We assessed the predatory potential of the common carp on the apple snail at a low density of carp. We released 1 carp (average body length : 16cm) and 1, 400 snails (average weight 0.12g) in each of 23 m2 plots set in a paddy field without rice in August 2000. Seven days after release, snail density was significantly lower in carp-occupied plots than in carp-free ones. However, in a 1 x 1 m cage set in each plot, which excluded carp, neither density of surface snails nor survival rate was different between carp-occupied and carp-free plots. This indicates that predation by carp was responsible for the reduction of the density of surface snails in carp-occupied plots, rather than the increase in the proportion of snails buried or natural snail mortality in the presence of carp. The average predation rate of a carp was estimated to be 54 snails/day. which was about one-fourth of the rate observed in the laboratory. However, the predation rate in the field was high enough to regard carp as biocontrol agents for the snail, not only in rice fields but also in other freshwater systems.
