INFLUENCE OF MASS DEVELOPMENT OF CONCHOST RACA ON REARING CARP FRY

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.