Influence of common carp on apple snail in a rice field evaluated by a predator - prey logistic model

The hypothesis that common carp can be used for the control of apple snails in rice fields was tested experimentally. In a rice field, 12 plots of 4 2 5 m were set and enclosed by plastic walls to prevent snail emigration and immigration. The experiment continued from June to September. Three replicated treatments were used for the plots: zero, four and 12 carp were released, giving carp densities of 0.0, 0.2 and 0.6 m⁻², respectively. Snail densities were estimated by the Jolly-Seber mark-recapture method. Newly laid egg masses were counted and measured for size, and hatching was monitored. The numbers of eggs per egg mass (y) were estimated using a regression equation obtained from the product of the maximum length and width of the egg mass (x): y =0.10x^1.24. Using these measures and the monthly mean hatching rate obtained from eggs laid in two outdoor aquaria from April to September, the number of hatched eggs was used to estimate the birth rate over a given time. A logistic model incorporating these estimates revealed that the snail population proliferated only in the zero-carp plot throughout the experiment. The study, together with other reports on snail longevity, predicts that a snail population would be eliminated in 2 years at a stocking density of 2000 carp hectare⁻¹, if no immigration of the snail occurred.