Are rice seedlings affected by changes in water quality caused by crayfish?

¹ IMAR : Institute of Marine Research, c/o Department of Ecology, University of Évora, Rua Romão Ramalho, n°59, 7000-671 Évora, Portugal
² Universidade de Lisboa, Museu Nacional de História Natural - Museu Bocage (Departamento de Zoologia e Antropologia), Centro de Biologia Ambiental (CBA), Rua da Escola Politécnica 58, 1269-102 Lisboa, Portugal
³ INIA Estação Agronómica Nacional, Quinta do Marquês, 2784-505 Oeiras, Portugal

Auteur de correspondance : anast@uevora.pt

Abstract

Crayfish are considered pests in most wet seeded rice fields, although in some parts of the world, namely in Louisiana (USA), double cropping crayfish and rice can be practiced with success. The direct negative effects of crayfish on rice seedlings are well known, but the indirect effects of crayfish are still unexplored. This paper tests the indirect effects on rice (Oryza sativa) seedlings via modifications in water quality caused by crayfish (Procambarus clarkii) activity. The experiment was performed in open air tanks divided by a mesh which prevented crayfish in the treatment tanks from moving to the other side of the tank. Control tanks were also divided but contained no crayfish. Statistical tests showed that the effects of crayfish on water quality are effectively transmitted across the mesh and into the other side of the tanks. A crayfish density of 1.7 ind. m-2 does affect water quality by increasing turbidity and decreasing dissolved oxygen. Although crayfish changed water quality no statistically significant effects were noted on the final values of: total rice biomass, biomass of not germinated seeds, seedlings biomass, number of not germinated seeds, number of seedlings and seedlings height. We conclude that indirect effects of crayfish on the studied rice variety through water quality changes, are not relevant to early developmental stages.