The effect of rice seeding rate and fish stocking on the floodwater ecology of the rice field in direct-seeded, concurrent rice-fish systems

Concurrent, rice-fish systems in the Mekong Delta, Vietnam, are characteriz ed by a rice field surrounded by a trench. Rice is direct seeded and fish a re reared in polyculture. The most common species for polyculture are silve r barb Barbodes gonionotus (Bleeker), common carp Cyprinus carpio L. and Ni le tilapia Oreochromis niloticus (L.). In a 1996-97 rice-fish experiment, t he effects of (1) absence or presence of such a polyculture, and (2) differ ent rice-seeding rates (100 and 300 kg pre-germinated rice per ha) on the f ield floodwater ecology were studied. Fish obtained from local nurseries we re stocked in six out of 12 rice-fish plots at the Co Do experimental rice- fish station (10 degrees 10' N and 105 degrees 20' E). Silver barb, common carp and Nile tilapia were stocked in polyculture at densities of 3150, 310 and 550 fish ha(-1), respectively. About 50% of the observed variation in the floodwater could be attributed to the fish stocked and to rice-seeding rate. The above-ground dry weight of the rice plants increased in the prese nce of fish and by an increase in rice-seeding rate. Greater shading by den ser plants resulted in changes in variables related to aquatic primary prod uction, including the amount of chlorophyll-a, dissolved oxygen concentrati on and pH. The presence of fish quadrupled the ammonium concentration, but decreased that of ortho-phosphate. Though fish increased the ammonium conce ntration, this did not result in a higher aquatic photosynthesis. Light, no t ammonium, was the most limiting factor. However, increased ammonium conce ntrations in the presence of fish did have a positive effect on the density of Euglenophyta and the amount of chlorophyll-a, though only at the lower rice-seeding rate. At the higher rice-seeding rate, light seemed to be lack ing. At the lower rice-seeding rate, fish also had a significant effect on the Cladocera-Rotifera ratio. Rotifera numbers increased as fish stimulated phytoplankton. We also demonstrated that fish most probably do not reduce ammonia volatilization losses. The lower rice-seeding rate seemed better fo r fish production than the higher rate: at the lower rate, dissolved oxygen concentrations were higher throughout the rice cycle. In addition, the amo unt of chlorophyll-a and the density of Euglenophyta were higher at the low er rice-seeding rate. Finally, we could show that even at high rice-seeding rates, fish had an impact on the aquatic environment. Fish access to the f ield is not hindered by dense rice stands.