Introduction
A large number of reservoirs have been built in Indochina, mostly for irrigation, electrical generation, and domestic water supply. With few exceptions the fish of these reservoirs provide an important source of animal protein and livelihood for people residing nearby. However, most reservoirs in the region are relatively unproductive with catch ranging from less than 10 to 65 kg ha-1 yr-1 (Anon., 1998). In Vietnam, many reservoirs exist in the central highland and some are used for fish production by enhancement of stocking and cage culture. In large reservoirs, catch per unit effort of wild fisheries is quite low due to low productivity of pelagic water. Cage culture in large reservoirs often suffers from heavy mortality and usually requires protein-rich feed, such as small fish caught from the reservoir itself. An alternative means to enhance fish production would be to pen semi-enclosed shoreline areas with barrier nets to isolate them from the main reservoir. Such a system is termed cove culture. Dendroid coves are a prominent feature of most mountain reservoirs. The ideal cove for aquaculture is one that contains mainly a littoral zone at 1 to 2 m depth with flooding during the wet season and exposure during the dry season. Cove culture has been widely used in Chinese reservoirs
(Sifa and Senlin, 1995). Some advantages of cove culture are ease of access from shoreline (compared to pelagic cage culture), more available food sources in the littoral zone, ease of harvest during the dry season, and low cost.
Tri An Reservoir, located 75 km from Ho Chi Minh City, contains 50 coves of various sizes within its surface area of
324 km2. At least a dozen cove culture systems already exist in this area. However, fisheries development of Tri An Reservoir is threatened by water pollution from runoff, organic matter input from surrounding vegetation, and wastes from cage culture (Luu, 1998). Development of cove culture relying on natural foods within the reservoir is ecologically sound and most likely more sustainable than systems using extensive food inputs.
Locally, the most common method of cove culture is to stock mainly herbivorous and detritivorous species such as common carp (Cyprinus carpio), silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis), Nile tilapia (Oreochromis niloticus), and grass carp (Ctenopharyngodon idella). Stocking densities and species ratios are based on those practiced for pond culture (Yang et al., 1990). As Tri An Reservoir is rich in benthic invertebrates, small fishes (such as clupeids), and freshwater shrimp, stocking of carnivorous species with high food value, such as marble goby (Oxyeleotris marmorata) and snakehead (Channa striata), could enhance production and economic gain from cove culture. Many efforts have been made to collect marble
goby fingerlings from the reservoir for use in cage culture, but these efforts have failed due to mass mortality after stocking marble goby in cages. An alternative method may be to stock marble goby in coves.
So far, little information is available on natural food productivity of a cove. Such information could enable us to estimate productivity of natural food at various trophic levels, predict carrying capacity of stocked fish species of different trophic levels, and thus determine the quantity and combination of fish to be stocked based on food availability. An ecosystem model could be constructed to assess trophic status for a cove. An average system can be described with a steady-state model (Christensen and Pauly, 1993). Such static models can give important information on energy flow and biomass storage, which should be measured to validate such a model. However, coves within a reservoir are often not in a steady state, so information gained from such an analysis is most likely a rough approximation of the fishery potential for the cove.
The purposes of this study were to:
1) Determine biomass production of various trophic levels in the fish culture cove;
2) Construct a trophic box model for the cove; and
3) Recommend ecologically sound stocking and management strategies for cove aquaculture.
Methods and Materials
This study was conducted at Truong Dang Cove of Tri An Reservoir from June 2002 to March 2003. Truong Dang Cove is located at 17 05' N and 11 13' E and has many characteristics that facilitate cove aquaculture. The cove's shape is long with a tortuous shoreline and narrow mouth. The surface area of the cove is 56,800 m2 with a mean water level of 5.4 m. Monthly water level in the cove varied from 0 to 10 m. The fish culture season in the cove ranges from 6 to 8 months per year, depending upon the annual hydrological regime of the reservoir. The cove is usually flooded from August or September and empty in June. The season with the highest water level spans from October to January.
This study consisted of two components: field measurements and model development using the collected data.
Field Measurements
From the middle of June to the end of July 2002, when the cove was empty of water, ten plots (1 m2 quadrat) were randomly selected. Ground vegetation in the selected plots was harvested biweekly four times and was identified to species. Subsamples of terrestrial vegetation were taken for the