Trials of cage fish culture in Ea Soup reservoir, Dak Lak province, central highlands of Vietnam.

Reservoirs in the Central Highlands of Vietnam are numerous and growing in number. Most of these reservoirs were built mainly for irrigation, with flood control and occasionally, hydroelectricity as secondary functions. Ea Soup reservoir covers an estimated 240 ha at the upper storage level, and has a drawdown of only 2.5 m. Probably for this reason, there is an abundant supply of macrophytes in the reservoir. While a number of species have cage culture potential, grass carp (Ctenopharyngodon idella) is particularly popular in many parts of Vietnam. The species feeds almost entirely on macrophytes, so cash requirements for feed are minimal. This is suitable for most poor farmers in the area. The market price, about VND 15,000 kg-1 (US $1=VND15,000 approximately) is higher than that for many species. In Ea Soup reservoir, cage culture of grass carp commenced in 1993, and rapidly expanded. By 1996, there were 152 cages. Stocks were hit by disease, and were decimated late in the year. These losses were a strong discouragement to continuation of the practice. Many owners suffered heavy financial losses. Pollution, poor circulation in the cages, high stocking density and overcrowding of cages may have contributed to the disease problem. From June 1998, the Project, jointly with three fishers has tested the culture of grass carp combined with small numbers of Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio) in cages with new materials, somewhat larger mesh size, and lower stocking density. The economic viability of this modified technology will depend mainly on the lifetime of the cage. Nevertheless, fishers are continuing to culture at their own expense. This suggests that the technology has some sustainability in the reservoir. The extent to which cage culture can be practiced remains to be seen, but any expansion must be done with caution.