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 ODRARIVER
Maria Wolska
Waldemar G.Piasecki
Kinga Mazurkiewicz-Zapałowicz
Department of Hydrobiology,
WestPomeranianUniversity 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
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:
- 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, foil-bedded 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.
- 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 lakeGłębokie;
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
Reservoir / Date / Mean body length of zooplankton [mm] / Mean body length in principal taxonomic groups [mm]Cladocera / Copepoda / Rotatoria / Larwy Dreissena
Habitat without fishes / I / June 27th / 0,805 ± 0,754 / 1,457 ± 0,588 / 0,192 ± 0,019
II / June 27th / 0,564 ± 0,399 / 0,771 ± 0,375 / 0,278 ± 0,200 / 0,132 ± 0,024
III / May 15th / 0,945 ± 0,694 / 1,444 ± 0,507 / 0,337 ± 0,204 / 0,268 ± 0,025
October 8th / 0,861 ± 0,457 / 1,021 ± 0,365 / 0,317 ± 0,291 / 0,242 ± 0,137
IV / May 7th / 0,646 ± 0,649 / 1,136 ± 0,758 / 0,469 ± 0,460 / 0,188 ± 0,034
Mean / 0,744±0,604 / 1,155±0,590 / 0,321±0,240 / 0,1939±0,067
Habitat with fishes / V / July 2nd / 0,436 ± 0,237 / 0,320 ± 0,065 / 0,769 ± 0,234 / 0,218 ± 0,011
VI / June 26th / 0,306 ± 0,147 / 0,296 ± 0,106 / 0,311 ± 0,155 / 0,210 ± 0,110
July 2nd / 0,205 ± 0,138 / 0,368 ± 0,097 / 0,190 ± 0,126 / 0,123 ± 0,100
VII / June 26th / 0,245 ± 0,122 / 0,269 ± 0,050 / 0,365 ± 0,192 / 0,185 ± 0,037
July 2nd / 0,235 ± 0,105 / 0,278 ± 0,038 / 0,402 ± 0,206 / 0,161 ± 0,046
Delta channel of the OdraRiver / June 19th / 0,182 ± 0,298 / 0,847 ± 0,935 / 0,269 ± 0,237 / 0,090 ± 0,0289 / 0,131 ± 0,0446
July 3rd / 0,353 ± 0,721 / 2,247 ± 1,573 / 0,444 ± 0,253 / 0,089 ± 0,071 / 0,138 ± 0,026
Mean / 0,274±0,252 / 0,425±0,591 / 0,361±0,248 / 0,130±0,070 / 0,1313±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 MarneRiver, 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
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
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:
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.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.
Brooks J.L., Dodson S.I., 1965, Predation, body-size and compositin of plankton. Science, 150: 28-35.
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.
Opuszyński K., 1997, Wpływ gospodarki rybackiej, szczególnie ryb roślinożernych, na jakość wody w jeziorach. PIOŚ, WIOŚ, Biblioteka Ochrony Środowiska, Zielona Góra.
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.
Szczerbowski J., 1993, Rybactwo śródlądowe. Wydawnictwo IRŚ, Olsztyn.
Рецензент профессор Вавжиняк В.
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