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Environmental management of mariculture in Hong Kong

by Choi, King-wah

Abstract (Summary)
(Uncorrected OCR) Abstract of thesis entitled ?nvironmental management of mariculture in Hong Kong?submitted by CHOI King Wah, David for the degree of Doctor of Philosophy at the University of Hong Kong in July, 2002 Sustainable mariculture management requires proper siting of fish farms and stocking density control. Both are related to the carrying capacity and flushing characteristics of the water body concerned. The present study is aimed to develop a systematic approach to determine the carrying capacity of a fish farm using robust three-dimensional (3D) numerical models and its effective coupling with a diagenetic water quality model. A methodology using numerical tracer experiment is formulated to accurately determine the flushing time in a fish farm typically located in a shallow tidal inlet. The time variation of tracer mass in the fish farm is found to be well represented by a double exponential curve that can also be derived from an analytical two-segment model. The coefficients of the curve are related to the exchange flows and bay geometry and can be obtained numerically by best-fitting the computed mass changes. The flushing time can be determined from the coefficients. A 3D finite difference hydrodynamic circulation model incorporated with mixing length hypothesis for turbulence closure has been developed. The model is validated against available laboratory data and field observations and is capable of simulating the wet season density stratified flow observed in Hong Kong waters. Coupled with two accurate, mass-conservative transport models developed for this study, tracer experiments are conducted to determine the flushing time. Fish farms are usually situated in well-sheltered embayments that may not connect directly to the open water. Both ?ocal?and ?ystem-wide?flushing time are needed. The former is for assessing the impact of short-term pollution upon its vicinity. The latter is related to the long-term water quality and used in the determination of the carrying capacity of the fish farm. To predict the long-term water quality in the fish farm, a steady state model that simulates the eutrophication kinetics in the water column and the diagenesis reactions in the sediment is developed. With known pollutant loading, reliably computed flushing rate and the environmental conditions, the carrying capacity of the fish farm can be determined in terms of key water quality parameters: chlorophyll-a, dissolved oxygen, organic nitrogen and potential lowest dissolved oxygen level on a day of negligible photosynthetic production. The developed methodology has been successfully applied to six fish culture zones that represent a wide range of hydrographic and fish farm loading conditions. The computed hydrodynamic circulation, flushing characteristics and water quality are well supported by the available field data. Ma Wan, Sok Kwu Wan and Lo Fu Wat are found to be well flushed and have extra capacity for higher stocking level. Yung Shue Au is marginally satisfactory, while Yim Tin Tsai and Yim Tin Tsai East are under stress and the current loadings need to be reduced. In general the wet season flushing time is about 2 ? 3 times smaller than that in the dry season, due to more rapid exchanges resulting from a two-layer gravitational circulation. However, for Sok Kwu Wan, a three-layer diffusion-induced circulation is discovered in the wet season; the flushing time is about an order of magnitude smaller. Numerical studies suggest that this is mainly related to the shape and short length of the bay.
Bibliographical Information:

Advisor:

School:The University of Hong Kong

School Location:China - Hong Kong SAR

Source Type:Master's Thesis

Keywords:fish culture environmental aspects china hong kong water quality biological assessment mariculture

ISBN:

Date of Publication:01/01/2003

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