Dr. Éva Murai szerk.: Miscellanea Zoologica Hungarica 4. 1987 (Budapest, 1987)
Kiss, I.: Ecological studies on zooplankton species important for fish farms. II. Interspecific relations between zooplankton species
individuals on every sampling occasion. The index, calculated from the dimension of the two measurements I = PL/L, can be used to demonstrate the evolving cyclomorphosis, the appearance of Asplanchna (HALBACH, 1971). RESULTS AND EVALUATION A. Cladocera breeds In the Cladocera breeds (Figs 1 and 2), in the starting slow period of reproduction we observed a significant Rotatoria stock, the quantity of which gradually decreased parallel with the reproduction of the Moina and Daphnia populations, respectively. Among the Rotatoria species, the reproduction of Brachionus calyciflorus started first multiplying very quickly, and it was followed by the consumer, the predator A splanchna species. In the Moina breeds (Fig. 1), parallel with the aging of the stock the food competing Daphnia species with better body construction and more intensive filtering activity, as well the Cyclops species increased significantly in number. In the Daphnia breeds (Fig. 2) during the starting period, it could be observed that besides reproduction of the Rotatoria species, after the introduction of the Daphnia basic population only for five days and only with a low number of individuals (50 ind./lit.) was the Moina population present. Presumably the inoculum was accompanied by some individuals, but it is more probable that the individuals born from the ephippial eggs stuck to the plants at the edges of the pond and left from the previous fish breeding period, multiplied. They could not increase in number significantly, because the food competitor Daphnia magna, with better body construction, and quicker consumption proved to be stronger and soon multiplied in large numbers. B. Rotatoria breeds In every case following the flooding, the individuals of the Brachionus calyciflorus species multiplied, then between the 10th and 15th days Asplanchna spp. also appeared in the water habitat. It can be observed in Figs 3 and 4 how sensitively the young Brachionus calyciflorus individuals reacted to the very low density of Asplanchna. Very few Asplanchna individuals per litre (3-5 ind./lit.) can already induce the growth of the postero-lateral spine, thus, the value of the "I" index increased. It is worth mentioning that the change of the proportion of the body size can already be observed when Asplanchna could not yet be identified in the water-sample. As plan c hna spp. are able to co-exist for a long time in large numbers with their prey animals: this is made possible by their ability to swallow some smallspined individuals with low "I" index of the Brachionus calyciflorus species, and the spineless, common individuals of B. angularis and B. rubens . In our breeds B. rubens individuals multiplied, and provided the Asplanchna spp. with easily available food. This is why the Asplanch na spp. survived in large numbers in spite of the marked decrease in numbers of Brachionus calyciflorus in two of our breeds (Fig. 4). If only Brachionus calyciflorus individuals can be found in the water habitat, Asplanchna spp. are able to exist only in small numbers as a consequence of the food shortage. They survive longer in comparison with the case when they co-exist with spineless prey animals (HALBACH, 1971). This is the result of the fact that there are individuals with low "I" index value in every Brachionus calyciflorus population as the consequence of genetic variability. However, considering that the selectional pressure of the Asplanchna spp. is aimed at the increase of the "I" index, the mechanisms of this constant co-existence need further investigations. SUMMARY The investigated Rotatoria (Brachio nus c alyciflorus, Asplanchna priodonta , Asplanchna bri gthwelli) and Cladocera (Moina brachiata, Daphnia magna) species are the most important members of the Zooplankton stock of several natural waters and fishponds in Hungary from the point of view of fish-breeding. The Cladocera species are able to exclude the Rotatoria species from the biocenosis
