9.
FISHERIES IMPACT
9.1.
Criteria for evaluating fisheries
impact are provided in Annex 9 of the Technical Memorandum on Environmental
Impact Assessment Process (EIAO-TM).
Annex 17 of the same document provides guidelines for fisheries impact
assessment. These guidelines and
criteria were followed in this assessment.
9.2.
Water quality within Marine FCZ is
regulated under the WPCO and its supporting regulations and statements. Within
Fish Culture Subzones, the dissolved oxygen level should not be less than 5 mg
l-1 for 90% of the sampling occasions during the year; values should
be calculated as water column average (arithmetic mean of at least 3
measurements at 1 metre below surface, mid-depth and 1 metre above seabed). In
addition, the concentration of dissolved oxygen should not be less than 2 mg l-1
per litre within 2 metres of the seabed for 90% of the sampling occasions
during the year, and the annual geometric mean of E. coli should not exceed 610/100 ml.
Sensitive
Receivers
9.3.
The Study Area encompasses fish fry
resources and adult fish resources. Sok
Kwu Wan also supports a gazetted FCZ and commercial fishing activity. These natural resources and the sections of
the community exploiting these resources are the sensitive receivers that need
to be evaluated by this assessment. In
terms of the fishing community it is the mariculture operators who rate the
most attention. These operators are
spatially fixed to their rafts and to the gazetted zone, while resource fishers
move over a wider area and are more adaptable to potential point source impacts
such as that of a sewage outfall.
Baseline
Conditions
Review of
Existing Information
9.4.
Information from the Port Surveys of
1989-91 and 1996-97 commissioned by AFCD provide the best source of existing
information about fisheries resources and their exploitation at Sok Kwu Wan. A
comparison of the two time periods (approx. 5 years apart) is given in Table
9.1.
Table 9.1 Comparison
of fish catches and fry production at Sok Kwu Wan (Area 101) between 1989-91
and 1996-97 for vessels of length less than or equal to 15m
|
Catches
|
Adult Fish
(tonnes)
|
Fry (tails)
|
|
1989-91
|
1996-97
|
1989-91
|
1996-97
|
|
Overall
|
145
|
98.4
|
175,000
|
28,551
|
|
Per Hectare
|
0.426
|
0.289
|
513
|
84
|
|
HK Average (per ha)
|
0.138
|
0.060
|
452
|
35.11
|
|
Difference
|
+67.6%
|
+79%
|
+12%
|
+58%
|
9.5.
Catches at Sok Kwu Wan are still above
average for Hong Kong on a per area basis (ranked 26th out of 189
fishing areas), however, this masks the overall significant decline in both
reported catches of adult fish and fish fry numbers over the past 5 years. At Sok Kwu Wan this has amounted to a
decline of 32% and 84% respectively.
9.6.
This situation is widespread
throughout Hong Kong and reflects the continuing uncontrolled exploitation of a
declining resource. In this regard, Sok
Kwu Wan is average. Further human
impact such as the proposed sewage outfall would likely compound this resource
health problem and these potential impacts need to be considered.
9.7.
The 1996-97 data reveal that the
dominant taxa reported from the area as landed catches are typically low
value. In rank order (by weight), the
top 10 fish types caught in the Sok Kwu Wan fishing area were mixed species, Acetes spp. (silver shrimp), Caranx spp. (scad/crevelle), Sardinella jussieu (sardine), Sebasticus marmoratus (rockfish), Stolephorus spp. (anchovy), Siganus oramin (rabbitfish), Ambassis gymnocephalus (glass perchlet),
Sparidae spp. (sea breams) and Pseudosciaena crocea (yellow croaker). Almost all the fishing in the area (at least
98% off approximately 155 vessels) is conducted by small vessels (less than 15m
in length) using gillnets, handlines, purse seines, cages and longlines.
However, these techniques are particularly suited for higher value food fish
such as snappers, breams and groupers as well as fry.
9.8.
While there has been a significant
decline in catches of adults and fry in the area over the past 5 years, dollar
values have remained relatively constant (Table 9.2).
Table 9.2 Relative
dollar values of catches at Sok Kwu Wan, 1989-91 (inflated as at 30/4/98) and
1996-97 (not inflated).
|
Component
|
Value of Combined Catch
(adults and fry)
|
|
1989-91
|
1996-97
|
|
Overall
|
2,474,600
|
2,201,671
|
|
per
hectare
|
7,257
|
6,457
|
|
HK
Average (per ha)
|
3,424
|
1,451
|
|
Difference
|
+53%
|
+78%
|
9.9.
This implies that unit prices
(particularly for fry) have greatly increased in response to demand. These data
suggest that while fry and adult fishing at Sok Kwu Wan may be lower in terms
of tonnage, the fishery is still financially viable. Protection of the fishery from other human impacts still needs to
be strongly considered.
9.10.
In addition to AFD Port data,
fisheries data are also available for Sok Kwu Wan from the AFD-commissioned
study on ‘Fisheries Resources and Fishing Operations in Hong Kong Waters’ (ERM
1998). Surveys conducted at Sok Kwu Wan
as part of this territory-wide fisheries study, have identified Sok Kwu Wan as
being part of a nursery area for commercial fisheries resources that
encompasses a large stretch of Hong Kong southern waters extending between
South Lantau (including the Sokos Islands) and the mid to southern portion of
Lamma Island waters (ERM 1998). In
general, this so-called South Lantau nursery area of which Sok Kwu Wan forms a
part, was identified as a nursery particularly for Oratosquilla anomala and Dictyosquilla
foveolata as well as Sciaenid and
Squilla fry. Furthermore, 6 trawl
surveys at Sok Kwu Wan conducted at 2 month intervals revealed that dominant
commercial species (by number) between June and October was the crustacean, Metapenaeopsis palmensis. Another crustacean, M. barbata was the dominant commercial species in April, while the
fishes, Thrissa kammalensis and Oxyurichthys tentacularis, were
numerically dominant in December and February respectively. Similarly, trawl
surveys in Sok Kwu Wan have also determined that catches of juvenile fish,
crustaceans and molluscs are highest in early to mid summer (up to 1200-1500
per trawl in June) but relatively low (0-300) at other times of the year. Other
surveys at Sok Kwu Wan using purse-seine netting conducted twice in the dry and
twice in the wet seasons found that mean monthly weight of captured fish,
crustaceans and molluscs was relatively low (<5kg) compared to other fishing
areas in Hong Kong although of similar species richness (fish – 22 spp.,
crustaceans – 4 spp., molluscs – 1 sp.). Dependency on Sok Kwu Wan is generally
low for all categories of vessels except P4 Vessels, which have a dependency
level of 86.6-100% for this area (ERM 1998).
9.11.
Of the 26 gazetted FCZs zones in Hong
Kong, two of them (Sok Kwu Wan and Lo Tik Wan) occur within or near the study
area. Sok Kwu Wan supports a FCZ in
three parts as shown in Figure 5.3. As
at November 2001, there were 86 licensed rafts operating at Sok Kwu Wan and the
area licensed for fish culture was 13,762m2. The Sok Kwu Wan FCZ itself comprises an area
of 14,200m2.
Assessment Methodologies
Desktop Study
and Interviews
9.12.
Information on the fisheries resources
of Sok Kwu Wan is shown in Table 9.1. These comprehensive data are based on
landed catches and interviews. In addition, findings from the study on
‘Fisheries Resources and Fishing Operations in Hong Kong Waters (ERM 1998) for
Sok Kwu Wan also provide relevant information for the assessment. In order to
assess the impact of the outfall on sensitive receivers, the following baseline
information is required and is available from the existing literature or
through interviews:
Ÿ
Reliable estimates of the existing resources and their spatial
distribution. Fry and adult fish both
need to be assessed, as both are relevant to the area (Table 9.1).
Ÿ
Characterisation of the Sok Kwu Wan area in terms of
fisheries significance (e.g. nursery and spawning).
Ÿ
Understanding of the requirements and opinions of the
mariculture operators who are most likely to be affected.
Interviews
9.13.
A questionnaire was designed to target
the mariculture operators in the area and to record facts of their operation
(species, source of fry, fishing activity) as well as their opinions about
water quality and its impact on their operations. A copy of the questionnaire sheet is attached in Appendix
9.1.
9.14.
Species under mariculture include:
Ÿ
Epinephalus areolatus
Ÿ
E. bruneus
Ÿ
Lutjanus
argentimaculatus
Ÿ
L. russelli
Ÿ
L. johnii
Ÿ
L. vitta
Ÿ
L. stellatus
Ÿ
Siganus
canaliculatus
Ÿ
Girella
melanichthys
Ÿ
Trachinotus
blochii
9.15.
Fry are almost entirely sourced from
overseas (PRC, Taiwan, Philippines, Thailand).
The only fry fishing actively conducted within Sok Kwu Wan are for S. canaliculatus. Some fry fishing is conducted within other areas of Lamma and
Lantau. Some operators fish in Sok Kwu
Wan for fish other than fry but such activity is marginally pursued. Water quality in Sok Kwu Wan was rated as
poor to average which they feel is indicative of many similar areas in Hong
Kong waters. Good water quality and low
costs were rated as the most significant factors to successful operation. Some operators feel that the outer part of
the bay around Mo Tat Wan has the best water quality within the bay at present.
9.16.
Potential impacts of the sewage
outfall could be predicted as:
Ÿ
Increased suspended sediment during construction phase
and its impact on fish resources, mariculture operation and significant marine
habitats.
Ÿ
Release of ammonia from sediments during construction
phase and its toxic impact on fish and mariculture operation.
Ÿ
Habitat loss due to submarine outfall pipeline.
Ÿ
Discharge of pollutants (including nutrients) during
operational phase and its impact on existing marine habitat, fisheries
resources and mariculture operation.
Construction
9.17.
Construction activities would raise
levels of suspended solids in the surrounding water column. It is known that
elevated suspended sediment levels may impact vital systems of fishes such as
gill function by clogging and eventually damaging the exchange surface of these
organs (direct). General physiological
stress caused by factors such as exposure to elevated sediment levels can also
degrade an organism’s ability to grow and reproduce (indirect). Young
life-history stages and fish contained within mariculture operations can be
expected to be the most vulnerable to this sort of disturbance.
9.18.
Minor dredging would be required
during the construction of about 240 m outfall pipeline and the diffuser zone
which would be no closer than 300m away from the gazetted FCZ. Release of sediment into the water column
due to the construction of inshore section of the outfall pipe (about 480 m
from shore) would be avoided through the adoption of the horizontal directional
drilling (no-dig method) (refer to Section 2 and Appendix 2.1).
9.19.
As detailed in Section 5, exceedences of WQO for SS concentration
at the FCZ were predicted without mitigation.
It is known that a high SS concentration would have the potential
impacts to fish species sensitive to SS, including fish-kill and inducement of
physiological stress. However, with the recommended mitigation
measures such as the use of closed grab dredger, silt curtains and reduction of
dredging rates, the elevated SS concentration from the offshore dredging works
would comply with the WQO. With
mitigation, changes in suspended solids within the bay or around the mariculture
areas due to construction are likely to be low and short-term. Significance of this potential impact is
therefore rated as minor.
9.20.
Dredging during the construction phase
may result in the release of ammonia from sediments which if in high
concentrations is known to be potentially toxic to fish. The release of ammonia has been assessed in
Section 5 and the result (see Table 5.4) indicates that the unionised ammonia
concentration during dredging activity was predicted to be well below the WQO
(0.021 mg/l). Hence, adverse impact on cultured fish due to the release of
ammonia would not be anticipated and the potential impact is rated as
minor.
9.21.
Dredging needed for the construction
of the submarine outfall pipe (approximately 26000 m3) would lead to
a temporary loss of epibenthic habitat. However, it is known that the seabed away from the coast is
largely devoid of macrofauna of importance to fisheries with only a small
number of seapens in evidence according to underwater dive surveys of these
areas (See Section 8). Significance of
this potential impact is therefore rated as minor.
Operation
9.22.
Effluent discharge from the proposed
submarine outfall would likely have impact the existing marine habitat, fish
resources and mariculture operations.
Modelling of the discharge plume (Section 5) shows that at the FCZ, SS,
BOD, NH3-N and E. coli
would be no greater than the WQOs.
Hence these parameters from the effluent discharge would not be
anticipated to impact the mariculture operations at Picnic Bay.
9.23.
The only discharge parameter that
would exceed WQO limits is Total Inorganic Nitrogen (TIN). This is despite the
fact that increases in TIN levels due to the effluent discharge would be minor.
The reason for the predicted exceedance is attributable to the high background
TIN levels which are close to or already exceed the WQO limit (up to 2x during
dry season, see Section 5.62). Although
the treated effluent from the outfall would only make a very small
contribution, exceedance in WQO TIN level may be unavoidable due to the high
background TIN level. During operation, TIN levels would be expected to fall
inside the range of annual background fluctuations. Besides, the Project would be anticipated to have the benefit of
reducing the TIN levels by eliminating untreated or partially treated sewage
discharges from the Sok Kwu Wan catchment (see Section 5.63).
9.24.
TIN levels in discharges deserve
careful attention because nitrogen is the most critical nutrient that limits
algal growth in the marine environment. As such, elevated nitrogen levels can
lead to increased algal growth which under ‘favourable’ growth conditions may
lead to dense concentrations of algae in the water column (called algal blooms
or red tide). The factors that lead to
the formation of red tide are complex and act synergistically but, in general,
red tides most commonly occur under calm and bright weather conditions with
high nutrient concentrations and supply and low mixing in the water column as
well as rising water temperature. Although red tides usually dissipate quickly,
they can be harmful to marine life and mariculture operations. The deleterious
effects largely depend on the type of algae in bloom but may include depletion
of dissolved oxygen levels leading to hypoxia; gill glogging and damage to
respiratory surfaces (also leading to suffocation); as well as exposure to
phycotoxins. In previous years, red tides have caused significant fish kills at
mariculture sites in Sok Kwu Wan and the relatively high background TIN levels
indicate the general susceptibility of Sok Kwu Wan to red tide.
9.25.
The potential for increased frequency
of red tide at Sok Kwu Wan due to discharges in the operational phase has
therefore been evaluated using water quality modelling predictions and an
understanding of phytoplankton dynamics and red tide processes. It was anticipated that the effluent
discharges would not have a significant impact on the frequency of red tide at
Sok Kwu Wan. The primary reason is
that, as discussed in Section 5, the discharge plume would not cause
significant elevations in TIN levels at Sok Kwu Wan. Although nitrogen
concentrations play a major role in determining the rate of growth of marine
algae, the small contribution of the discharge relative to ambient nitrogen
levels implies that uptake of nitrogen by algae and therefore their growth
would not be significantly enhanced by operational discharges. Besides, it was
anticipated that once the outflow becomes operational, the discharge of
untreated treated sewage into the more sheltered waters close to shore would
cease. In this way, it is expected that nutrient loading of these sheltered
inner bay waters, which would be more prone to red tide would diminish. The
potential impact for effluent discharges to induce red tide is therefore rated
as minor.
Emergency
Discharge
9.26.
The emergency discharge from the
pumping stations and STW may be as the result of pump failure, interruption of
the electrical power supply, Sequencing Batch Reactor (SBR) unts of the STW
failure or the blockage of the submarine outfall. Water quality impact on the FCZ from the emergency discharge
would be expected. Potential impacts on
the FCZ from the emergency discharge would primarily be toxic effects on fish
of released unionised ammonia (NH3-N) and potential depletion of
dissolved oxygen levels because of high BOD loading. It is known that fish are highly sensitive to unionised ammonia
and may suffer mortality when levels exceed 0.2 mg/L.
9.27.
As detailed in Section 5 of this EIA
report, extensive mitigation measures including provision of standby pumps and
generators, 24-hour temporary storage for sewage in case of emergency and
implementation of a telemetry system would be adopted. The provision of these measures would make
the risk of emergency discharge to be negligible. Although the chance very remote with the implementation of the
measures, in an extremely case that all measures fail, the recommended
contingency measures would minimize the water quality impact on the FCZ.
9.28.
It should be noted that the occurrence
of emergency discharge would be episodic and very short-termed. In comparison with the long-termed
improvement of water quality of Picnic Bay from the Project and given the
recommended measures to prevent and minimize the emergency discharge, the potential
impact due to emergency discharge is rated minor.
Table 9.3 Summary of
Impact Evaluation
|
Project Phase
|
Potential Impact
|
Impact Evaluation
|
|
Construction
Phase
|
Elevation
of SS and impact on fisheries sensitive receivers
|
Minor, with mitigation
|
|
Release
of ammonia toxic to fish
|
Minor
|
|
Loss
of epibenthic habitat
|
Minor
|
|
Operational
Phase
|
Discharge
of pollutants (including nutrients) and impact on marine habitat, fisheries
and mariculture operation
Emergency
discharge of pollutants and impact on marine habitat, fisheries and
mariculture operation
|
Minor
Minor
|
|
Overall
Impact
|
Minor
|
Construction
9.29.
Moderate impacts associated with
construction, which involves sediment dredging, can be mitigated by careful
construction practice and stringent mitigation. Closed grab dredger, deployment of silt curtains around the
immediate dredging area and low dredging rates have been recommended (Section
5) in order to minimise sediment release into the water column. As shown in Section 5, the implementation of
these measures would not result in exceedance of WQO limits at the gazetted
FCZ. In this way, moderate impacts of dredging to the nearby FCZ would be
mitigated to an extent that the works are environmentally acceptable to
fisheries.
Operation
9.30.
The high initial mixing or dilution of
the STW effluent at the proposed outfall location would be sufficient to ensure
that the effluent discharge parameters would not pose any significant increase
in pollutant loading on the ambient water quality. Owing to the high ambient TIN level, the TIN levels were
predicted to exceed the WQO during dry season without stratification and wet
season with stratification. However,
the predicted values were not taken account of the benefit of the Project by
eliminating untreated or partially treated sewage discharges from the Sok Kwu
Wan catchment. As indicated in Section
5, the Project would be anticipated to have the benefit of reducing the TIN
levels by eliminating the discharges of untreated or partially treated sewage
into Picnic Bay. Hence, no mitigation
measures would be required.
9.31.
With the implementation of the
extensive mitigation measures and contingencies measures as recommended in the
Section 5, the emergency discharges would be unlikely happened and even in the extremely
unlikely case that all the measures fail, the impact on the FCZ from emergency
discharge would be minimized.
Fisheries Monitoring and Audit
Programme
9.32.
Measurements of SS and other relevant
water quality parameters should be undertaken during the construction phase of
the Project. Due to the predicted exceedence of
TIN levels during operation of the Project, water quality monitoring would be
required during the initial operation stage.
These parameters are relevant to the mariculture operations and to the
subtidal habitats. These parameters
would be best measured within the water quality part of an EM&A programme
rather than within a fisheries programme.
However, data collected should be interpreted with respect to biological
and fisheries impacts and this should be documented separately in the EM&A
reports. Detailed EM&A requirements
are provided in a separate EM&A Manual.
9.33.
With the implementation of mitigation
measures, no unacceptable residual environmental impacts on fisheries resources
would be expected.