Apple snails Pomacea canaliculata escaping from a paddy field into an irrigation canal in South Japan.

Abstract

As a part of our investigation to elucidate the population biology of the apple snail Pomacea canaliculata inhabiting irrigation canals, we monitored the number of snails escaping from a paddy field into an irrigation canal in Kamimine Saga, South Japan. We installed a net trap on one of the two drainage pipes in a paddy field(60 a). The number of snails trapped in the net was counted twice or three times a month during crop season (from late June to early October, 2008). Totally, 265 overwintered snails that had hatched in the previous season were trapped in the whole season. Snails of the new generation were trapped from August onwards and a total of 470 young juveniles with shell height more than 8 mm escaped from the paddy field until the end of the crop season. According to the size specific densities of snails in the paddy field, we estimated that more than several thousand snails with shell height smaller than 8 mm likely escaped through the wide mesh of the net trap and into the canal. The number of snails caught in the net had a significant correlation with the precipitation. The snails were considered to have escaped from the paddy field when paddy water overflowed into the canal. Many similar paddy fields were connected to the canal, and we therefore suspect that huge numbers of snails entered the canal from the fields. It raises a question how the escape of apple snails from paddy fields influences their population in the canals.