Factors inducing overland movement of invasive crayfish (Procambarus clarkii) in a ricefield habitat

The red swamp crayfish (Procambarus clarkii) is a widespread invader which can disperse overland. This study was conducted to determine which factors can induce overland dispersal. During 12 months, the numbers of P. clarkii out of water were monitored at an experimental rice research station. Data were obtained on preferential time period for overland dispersal; population structure on land and in water; direction of movement; and the relation between environmental variables and the occurrence of crayfish out of water. A main factor inducing overland dispersal was intentional drainage of the study area associated with rice cultivation. During post-drainage, crayfish dispersal overland was positively correlated with rainfall. Other variables significantly affecting the overland dispersal of crayfish were the relative humidity, air temperature and the period of the day. We found significant differences in the structure of the trappable population in the water and of the individuals out of the water, with a higher proportion of mature individuals dispersing overland. Because overland migrants were dominated by mature individuals, often with eggs or hatchlings attached to the abdomen, these may effectively colonize new areas.