Mass development of conchostracs occurs regularly in some localities o
f Southern Moravia. This event is connected with high mortality of car
p fry. Leptestheria dahalacensis is the commonest species of conchostr
acs in ponds. The conchostracs are typical inhabitants of periodical w
aters. These localities are dried in late summer, the bottom is frozen
in winter. The conchostracs are well adapted to these conditions. The
animals are filtrators which whirl the bottom. The low transparency o
f water reduces the assimilation of water plants and the amount of dis
solved oxygen has a decreasing tendency. Whirled mud clogs the gills o
f carp fry. This situation call up the mortality of fishes. Developmen
t of zooplankton of studied ponds in the years 1987 - 1988 is given in
Tabs. I and II. A relationship between conchostracs and carp fry was
studied also in aquarium experiments. No direct negative impact of con
chostracs on carp fry was found. The abundance of conchostracs and car
p fry at the beginning and the end of experiment in some variants see
Tab. III. Some possibilities exist for elimination of conchostracs: pl
oughing of pond bottom before the filling of ponds, hunting conchostra
cs by means of light traps, elimination of conchostracs by means of ch
emicals, predation pressure of stock of older fishes, keeping of ponds
at full water in winter season, repeatedly filling ponds by water.