USING MEAN ZOOPLANKTON…
M. WOLSKA
USING MEAN ZOOPLANKTON SIZES IN EVALUATION OF FRY
FEEDING PRESSURE IN VARIOUS BODIES OF WATER AND
THE ESTUARINE PART OF THE ODRA RIVER
Maria Wolska
Waldemar G. Piasecki
Kinga Mazurkiewicz-Zapałowicz
Department of Hydrobiology,
West Pomeranian University of
Technology in Szczecin, Poland
ABSTRAKT
The size resaerch of zooplankton in fishy and non-fishy basin showed that the
size of plankton and their specifics reflect the feeding pressure of tiny fishes on
zooplankton. At a term of maximal food necessity tiny fishes considerably limited
the zooplankton development in a Oder creek.
Keywords: zooplankton, baby fishes, feeding pressure
Introduction
Body size of prey is the main factor of food selection of fish (Brooks and
Dodson 1965). The largest plankton organisms are eaten in the first place, which
means that the size structure and species composition of zooplankton changes under
an increased feeding pressure of fish (see e.g. Akopian et al. 1999, Nielsen et al.
2000). According to this rule, the size of the planktonic organisms allow us to
evaluate their attractiveness as food items and the feeding pressure of the fish.
Stenson (1972) has demonstrated that lakes of low and high predation levels in
Sweden differed in terms of the body sizes of zooplankton organisms living
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USING MEAN ZOOPLANKTON…
M. WOLSKA
there. If we observe a high abundance of adult forms of planktonic crustaceans,
especially cladocerans, in a sample of zooplankton, we can deduce that the natural
food reserves in the given body of water remain underexploited. This means that
there is further potential of the water body to increase the fishing effort, subject to
prior necessary management measures, such as fish stocking (Szczerbowski 1993). In
order to estimate to effect of fish on zooplankton we first need to approximate the
feeding pressure, i.e. the ratio of the amount of plankton eaten and the number of fish.
Adult fish densities may be roughly estimated from fishing records. The problem is,
however, how to estimate the population of juvenile forms, i.e. larvae and fry, which
do not fall within the scope of commercial fishing and which exert the highest
feeding pressure on the zooplankton communities. This applies to all the species that
do not have other food resources available and, additionally, are present in large
densities (Opuszyński 1997). Therefore, alternative methods are sought to evaluate
the predation pressure of fish against zooplankton.
Materials and methods
It has been presupposed that the zooplankton organism mean body size is the
measure of its aptitude as a food item for fish. The study includes:
1. Measuring the zooplankton in 5 samples collected from bodies of water
void of fish populations located in Szczecin (Poland) and its vicinities: water body I –
an old, overgrowing pond on a damp meadow (in peat); water body II – a new, foilbedded garden pond; water body III – (Staw Bliźniak), a city pond, water body IV – a
politrophic reservoir in a city park (Lasek Arkoński). The survey was carried out from
April till October. Appart from the water body III, all the other ponds were rather
small and shallow, freezing to the bottom in winter, hence most probably fishless.
2. Measuring zooplankton in 7 samples from fish-containing water bodies,
also located within the Szczecin municipality, during June and July: water body V
(Syreni Staw), a pond located in a city park; water body VI – city lake Głębokie;
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USING MEAN ZOOPLANKTON…
M. WOLSKA
water body VII (Staw Łomot) – a city pond; an Odra delta channel (one of the
estuarine river branches).
In order to obtain the most representative data to estimate the mean body size,
we measured 400 individuals in each sample. Each sample was collected by dragging
a plankton net (0.044 mm mesh size) through the water column several times. The
measurements included all planktonic organisms, rotifers and crustaceans. The
organisms were randomly selected for measurements, i.e. individual organisms were
taken from the well mixed sample until the number of 400 was reached. Mean lengths
were calculated for each sample as well for the entire population.
Results and discussion
It has been presupposed that the fish predation level on zooplankton and
nutritional value of zooplankton for fry can be evaluated basing on the mean body
length of the plankton organisms. The mean body sizes between both groups of water
bodies differed substantially. In the fishless bodies of water, the values ranged
between 0.546 mm and 0.946 mm, whereas in those hosting fish populations the
range was between 0.182 and 0.436 mm (Table 1). Large species were found in the
fishless bodies of water, such as Daphnia magna and D. pulex, Simocephalus vetulus,
large copepodes Cyclops strenuus and C. vicinus, Eucyclops serrulatus, large rotifers
Asplanchna and Brachionus rubens, the latter species coexisting with Daphnia. A
similar dominant species composition of zooplankton living in water bodies with or
without fish communities was reported by Opuszyński (1997).
Table 1. Mean body length of zooplankton [mm] (± SD) in reservoirs without and with
fishes
Reserv
oir
Date
Mean body Mean body length in principal taxonomic groups
length of
[mm]
3
zooplankto
n [mm]
Larwy
Cladocera
Copepoda
Rotatoria
Dreissen
a
I
Habitat without fishes
II
III
June
0,805 ±
1,457 ±
0,192 ±
27th
0,754
0,588
0,019
June
0,564 ±
0,771 ±
0,278 ±
0,132 ±
27th
0,399
0,375
0,200
0,024
May
0,945 ±
1,444 ±
0,337 ±
0,268 ±
15th
0,694
0,507
0,204
0,025
October
0,861 ±
1,021 ±
0,317 ±
0,242 ±
8th
0,457
0,365
0,291
0,137
0,646 ±
1,136 ±
0,469 ±
0,188 ±
0,649
0,758
0,460
0,034
May 7th
IV
Mean
Habitat with fishes
V
VI
4
0
0
67
0,436 ±
0,320 ±
0,769 ±
0,218 ±
0,237
0,065
0,234
0,011
June
0,306 ±
0,296 ±
0,311 ±
0,210 ±
26th
0,147
0,106
0,155
0,110
0,205 ±
0,368 ±
0,190 ±
0,123 ±
0,138
0,097
0,126
0,100
June
0,245 ±
0,269 ±
0,365 ±
0,185 ±
26th
0,122
0,050
0,192
0,037
0,235 ±
0,278 ±
0,402 ±
0,161 ±
0,105
0,038
0,206
0,046
July 2nd
July 2nd
VII
0,744±0,60 1,155±0,59 0,321±0,24 0,1939±0,0
July 2nd
Delta
June
0,182 ±
0,847 ±
0,269 ±
0,090 ±
0,131 ±
channe
19th
0,298
0,935
0,237
0,0289
0,0446
4
l of the
Odra
July 3rd
River
Mean
0,353 ±
2,247 ±
0,444 ±
0,089 ±
0,138 ±
0,721
1,573
0,253
0,071
0,026
0,274±0,25 0,425±0,59 0,361±0,24 0,130±0,07 0,1313±0
2
1
8
0
,042
A comparison of mean lengths between the water bodies with fish shows that
the highest predation occurred in the Odra channel in June (Table 1). Zooplankton
specimens of as little as 0.182 mm in average length comprised organisms avoided by
fish, eaten as last. As a result of the strong predation, the zooplankton did not
represent a desirable food item. In July the situation slightly improved. The mean
length was then 0.353 mm and the resources had not been completely depleted;
however, we cannot conclude that the zooplankton managed to avoid strong predation
pressure. In both samples from the channel (June and July), the mean lengths of
cladocerans were high (0.847 and 2.247 mm, respectively), which resulted from the
presence of Daphindae and the large cladoceran Leptodora kindti. Even a very small
number of these specimens substantially influences the mean due to their large
bodies. The low overall mean lengths, despite the large sizes of the cladocerans, lead
to a conclusion that
the percentage of these crustaceans in the zooplankton
composition was very low. In the June samples, cladocerans represented in all 6% of
the zooplankton, with nearly 5% within length classes up to 0.6 mm (the most, 2%
within the class 0.41 to 0.6 mm), whereas 1% were represented by Leptodora kindti
(more than 1.4 mm). The July sample was in 7% composed of cladocerans, of which
4% were in the class longer than 1.4 mm (L. kindti). In the Marne River, a shift in the
species composition and size structure of zooplankton and a reduction in the
resources to 75% was mainly a result of selective predation by the perch (Perca
fluviatilis) fry (Akopian et al. 1999). Probably, also in the mouth of the Odra perch
fry exerted a strong pressure on zooplankton, since the authors saw large numbers of
perch fry.
A lower predation pressure was observed in the water body VII with
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USING MEAN ZOOPLANKTON…
M. WOLSKA
stickleback fry. We found small cladocerans in the pond, such as Bosmina
longirostris, Chydorus sphaericus, Alona guttata, A. rectangula, and juvenile forms
of Ceriodaphnia, which did not exceed 0.4 mm in length. Copepodes were most
abundantly represented by Thermocyclops oithonoides as well as copepodits and
nauplii. The copepode Eucyclops, which reaches more than 1 mm in length,
represented 1% of the total number of specimens. A similar level of predation was
found in the water body VI in July. Ciprinid fry depleted larger rotifers as well as
copepods of larger sizes; mainly copepodids remained. In the previous sample, from
June, we still encountered Asplanchna, adult forms of Thermocyclops, copepodits, as
well as Bosmina longirostris. Cladocerans found in July (15% of the total
zooplankton number) were represented by Ceriodaphnia, Scapholeberis, and
Diaphanosoma.
The highest mean body length of the zooplankton was found in the water body
V. We found very numerous (71% of zooplankton) Bosmina longirostris, with a
sporadic Ceriodaphnia, Scapholeberis, and Moina (in all 2%). Among copepodes
(26%), adult forms of Thermocyclops and Mesocyclops were most abundant (21%).
Rotifers, which represented less than 0.5% of the total zooplankton organisms, were
identified as Euchlanis. This implies the poorest predation and low densities.
Results by other authors, who studied the size of zooplankton under fish
predation pressure (including Nielsen et al. 2000, Bøhn and Amundsen 1998, Stenson
1972) confirm that the knowledge on qualitative and quantitative structure and mean
plankton organism sizes can be used to evaluate the trophic conditions of the given
body of water.
Conclusions
The zooplankton species that attain larger sizes were found in the fishless
bodies of water, whereas the species of smaller individual sizes, as a result of fish
pressure, were found in the water bodies inhabited by fish communities. The mean
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USING MEAN ZOOPLANKTON…
M. WOLSKA
lengths of plankton organisms reflect the feeding value of the zooplankton for fry as
well as the level of fish predation on zooplankton.During the period of the most
intensive feeding, the fry significantly restrain the growth of zooplankton populations
in the mouth of the Odra river, though it should be stressed that this pressure is not
the strongest one if compared with other bodies of water.
References:
1.
Akopian M., Garnier J., Pourriot R., 1999, A large reservoir as a source of
zooplankton for the river: structure of the populations and influence of fish
predation. J. Plankton Res., 21, 2: 285-297.
2.
Bøhn T. i Amundsen P.-A., 1998, Effests of invading vendance ( Coregonus
albula L.) on species composition and body size in two zooplankton
commumities of the Pasvik River System, northern Norway. J. Plankton Res.,
20, 2: 243-256.
3.
Brooks J.L., Dodson S.I., 1965, Predation, body-size and compositin of
plankton. Science, 150: 28-35.
4.
Nielsen D.J., Hillman T.J., Smith F.J., Shiel R.J., 2000, The influence of
a planktivorous fish on zooplankton assemblages in experimental billabongs.
Hydrobiol., 434:1-9.
5.
Opuszyński K., 1997, Wpływ gospodarki rybackiej, szczególnie ryb
roślinożernych, na jakość wody w jeziorach. PIOŚ, WIOŚ, Biblioteka Ochrony
Środowiska, Zielona Góra.
6.
Stenson J.A.E., 1972, Fish predation effects on the species composition of the
zooplankton community in eight small forest lakes. Rep. Inst. Fresh. Res.
Drottningholm, 52: 132-148.
7.
Szczerbowski J., 1993, Rybactwo śródlądowe. Wydawnictwo IRŚ, Olsztyn.
Рецензент профессор Вавжиняк В.
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