I discussed the issues you raised with my marine engineering friend again. He has kindly provided further views in respond specifically to them. He is a proponent of wider deployment of Anti-trawling Artificial Reefs along our coastline, as a means to restore our damaged seabed and depleted fish stock, to prevent further damage by illegal trawling, as well as reuse of our waste tyres as sustainable disposal solution. What is perplexing to us is why is this option not adopted by EPD to dispose of our waste tyres stockpile, and why is AFCD not charged with enriching our in fish stock through Anti-trawling ARs. However, it is also important to know that Waste-tyres AR is by itself a sustainable solution, hence these exchanges. My other enquiries reveal that AFCD is only charged with creating the ARs in Marine Parks and Fishery Protection Areas, as well as the methodology of creating the ARs; anywhere else is the responsibility of Lands Department, and the Marine Department would have a say from the navigation point of view. I don’t see the Lands Department initiating a sustainable solution for the seabed. Hence it would appear to me that the responsibility should fall on the Environmental Department which has to dispose of the waste tyres. EPD has apparently been adopting wrong methods of disposal. We are eager that EPD is motivated to initiate the use a more sustainable solution such as Anti-trawling ARs to dispose of our stockpiling waste tyres, and for the other departments, Lands, AFCD and Marine to co-operate.
and from engineering friend:
Apparently, some old issues have not been settled and some new ones are raised. Hopefully the new information below will help: –
1. “Structural Integrity of the tyre ARs is still untested. …. No direct hit by serious typhoon
yet”. Back in 1997, AFCD were equally cautious when they set out to choose an appropriate
tyre AR design. AFCD subjected the test tyre AR to the “flip tests” which simulated the
worse possible conditions where a severe typhoon would tumble the tyre ARs along the
seabed. These were very severe tests because the tyre AR was tested on land where its
weight is 4 times that in water, and therefore experienced 4 times the stresses actually
encountered in the sea. If the tyre AR passed the tests on land, it would have no problem
in the sea during typhoon conditions.
The chosen tyre AR design performed well during the test. All tyres were intact and the
whole AR structure maintained its shape. Thereafter, questions of structural integrity of
the chosen tyre AR design never arose again.
In fact, these tyre ARs have proven themselves over the years. Since their first
deployment in 1998, Hong Kong was hit by no fewer than 11 typhoons rated 8 or above.
Maggie rated 9, hit Hong Kong in June 1999 followed in September 1999 by York, rated 10. York, the most severe typhoon in the last twenty years lashed HK for over 11 hours
with wind speed up to 234 km/hour, the highest recorded wind speed in HK. Subsequent
dive surveys by AFCD showed that the deployed tyre ARs retained their structural
integrity and, no repair or maintenance work was required.
Two “Tyre nodules are (traditionally used as) fish habitats.” … They have not been tested as
Since AFCD began their AR Programs in 1995, HK’s AR technologies have evolved
rapidly from small pyramidal tyre nodules or sunken boats, to very sophisticated, high
profile structures designed to suit specific tasks and sites. The latest and most successful
anti-trawling ARs are such specialized structures, incorporating tyres or bio-filters,
sufficiently heavy and robust to resist the impacts of the largest trawlers in HK, and yet
light enough not to sink into HK’s muddy seabed.
Some of these third generation anti-trawler ARs standing 7 m above the sea floors, can be
found in the two marine parks in Hoi Ha Wan and Yan Chau Tong as well as the Long
Harbour Fishery Protection Area. From the shrill complaints AFCD receives since their
deployment and the large number of damaged trawler nets found on these anti-trawler
ARs, there is no doubt that these ARs are extremely effective.
Regular dive surveys by AFCD have also confirmed that these ARs provide excellent
feeding grounds, spawning sites and protective habitats for a wide variety of fishes,
including commercially valuable reef fishes.
3. “Anti-trawlers are usually ships/or derelict dolphins … much heavier obstructions”
Traditionally, ships or derelict dolphins were deployed as anti-trawlers. But derelict
dolphins from dismantled bridges or piers are not readily available and very expensive to
deploy due to their massive weight; while de-commissioned ships are expensive to buy,
clean out and prepare for deployment to avoid polluting the sea.
Furthermore, with the high prices of scrap steel in recent years, any mass over and above
the necessary weight (such as the over sized steel ships) to stop illegal trawlers is
redundant and a waste of resources. The modern anti-trawler ARs are designed to provide
just enough mass to stop the illegal trawlers and yet do not sink into the muddy seabed in
HK, unlike the massive dolphins.
HK’s experiences also showed that ships and derelict dolphins were not as productive as
purpose built anti-trawler ARs. They need additional enhancement ARs to raise their
productivity. So from an overall cost benefit point of view, purpose built anti-trawler
ARs are much better proposition than decommissioned ship, dolphin, or other dismantled
4. “The number of tyres that need disposing off could make the sea floor in HK look like a
scene from NT.” Some simple calculations may clarify the situation.
According to AFCD’s latest AR design with anti-trawling properties, one 8-9 m high tyre
anti-trawling AR will take about 4,000 tyres, to give the AR sufficient size, weight and
structural complexity. Just to put a protective ring of anti-trawler ARs around one marine
park (e.g. Tong Ping Chau) would require over 300 of these ARs. That comes to 1.2
Let’s consider the Outer Port Shelter Fishery Protection Area. To put a protective ring
around this area would require over 600 of these anti-trawler ARs. That comes to 2.4
Experiences also show that some trawlers attracted by the increased fish stock inside the
protected area, would lift their nets over the anti-trawler boundaries to fish inside. So
randomly dispersed AR deployments inside these zones are also necessary. Again, let’s
take the Outer Port Shelter FPA as an example. At a deployment rate of 1% of the sea
floor area, a further 1,800 ARs are needed. That adds up to 7.2 million tyres, or six years
supply of HK’s discarded tyre.
5. Does AR promote fish production or merely pull in fish stock from the nearby rocky
shores? …. The “natural habitat versus ARs” issue
AFCD’s own surveys and studies showed that the deployed ARs supported larger (sizewise)
and more commercially valuable reef fishes than the natural rocky shores in HK.
On the other hand, there were very few fish in HK’s muddy seabed.
Native species such as John’s Snapper, Painted Sweetlip and Long-tooth Grouper which
had disappeared in HK waters since the late 1950’s, have reappeared in and around the
ARs and in significant numbers. More interestingly, several commercially important fish
have used the ARs as spawning sites. These included the Blacktip Crevalle, Purple
Amberjack, Mangrove Snapper, Painted Sweetlips, Japanese Seabass, Black Seabream
and Goldlined Seabream.
These are usually reef fishes which prefer waters deeper than six meters (i.e. away from
the rocky shores) where HK’s natural hard-bottom substrates are already destroyed by
intensive trawl fishing in the past. Therefore, AFCD’s studies strongly suggest that the
deployed ARs do increase the overall fish stock within the HK waters rather than merely
pulling the fish away from HK’s rocky shores or muddy seabed.
6. “By deploying anti-trawler ARs, are we not pulling in fish stock to centralized locations
for them to be caught quicker?”
In 2000, AFCD commissioned a study to examine the impacts of various policy
approaches on HK’s fish stocks. The study concludes “the greatest improvement in (HK’s)
fishery (would be) achieved by the elimination of trawling”. Failing that, the simple
exclusion of trawlers in the various marine parks and Fishery Protection Areas (FPAs)
would be better than doing nothing. In addition, further improvements can be achieved by
deploying ARs in these protected zones even if we cannot exclude the indigenous
fishermen using gill nets and lines from these sites.
Therefore, the answer is very clear. HK’s fish stock will benefit much more if anti-trawler
ARs are deployed to keep out the trawlers in these protected areas.
Ideally, we should ban all form of fishing inside the existing 2 PFAs, the 4 marine parks,
the D’Aguilar Marine Reserve and the Airport Marine Exclusion Zone. But political
reality dictates that we have to move one step at a time if we hope to make steady
headway in helping our marine environment.
7. There are still some doubts. Why don’t we wait until there are conclusive scientific proofs
before taking the plunge?
It is important to realize that HK’s hard bottom substrates at 6 meters and deeper were
virtually eliminated by indiscriminate trawling in the past. The anti-trawler ARs deployed
in water deeper than 10 meters (to allow for a minimum draft of 5 m Chart Datum) in the
PFAs and marine parks are simply an attempt to partially restore this portion of this
marine ecology. HK cannot afford the luxury to sit back and do nothing.
8. West Kowloon and Victoria Harbour are not particularly good examples … these are
beyond any form of short/long term remedial action.
A practical approach would be to require land reclamation projects to contribute 1%, of
the project costs to a “Marine Mitigation Fund” to be used to partially restore the damage
marine ecology. The mitigation effort can be carried out in HK’s marine parks, marine
reserve, the Airport Marine Exclusion Zone and the PFAs. “Carpeting the (sea) bottom”
with ARs is not practical and has never been considered.