Dr. Éva Murai szerk.: Miscellanea Zoologica Hungarica 3. 1985 (Budapest, 1985)

Kiss, I.: Ecological studies on zooplankton species important for fish farms. I. Population biological investigations on zooplankton breeds

In the tables we indicated the above-mentioned data according to the dates of the individual times of sampling, and to the number of days after the introduction of the basic population In the 3rd breeds of the Rotatoria and Cladocera species, under unfavourable temperature and nourishment conditions, we found so small number of individuals that we could not' even use them for the population structure investigations. So the change in the number of individuals is indicated only in Table 1. In the case of each breed, the growth of population, until it reached the maximum number of individuals, was compared using the method of linear regression analysis (Figs 2 and 3). We supposed that, at the starting period, the population growth can be described by sigmoid curve. We expressed the indicated data in logarythm values on both axes, and on the basis of the equation y = ax+b, we made the linear adjustment. RESULTS AND DISCUSSION A ) Rotatoria breed I recorded the changes observed in the first experimental pond in Table 1 and Figure 1. In this breed, in the period from 14.07 to 01.08, the average water temperature was between 22 and 24°C, which was very favourable for the Rotatoria. In the period following the flooding and the addition of chemicals, in the Zooplankton stock of the pond, Brachionus calyciflorus was overwhelming, whereas, later, the multiplication of the predator Asplanchna species followed. The Brachionus gradation reached its maximum on the 5th day following the flooding of the pond, then the value was 1352. 8 individuals/ litre. After this, as a result of the relative shortage of nourishment and the growing Asplanchna population, their number decreased rapidly, and on the 18th day I found hardly.any surviving individuals. The Asplanchna stock reached the maximum number of individuals (1122. 8 ind./ lit. ) on the 11th day. Parallel with the Rotatoria stock' s growing old in the pond, there appeared the nourishment-rival Copepoda and Cladocera species. With the Brachionus calyciflorus, the average number of eggs was 2. 0 during the time of investigation, even at the time of intensive reproduction. This can be explained by the following: the species react to the favourable environmental conditions not with increasing the number of eggs but with the acceleration of the rhythm of the production and ovulation of eggs. The time span between the oviposition cycles is very short: from 18 to 22 hours if the temperature is 21 to 24°C. If the environment is rich in nourishment, the reproduction of the Rotatoria is more intensive, but their life-span also becomes shorter, they live for 4 to 7 days only (VARGA, 1966). The juvenile, individuals not producing eggs comprise 50% of the Brachionus population at the time of the maximum number of individuals. It can also be observed that there are mictic females already in the booming period of the population, but their number is the biggest at the time of the maximum number of individuals of the population and immediately after it, as if forecasting the rapid decrease in number, well shown by the appearance of the ephippial females. Afterwards we experienced one more much smaller maximum of population, the appearence of which can be contributed to the remultiplication of some surviving female individuals. The arising relative plentitude of nourishment originates from the organic-material recuperative processes of the so-called "plankton-fall" consisting of dead individuals (SEBESTYÉN, 1963). The rapid multiplication, the big starting number of the individuals of the Brachionus population provided good opportunity for the upswing of the populations of the Asplanchna species. The most frequent species are: Asplanchna priodonta Gosse, Asplanchna brightwelli Gosse. The sudden collapse of the Brachionus population, however, was followed by the decline of the Asplanchna populations consuming it. In the second experimental pond (Rotatoria breed II, Table 2, Fig. 1) in the period from 15.08 to 09.09, the average water temperature was between 20 to 22°C, which can be considered more unfavourable in comparison with the conditions of the first breed. Here the Brachionus population reached its maximum (736. 0 ind. /lit. ) only on the 10th day, which shows significant decrease in the number of individuals and time shift in comparison with the results of the previous breed (Table 2, Fig. 1). The breed lived on for a longer period of time (25 days), and the two species co-existed for a longer period, too.