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Development of a Trophic Box Model to Assess Potential of Ecologically Sound Management for Cove Aquaculture Systems in Tri An Reservoir, Vietnam 10ASMR1

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Development of a Trophic Box Model to Assess Potential of Ecologically Sound Management for Cove Aquaculture Systems in Tri An Reservoir, Vietnam

Aquaculture System Modeling Research (10ASMR1)/Study/Thailand and Vietnam

Collaborating Institution
Asian Institute of Technology, Thailand
     C. Kwei Lin
     Yang Yi

University of Agriculture and Forestry, Vietnam
     Le Thanh Hung

University of Michigan
     James S. Diana

Objectives
1) Determine biomass production of various trophic levels in the fish culture cove.

2) Construct a trophic box model for the selected cove.

3) Recommend the ecologically sound stocking and management strategies for cove aquaculture.

Significance
A large number of reservoirs have been built in Indochina, mostly for irrigation, electrical generation and domestic water supply. With few exceptions, the fishery of these reservoirs also provides an important source of animal protein and livelihood for people living around the reservoirs. However, most reservoirs in the region are relatively unproductive with catch ranging from <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 through enhancement 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 in the reservoir itself. An alternative means to enhance fish production would be to pen semi-enclosed shoreline areas with barrier nets between 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 of 1-2 m depth with alternating flood and exposure during rainy and dry seasons. Cove culture has been widely used in Chinese reservoirs (Li and Xu, 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 size in its 324 km2 surface area. 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.

The most common local method of cove culture stocks mainly herbivorous and detritivorous species such as common carp Cyprinus carpio, bighead carp Aristichthys nobilis, tilapia Oreochromis niloticus, and grass carp Ctenopharyngodon idella. Stocking density and species ratios are based on those practiced for pond culture (Yang et al., 1990). As Tri An Reservoir is rich in benthic invertebrates and small fishes (such as Clupeids and freshwater shrimp), stocking of carnivorous species with high food value, such as sand goby Oxyeleotris marmoratus and snakehead Channa striata, could enhance production and economic gain from cove culture. Many efforts have been made to catch sand goby fingerlings from the reservoir for cage culture, but all failed because of mass mortality after moving sand goby to cages. An alternative method may be to culture sand goby in coves.

So far, little information is available on the natural food productivity of a cove. Such information could enable us to estimate the productivity of natural food at various trophic levels, predict carry 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 that information gained from such an analysis is most likely at best an approximation of the fishery potential of a cove.

Anticipated Benefits
The results of this study could be used to enhance fish production with greater economic return from cove culture. The study will also establish a case of sound management of fish culture in reservoirs that is more ecologically sustainable than current cage culture. The development of realistic ecological models would provide a new strategy for development and management of reservoir systems, and would facilitate extension of cove culture systems throughout the region. Use of Ecopath and a steady state analysis is only the first step in the development of realistic models, but is a necessary first step.

Research Design
Location: Truong Dang (TD) Aquaculture Cove, Tri An Reservoir, Vietnam; lab work at Aquaculture Laboratory, University of Agriculture and Forestry, Vietnam

Methods: Construct a trophic box model for TD Cove using the ECOPATH 3.0 approach and software, developed by ICLARM (Dalsgaard and Oficial, 1998). The model will be made on a dry weight (DW, g) per area basis.

Parameters to Be Measured for Constructing the Model

Sampling Schedule for Above Parameters

Sampling period: 8 months from August 2002 to February 2003.

Water quality (pH, DO, water transparency, and temperature) and biomass (detritus, phytoplankton, zooplankton, and benthos): measured for four distinct periods in August, October, December 2002 and February 2003. Each period has three sampling times in two weeks.

Biomass of terrestrial plants: measured for two distinct periods in June and July 2002. Each period has three sampling times in two weeks.

Biomass of different cultured fish groups: are assessed at stocking and at harvest time.

Regional Integration
In many Asian countries, reservoirs have been developed for fish production, making a significant contribution to overall inland fish production. However, the production is usually low and faces increasing fishing pressure. Development of cove culture will enhance fish production and provide an alternative strategy for nature-based aquaculture in reservoirs in the region.

Schedule
June 2002 to February 2003
Report submission: April 2003

Literature Cited
Anon., 1998. Workshop on reservoir fisheries in Indochina region. Mekong River Commission, Bangkok, Thailand.

Bernardi, R.D., 1984. Methods for the estimation of zooplankton abundance. In: J.A. Downing and F.H. Rigler (Editors), A Manual on Methods for the Assessment of Secondary Productivity in Fresh Waters. Blackwell Scientific Publication, Oxford, London, pp. 59­83.

Boyd, C.E., 1995. Bottom Soils, Sediment and Pond Aquaculture. Chapman and Hall, New York, 348 pp.

Christensen, V. and D. Pauly, 1993. On steady-state modelling of ecosystems. In: V. Christensen and D. Pauly (Editors), Trophic Models of Aquatic Ecosystems. ICLARM/DANIDA, Manila, Philippines, pp. 14­19.

Dalsgaard, J.P.T. and R.T. Oficial, 1998. Modeling and analyzing the agroecological performance of farms with ECOPATH. ICLARM Tech. Rep. 53, 54 pp.

Keckler, D., 1997. SURFER for Windows, Version 6.0 ­ User's Guide: Contouring and 3D Surface Mapping. Golden Software Inc., USA, 483 pp.

Li, S.F. and S. Xu, 1995. Culture and capture of fish in Chinese Reservoirs. Southbound, Penang, Malaysia and IDRC, Ottawa, Canada, 128 pp.

Lin, O.T., 1974. Handbook of common methods in limnology. The C.V. Mosby Co., USA, 154 pp.

Luu, T.T., 1998. The survey of pollution of cage aquaculture in Tri An Reservoir in 1997-1998. Department of Technology and Environment of Dong Nai Province, HCMC, 42 pp. (in Vietnamese)

Ruddle, K. and V. Christensen, 1993. An energy flow model of the mulberry dike-carp pond farming system of the Zhujiang delta, Guangdong province, China. In: V. Christensen and D. Pauly (Editors), Trophic Models of Aquatic Ecosystems. ICLARM/DANIDA, Manila, Philippines, pp. 48­55.

Winberg, G.G., 1971. Methods for the estimation of production of aquatic animals. Academic Press, London, 167 pp.

Yang, H.Z., Y.X. Fang, and Z.Y. Liu, 1990. The biological effects of grass carp (Ctenopharyngodon idella) on filter-feeding and omnivorous fish in polyculture. In: R. Hirano and I. Hanyu (Editors), The Second Asian Fisheries Forum. Asian Fisheries Society, Manila, Philippines, pp. 197­200.

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