Impact of topmouth gudgeon (Pseudorasbora parva) on production of common carp (Cyprinus carpio) - question of natural food structure

Four production fishponds in the Czech Republic were investigated in 2003 and 2004 during a feeding experiment on common carp (Cyprinus carpio). In 2003, topmouth gudgeon (Pseudorasbora parva) was detected in enormous amounts in all of the investigated fishponds. P. parva got into the fishponds spontaneously by water inflow from connecting channels. The objective of this paper is to describe the condition of natural food in the presence of P. parva and its subsequent effect on carp production. The estimation of the population density of P. parva achieved at least 44 kg ha(-1) in 2003. In 2004, precautions against P. parva invasion were taken and its presence wasn't recorded during the season. The impact of P. parva on natural food structure was described in terms of zooplankton and zoobenthos amounts and main fish production parameters. A special focus has been taken on the density of Daphnia genus and chironomid larvae, the most preferred zooplanktonic and zoobenthic groups in feed of P. parva, respectively. In 2003, P. parva suppressed the zooplankton populations significantly, especially large cladocerans of the Daphnia genus. Influence of P. parva on zoobenthos structure and density was not detectable. Unfavorable natural food condition in 2003 caused extremely low carp production which fell to the mean value of 283 kg ha(-1) and food conversion ratio reached 3.5. On the contrary, in 2004 the mean carp production and food conversion ratio attained 634 kg ha(-1) and 1.6, respectively. The results described harmful competitive effect of huge populations of P. parva and its surprising economic consequences. Costs per 1 kg of growth were increased by approximately 100% in 2003 compared with results from season 2004.