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Aquaculture CRSP 21st Annual Technical Report
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69
Development of a Trophic Box Model to Assess Potential of Ecologically Sound
Management for Cove Aquaculture Systems in Tri An Reservoir, Vietnam

Tenth Work Plan, Aquaculture System Modeling Research 1 (10ASMR1)
Final Report

Vu Cam Luong and Yang Yi
Aquaculture and Aquatic Resources Management
School of Environment, Resources and Development
Asian Institute of Technology
Pathumthani, Thailand

Le Thanh Hung
Faculty of Fisheries
University of Agriculture and Forest
Ho Chi Minh City, Vietnam

C. Kwei Lin and James S. Diana
School of Natural Resources and Environment
The University of Michigan
Ann Arbor, Michigan, USA

Abstract

This study was conducted at Truong Dang Cove of Tri An Reservoir from June 2002 to March 2003 to determine biomass production of various trophic levels in the fish culture cove, to construct a trophic box model for the selected cove, and to recommend ecologically sound stocking and management strategies for cove aquaculture. From June to July 2002, ground vegetation in the selected plots was harvested biweekly four times to estimate terrestrial vegetation biomass before flooding. Water quality parameters including pH, dissolved oxygen, temperature, and Secchi disk depth were measured in situ. Monthly water samples were taken for analyses of total alkalinity and chlorophyll a and to determine the biomass of phytoplankton and zooplankton. Biomass of benthic organisms and detritus were estimated monthly, while fish biomass was determined at stocking and harvest.

A trophic box model of cove aquaculture in Truong Dang Cove of Tri An Reservoir was constructed using Ecopath 3.0 approach and software. The energy flows through the system, which included five cultured fish species (Cyprinus carpio, Aristichthys nobilis, Hypophthalmichthys molitrix, Ctenopharyngodon idella, and Oxyeleotris marmorata) were quantified. Fish stocking density ranged from 0.018 to 0.107 fish m
-2, and there was no addition of feed or fertilizer. In terms of natural food sources, the mean biomass [g dry weight (DW) m-2] and mean production (g DW m-2 per season) were 1.21 and 191.25 for phytoplankton, 0.34 and 2.22 for zooplankton, 1.88 and 4.00 for benthos, 257.30 and 264.29 for terrestrial vegetation, and 1.41 and 0.92 for small wild fish, respectively. Detritus biomass was determined to be 1,066.90 g DW m-2.

The trophic model shows that the system had rather few trophic levels, and the bulk of flows were at trophic level II (herbivores and detritivores). Although marble goby, the main cultured species, had an average trophic level of 3.24, its chain linked directly to trophic level II. Phytoplankton and zooplankton became important limiting factors affecting productivity of trophic level II, while their biomasses were rather low. Effective use of natural plankton sources is the best strategy to achieve greater economic return from cove culture.

Species selection for stocking was developed to make full use of food resources and ecological niches in the cove aquaculture. Some high-value predatory fish, climbing perch (Anabas testudineus), and bronze featherback (Notopterus notopterus), which produce minimal predation on other stocked fish could be introduced into the system. Freshwater prawns (Macrobrachium spp.) should also be considered as potential species to be stocked in coves for better utilization of all food resources there.