Crayfish, Procambarus clarkii, effects on initial stages of rice growth in the lower Mondego River valley (Portugal)

Effects of crayfish (Procambarus clarkii) on the survival of rice seedlings were investigated. In field experiments, 12 enclosures with 1m(2) surface area each were used to test several crayfish densities (0, 1, 3, 5 and 10 individuals m(-2)). Three levels of replication were considered, respectively, time, field and density. Adult crayfish were stocked in the enclosures after flooding the fields, two days prior to seeding rice. Eighteen grams of rice seed variety Ariete were sown in each enclosure. Experiments lasted 14 to 30 days. One single density, 1 crayfish m-2, allowed some rice to survive and grow; higher densities prevented survival of the seedlings. Results suggest that most of the destruction takes place in the first days after sowing, which could be partially avoided if fields were sown 2 to 3 weeks before flooding. In the laboratory 5 control and 5 treatment (5.7 crayfish m(-2)) aquariums were used. After two weeks acclimation to laboratory conditions, and 48h prior to seeding rice, adult crayfish were stocked in the aquariums. One hundred seeds of the rice variety Ariete were sown in each container and the experiments lasted 30 days. Both the survival of rice seedlings and the turbidity of the water were greatly affected by crayfish. it is hypothesised that the most important mechanisms of crayfish destruction of rice seedlings are not only the direct grazing, but also the indirect effect of a increase in water turbidity.