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Environmental Management of Mariculture in Hong Kong David KW Choi Department of Civil Engineering The University of Hon

Environmental Management of Mariculture in Hong Kong David KW Choi Department of Civil Engineering The University of Hong Kong. Hong Kong and the Pearl River Estuary (locations of marine fish culture zones and algal dynamics field station indicated).

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Environmental Management of Mariculture in Hong Kong David KW Choi Department of Civil Engineering The University of Hon

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  1. Environmental Management of Mariculture in Hong Kong David KW Choi Department of Civil Engineering The University of Hong Kong

  2. Hong Kong and the Pearl River Estuary (locations of marine fish culture zones and algal dynamics field station indicated)

  3. Mariculture (or marine fish farming) causes local nutrient enrichment but can also be a victim of existing pollution. A robust quantitative methodology is needed for mariculture management (site selection; impact assessment; determine carrying capacity) • Organic loading • Flushing rate • Hydro-meteorological conditions “Environmental management of marine fish culture in Hong Kong” (Lee, Choi and Arega, Marine Pollution Bulletin, 2003)

  4. Framework for modeling of carrying capacity of a fish culture zone Potential/existing fish culture zone (FCZ) Bathymetry, tidal conditions, salinity Hydrodynamics model Pollution loading, ambient water quality Mass transport model Numerical tracer experiments Water quality model (sediment – water exchange) Flushing time Carrying capacity of FCZ

  5. Flushing time (Tf) It is an important indicator of the self-cleaning capacity of a water body due to tidal exchanges and dispersion. In the present study, it is defined as “The average lifetime of a particle in the given volume of water body.” (Officer and Kester 1991) It is determined by conducting numerical tracer experiments using the 3D hydrodynamic and mass transport models. “Numerical determination of flushing time for stratifiedwater bodies” (Choi and Lee, J. Marine Systems, 2004)

  6. 3D models and laboratory experiments show that the tracer mass removal process due to tidal flushing can be approximated by a double-exponential decay curve that is described by 3 flushing parameters only, from which the flushing time (or flushing rate) can be uniquely determined. Tracer mass Flushingtime The three flushing parameters , k1and k2 can be interpreted in terms of the size of the fish farm, the exchange flows between the fish farm and its surrounding and that between the water system and the external “clean ocean” using a two-segment model

  7. Laboratory tracer experiment to determine residence time of semi-enclosed rectangular typhoon shelter Measured tracer concentration with time in typhoon shelter best described by double-exponential decay (Li and Ip 1999)

  8. Numerical determination of Flushing time: Release tracer in fish farm and track changes Computation via 3D hydrodynamic and mass transport (particle tracking) model System = fish farm open boundary Flushing time =average lifetime of a particle in the given water system

  9. Computed concentration 12 hr after release of tracer mass in fish culture zone in dry season Local flushing The adjoining waterbody is assumed “clean” Yung Shue Au fish culture Zone, Three Fathoms Cove, Tolo Harbour, Hong Kong

  10. Wet Season Yung Shue Au fishfarm Three Fathoms Cove Tracer mass removal (flushing process) can be described by a double exponential decay of a two-segment system Tracer mass in fishfarm time Dry season The flushing time or rate can be accurately determined from the tracer mass removal curve Local flushing time!

  11. Local vs “System-wide” flushing time Segment 1 Tracer releaseover entire bayrather than just fish farm! Segment 2 Fish farm To determine “system-wide” flushing, the edge of the system must be chosen at a location near the open sea, where pollutants discharged in the ebb will not likely return during the following flood System =entire bay

  12. Yim Tin Tsai Fish Culture Zone Tolo Harbour Tracer experiment for determining the flushing time dry season M/Mo wet season YTT FCZ time (days) dry season

  13. Sok Kwu Wan - dry season Tracer Mass in fish farm “System-wide” flushing “Local” flushing Time

  14. Residence time = mass weighted average of the time taken by individual particles to leave the system through the open boundary = “system-wide” flushing time Tolo Harbour Movement track for individual particles

  15. Schematic diagram of water quality model for a fish farm

  16. Organic loads, flushing times and key water quality indicators at representative fish culture zones

  17. Allowable organic loading dictated by DO and algal biomass limits.

  18. Conclusions • A general method for the numerical determination of flushing time of stratified water bodies is developed. • A simple and robust methodology for mariculture management can be based on flushing time and a seasonal average water quality model. Using water qualityindicators such as the potential lowest dissolved oxygen (PLDO) and Chorophyll-a level, the carrying capacity can be determined in term of the allowable organic loading. • The determination of flushing rates must be basedon the concept of “system wide” flushing. • The flushing rates, water quality, and relative carrying capacity of six representative fish farms so determined are supported by field observations.

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