Environmental Management of Mariculture in Hong Kong

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Environmental Management of Mariculture in Hong
Kong David KW Choi Department of Civil
Engineering The University of Hong Kong
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Hong Kong and the Pearl River Estuary (locations
of marine fish culture zones and algal dynamics
field station indicated)
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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)
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Framework for modeling of carrying capacity of a
fish culture zone
Potential/existing fish culture zone (FCZ)
Bathymetry, tidal conditions, salinity
Hydrodynamics model
Mass transport model
Pollution loading, ambient water quality
Numerical tracer experiments
Water quality model (sediment water exchange)
Flushing time
Carrying capacity of FCZ
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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)
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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 ?, k1 and 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
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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)
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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
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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
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Yung Shue Au fishfarm Three Fathoms Cove
Wet Season
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!
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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
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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
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Sok Kwu Wan - dry season
Tracer Mass in fish farm
System-wide flushing
Local flushing
Time
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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
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Schematic diagram of water quality model for a
fish farm
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Organic loads, flushing times and key water
quality indicators at representative fish culture
zones
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Allowable organic loading dictated by DO and
algal biomass limits.
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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.