During the last few years, the spread of the silver crucian carp caused significant problems in pond fish culture. One of the main challenges of the successful fish production in fishponds, is to rid ponds of the silver crucian carp. In the course of the experiment, carp fry were stocked in 5 nursing ponds at the same stocking rate, with different (0-, 50-, 100-, 150-, 200%) stocking rates of silver crucian carp fry, respectively. During the breeding season, weekly test fishings were made to examine the growth of the fishes. At the end of the experiment, the yield/hectar and the growth rates were calculated. The experiment proved that the presence of the silver crucian carp in the fishponds causes lower productivity and growth rates in common carp. In the control pond (with no silver crucian carp), the average weight of the common carp was over 20 g and the yield/hectar was over 700 kg, while in the pond with the most silver crucian carp, the average weight of the common carp was 4.5 g and the yield/hectar was only 123 kg.