CHANGES IN THE LOWER TROPHIC LEVELS AS A CONSEQUENCE OF THE LEVEL OF FISH MANIPULATION IN THE PONDS

Cyprinid fish of different mature age classes (3+-4+) and stocks (100, 300 and 500 kg/ha) were introduced into each of three experimental po nds with area of 0.3 ha (average depth ca 1.7 m) while the fourth pond was left free of fish. Bream (Abramis brama L.), white bream (Blicca bjoerkna L.) and roach (Rutilus rutilus L.) made up 75% of the total c yprinid biomass, with wild carp (Cyprinus carpio L.) as the remaining 25%. The introduced fish spawned successfully. The high (above 300 kg/ ha) planktivorous and benthivorous fish stocks resulted in several qua litative and quantitative alterations of the food chain structure in o ur simulation pond experiments. These alterations must primarily be as signed to changes caused by both the zooplanktivory and benthivory nat ure of the stocked fish populations. At the higher levels of fish biom ass, Secchi depth was influenced significantly by chlorophyll-a concen tration. Most of the variance in suspended solids concentration could be explained by the biomass ratio of the mature benthivorous fish. The re was a clear shift in algal cell size in the ponds with the higher f ish stocks: ponds with more fish had larger cells later in the summer. The relative influence of young cyprinid fish on crustaceans species composition and biomass, and mature populations on benthic fauna abund ance and biomass, was sufficiently greater at higher (300-500 kg/ha) f ish stock rates.