanipulation strategies
fostering Daphnia grazing might be limited, however,
if phytoplankton community structure shifts towards
large inedible algae or if Daphnia are preyed upon
by fish or invertebrates (Gliwicz & Pijanowska,
1989; Reynolds, 1994; Benndorf, 1995; Drenner &
Hambright, 1999). Silver carp, together with large
cladocerans, might reduce total phytoplankton biomass effectively, but have also been shown to
suppress herbivorous zooplankton (Kajak et al., 1975;
Opuszynski, 1979; Domaizon & Dévaux, 1999). Consequently, Smith (1993) suggested using a series of
alternate ponds stocked with either silver carp or
large zooplankton as an approach to reduce phytoplankton biomass in wastewater treatment ponds. For
lakes, Domaizon & Dévaux (1999) proposed a threshold density of silver carp of 8 g wet weight m)3
(200 kg ha)3) below which negative effects on herbivorous zooplankton should be minimised and beneficial effects on phytoplankton yet be effective.
We performed an enclosure experiment in a mesotrophic reservoir used for drinking water supply.
Silver carp had been stocked in this reservoir, and a
number of others in eastern Germany, in the late 1980s
to improve water quality (seston concentration). After
the stocking, the summer mean cladoceran biomass
(almost entirely Daphnia galeata) decreased in the
reservoir (Horn & Horn, 1995) and mean phytoplankton biomass remained unchanged (Horn, Paul &
Horn, 2001). Although the decrease in cladoceran
biomass was apparently correlated with the stocking
of silver carp, this decrease might have been related to
other planktivorous fish species in the reservoir, such
as roach, Rutilus rutilus (L.), the dominant native
planktivore (Radke, unpublished data). Thus, the aim
of this study was to test the effect of a moderate
density of silver carp on the phytoplankton and
crustacean zooplankton community while excluding
the influence of other planktivorous fishes.
Methods
The study was performed in mesotrophic Saidenbach
Reservoir, Saxony, Germany. The reservoir is primarily used for drinking water supply and to a limited
extent for recreational fishing. The reservoir has a
surface area of 1.46 km2, a mean depth of 15.3 m and a
maximum depth of 45 m. The mean annual concentration of total phosphorus declined from 25 to
15 lg L)1 within 2 years after the use of phosphatefree detergents became compulsory in 1990 and it has
remained stable since (Horn, Horn & Paul, 1994; Horn
et al., 2001).
Six cylindrical enclosures, 1 m in diameter and
12 m deep, were made of transparent polyethylene
and stabilised with four stainless steel rings to prevent
collapsing. The bottom of the enclosures was covered
with a 30-lm mesh gauze. The enclosures were hung
from a floating aluminium frame attached to the
bottom of the reservoir. They were positioned in the
centre of the main basin above a depth of 30 m.
The enclosures were filled with unfiltered lake water
by pulling the folded enclosures up from a depth of
12–15 m 1 week before the start of the experiment. On
22 June 1999, three enclosures were randomly selected
and each stocked with two silver carp. Individual
biomass was 48.1 ± 6.7 g (mean ± 1 SD). The resulting
initial biomass in enclosures with fish was
10.2 ± 1.4 g m)3 and the final biomass was 10.1 ±
1.5 g m)3. We chose this biomass level, because it is
close to the threshold value of 8 g m)3 found by
Domaizon & Dévaux (1999). The remaining three
enclosures served as controls with no fish.
Enclosures were sampled twice a week until 16
July. Water temperature was measured with a digital
probe at depth intervals of 1 m. Water samples for the
analysis of total phosphorus, particulate organic
carbon (POC) and phytoplankton biomass were taken
at 2-m intervals from the entire water column with a
2-m tube sampler and mixed before taking subsamples. Phosphorus concentrations were determined
according to standard methods (Institut für
2002 Blackwell Science Ltd, Freshwater Biology, 47, 2337–2344
Effects of silver carp 2339
Wasserwirtschaft Berlin, 1986). The concentration of
POC <30 lm was determined with a carbon analyser
C-200 (LECO, St Joseph, MI, U.S.A.) after passing 250–
500 mL of water through a 30-lm mesh screen and
filtering both size fractions on pre-ashed (500 C)
Whatman GF ⁄ F filter (Whatman, Maidstone, UK)
under gentle vacuum (200 mbar). The 30-lm size
threshold for POC and phytoplankton fractions was
used, because it corresponds to the maximum particle
size that is effectively filtered by D. galeata Sars
(Burns, 1968), the dominant cladoceran species in
Saidenbach Reservoir (Horn & Horn, 1995).
Phytoplankton was preserved with Lugol’s iodine
solution immediately after sampling. Cells were
counted under an inverted microscope and sized to
derive biovolumes from appropriate geometric shapes
(Arbeitsgemeinschaft Trinkwasser e.V. Arbeitskreis
Biologie, 1998). Biomass (wet weight) was calculated
assuming a wet weight density of 1 g cm)3. Crustacean zooplankton was sampled by making vertical
hauls over the entire water column with a plankton
net of 100 lm mesh size, 25 cm in diameter and
equipped with a flow meter. Samples were immediately preserved in sucrose-formaldehyde (3%) solution. Taxa were counted in three subsamples and at
least 100 specimens of D. galeata and 50 for all other
taxa were measured. Biomass (wet weight) was
calculated with length–mass regressions according
to Bottrell et al. (1976) and size at maturity (SAM) of
D. galeata was determined as the smallest egg carrying
female in a sample.
A t-test was performed to test for significant
differences between treatments (fish presence versus
absence) of all measured variables at the start of the
experiment, and repeated-measures A N O V A was performed to test for effects of fish presence and time on
seven sampling dates. Statistical analysis was performed with S T A T I S T I C A (StatSoft Inc., Tulsa, OK,
U.S.A.).
Results
Surface temperature ranged from 16.6 C at the start
of the experiment to 20.0 C at the end. The lake was
stratified (thermocline at 8 m at the start of the
experiment and 11 m at the end) and the temperature
difference between the water surface and enclosure
bottom ranged from 6.5 to 7 C. Differences in
temperature between the lake and enclosures were
2002 Blackwell Science Ltd, Freshwater Biology, 47, 2337–2344
less than 0.5 C. On day 1 of the experiment, no
significant differences in any of the measured variables was detected between control and treatment
enclosures (Table 1). Total phosphorus concentrations
declined during the experiment simultaneously in fish
and no-fish control enclosures (Fig. 1), and repeatedmeasures A N O V A revealed neither significant differences between treatments nor sampling dates
(Table 2). Secchi depth was significantly higher (up
to 2 m) in the no-fish enclosures than in the fish
enclosures (Table 2). While the biomass of phytoplankton <30 lm was significantly higher in the fish
enclosures, no difference was found in the biomass of
the size fraction ‡30 lm. During the first week of the
experiment, the biomass of the larger phytoplankton
fraction declined strongly in both treatments and
subsequently remained low except for 1 day where
higher biomass levels were found in the no-fish
enclosures. Concentrations of POC <30 lm were
significantly higher in the fish enclosures than in
no-fish enclosures. Although daphnid biomass was
always higher in the controls (apart from the first day
of the experiment), differences were not significant.
Similar to daphnid biomass, copepod biomass
decreased in both treatments over time, with no-fish
enclosures showing a significantly higher biomass.
The effect of fish on total crustacean biomass was
slightly higher than the effect on daphnid biomass,
but it was not significant either (Table 2). Other
cladocera, including Diaphanosoma brachyurum
(Lievin), Bosmina spp. and Chydorus spp., accounted
for <2% of the total crustacean biomass. The SAM of
D. galeata remained >1.1 mm in no-fish enclosures,
but declined to values <0.9 mm in fish enclosures.
This highly significant change led to differences in
SAM of more than 0.4 mm between treatments on two
sampling dates.
Discussion
Our results show that total phytoplankton biomass
could not be reduced by silver carp at the chosen
moderate biomass level. The inability of silver carp to
filter particles <10 lm effectively has been observed in
several experimental studies (Herodek et al., 1989;
Smith, 1989; Dong & Li, 1994; Vörös et al., 1997) and
explains why this fish species was unable to suppress
total phytoplankton biomass in other studies (Miura,
1990; Lieberman, 1996; Wu et al., 1997). In our
2340
R.J. Radke and U. Kahl
t-Value
P-value
6.2 ± 0.4
6.0 ± 0.0
1.0
0.37
NF
F
8.4 ± 2.1
14.0 ± 3.2
2.53
0.06
Particulate organic
carbon <30 lm (mg L)1)
NF
F
0.46 ± 0.01
0.50 ± 0.12
0.61
0.57
Biomass of phytoplankton
<30 lm (mg L)1)
NF
F
0.21 ± 0.08
0.18 ± 0.07
0.44
0.69
Biomass of phytoplankton
‡30 lm (mg L)1)
NF
F
0.08 ± 0.01
0.11 ± 0.03
1.48
0.21
Biomass of Daphnia
galeata (mg L)1)
NF
F
0.09 ± 0.04
0.10 ± 0.05
0.31
0.77
Copepod biomass
(mg L)1)
NF
F
0.050 ± 0.005
0.048 ± 0.003
0.59
0.60
Crustacean biomass
(mg L)1)
Size at maturity of
Daphnia galeata (mm)
NF
F
NF
F
0.26
0.81
1.08
0.34
Variable
Treatment
Secchi depth (m)
NF
F
Total phosphorus (lg L)1)
Mean ± SD
0.14
0.15
1.28
1.33
±
±
±
±
0.04
0.05
0.05
0.07
experiment, the algae fraction <30 lm was actually
dominated by forms <10 lm, accounting for more
than 99% of the total cell number of this size class
during the whole experiment. The small size of algae
in the fraction <30 lm and the low concentration of
phytoplankton ‡30 lm may also account for the
failure of silver carp to gain weight during our
experiment. This explanation is in accordance with
the observation that cases in which silver carp were
successfully used for biomanipulation (e.g. Kajak
et al., 1975; Starling, 1993; Datta & Jana, 1998;
Starling et al., 1998; Fukushima et al., 1999) are characterised by phytoplankton communities dominated by
large species or forms [e.g. Microcystis spp., Cylindrospermopsis raciborskii (Woloszynska) Seenaya et Subba
Raju]. Nevertheless, it remains unclear why small
algal species failed to profit from their size refuge in
the studies cited above.
The decrease of Daphnia biomass in the fish enclosures may have been a direct effect of predation by
silver carp (e.g. Kajak et al., 1975; Opuszynski, 1979;
Spataru & Gophen, 1985; Domaizon & Dévaux, 1999).
The decrease in the no-fish enclosures, however, was
unexpected. The most likely explanation for this
finding is that food limitation occurred in the no-fish
enclosures, as indicated by low POC <30 lm levels
(<0.3 mg L)1) on all sampling dates after the first
Table 1 Response variables (means ± 1
SD) measured on the first day of the
experiment (22 June 1999) in triplicate
enclosures with fish (F) and control
enclosures without fish (NF). t- and
P-values of t-test (d.f. ¼ 4)
week. This value is well within the limiting food
concentration for D. galeata, below which growth and
clutch size are reduced (Müller-Navarra & Lampert,
1996). The significantly larger maximum clutch size
(7.7 ± 1.5 versus 3.7 ± 0.5; mean ± 1 SD; t-value ¼ 4.2,
P ¼ 0.02, d.f. ¼ 3) and the lack of a significant difference in maximum body length (1.74 ± 0.14 versus
1.80 ± 0.15; t-value ¼ 0.51, P ¼ 0.64, d.f. ¼ 3) in enclosures with and without fish on day 18 of the
experiment supports this assumption.
Similar explanations as for daphnids might hold
true for the low copepod biomass observed in both
treatments. While adult copepods have higher evasive
abilities than cladocerans, copepod nauplii are very
vulnerable to predation by suction feeders (see review
in Lazzaro, 1987) such as silver carp (Dong et al.,
1992). Accordingly, nauplii biomass has been found to
be negatively affected by silver carp in several studies
(e.g. Kajak et al., 1975; Starling & Rocha, 1990; Starling,
1993; Domaizon & Dévaux, 1999). Food limitation is
the most reasonable explanation for the decline of
copepod biomass in the no-fish enclosures. In general,
copepods feed on a particle size spectrum slightly
larger than that used by daphnids (Sterner, 1989).
Assuming that only a part of the phytoplankton size
fraction < 30 lm was available to the adult copepods
and taking into consideration that the larger size
2002 Blackwell Science Ltd, Freshwater Biology, 47, 2337–2344
Effects of silver carp 2341
Fig. 1 Dynamics of total phosphorus,
Secchi depth, the biomass of phytoplankton <30 and ‡30 lm, particulate organic
carbon (POC), Daphnia biomass, copepod
biomass and Daphnia size at maturity
during an enclosure experiment from 22
June to 16 July 1999. Values are means ± 1
SD of three replicate enclosures with or
without silver carp. Significance of treatment effects (fish versus no fish) was
analysed with a repeated-measures
A N O V A : NS ¼ non-significant; *P < 0.05;
**P < 0.01; ***P < 0.001. Total phosphorus
values of day 5 of the experiment were
identified as outliers and omitted from
statistical analysis.
fraction was scarce, it seems most probable that food
limitation of adult copepods in the no-fish enclosures
was even more severe than for D. galeata.
The reduction of SAM in Daphnia may be caused by
selection of certain genotypes as a result of size 2002 Blackwell Science Ltd, Freshwater Biology, 47, 2337–2344
selective predation (e.g. Pace, Porter & Feig, 1984), by
a phenotypically plastic response to chemical stimuli
exuded by fish (Stibor, 1992), or by indirect effects of
predation (Lampert, 1993). Whereas the first mechanism relies on genotypic variability and the presence of
2342
R.J. Radke and U. Kahl
F
P
Variable
Factor
Secchi depth (m)
Fish
Time
Fish · time
51.7
1.67
4.67
0.002
0.17
0.003
Total phosphorus (lg L)1)
Fish
Time
Fish · time
0.01
2.36
0.16
0.93
0.08
0.97
POC <30 lm (mg L)1)
Fish
Time
Fish · time
14.0
2.70
2.32
0.04
0.047
0.08
Biomass of phytoplankton <30 lm (mg L)1)
Fish
Time
Fish · time
14.3
1.51
1.21
0.02
0.22
0.34
Biomass of phytoplankton ‡30 lm (mg L)1)
Fish
Time
Fish · time
0.35
2.21
0.94
0.58
0.08
0.48
Biomass of Daphnia galeata (mg L)1)
Fish
Time
Fish · time
5.10
10.4
1.43
0.09
<0.001
0.24
Copepod biomass (mg L)1)
Fish
Time
Fish · time
13.0
24.9
0.24
0.02
<0.001
0.96
Crustacean biomass (mg L)1)
Fish
Time
Fish · time
7.0
16.7
1.12
0.06
<0.001
0.38
Size at maturity of Daphnia galeata (mm)
Fish
Time
Fish · time
167.3
6.81
5.34
<0.001
<0.001
0.001
well-adapted individuals within the population, the
second will influence SAM in the next generations
even if no individuals showed the trait before
the stimuli became effective. The third factor also
depends on selective predation on large individuals,
which is rather unlikely in the case of a filter-feeding
fish that selects prey on the basis of prey evasiveness
(Dong & Li, 1994). Results of the present study point
to the second mechanism, a phenotypically plastic
response, because no small-sized individuals reproducing for the first time were found at the start of the
experiment, nor later in no-fish enclosures, and
because D. galeata responded to the presence of fish
within 1 week after the start of the experiment. This
non-species specific response of Daphnia might be an
advantage in the presence of a visually oriented
predator (the more common case in temperate lakes)
selecting large individuals, but is a disadvantage if the
predator is a filter-feeder such as silver carp, which
Table 2 Results of repeated-measures
A N O V A testing for effects of fish presence
(between) and time (within) from days
5–25 of the enclosure experiment. Degrees
of freedom were 4 and 24 for all variables
except for total phosphorus (4, 20), where
all values of day 5 of the experiment were
outliers, and for particulate organic carbon (POC; 3, 18), where one value was
missing on day 15
selects its prey on the basis of the species and sizespecific escape abilities (see review in Lazzaro, 1987;
Dong & Li, 1994). Consequently, the reduction of
SAM observed in our study may have even increased
the vulnerability of daphnids to silver carp predation.
Based on the results of our experiment and earlier
studies, we conclude that the use of silver carp in
biomanipulation is only appropriate if the primary
aim is to reduce nuisance blooms of large algal species
(e.g. cyanobacteria), which cannot be effectively controlled by large herbivorous zooplankton. The combined (alternating) use of filter-feeding fish and
daphnids as suggested for postwastewater treatment
(Smith, 1993) is not feasible in larger lakes, because
adequate refuges for the cladocerans are lacking.
Consequently, stocking of silver carp should be
restricted to tropical lakes that are highly productive
and naturally lack large cladoceran zooplankton
species (Nielssen, 1984).
2002 Blackwell Science Ltd, Freshwater Biology, 47, 2337–2344
Effects of silver carp 2343
Acknowledgments
We thank G. Egerer for phosphorus analyses and
help in the laboratory, M. Bollenbach for phytoplankton analysis and F. Wieland for technical
assistance with the enclosure frame. T. Schulze and
K. Rinke kindly helped in the field. S. Hülsmann, M.
Gessner and two anonymous referees gave helpful
comments on the manuscript. This study was supported by Landestalsperrenverwaltung Sachsen and
Arbeitsgemeinschaft
Trinkwassertalsperren
e.V.
(ATT).
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