by Keith Wilson
Artificial Reefs (ARs) are popular tools to improve marine resources. High catch rates and huge numbers of fish associated with ARs give the impression that fish stocks have been greatly improved. Many fishery scientists however challenge this view and consider ARs to be a waste of money, which do little if anything to enhance fishery resources. Some go further and theorise that ARs actually do more damage than good by aggregating fish, which makes them easier to catch, thus exacerbating over-fishing. These arguments are here examined and applied to the Hong Kong fishery and considered alongside the artificial reef programme and the Consultants' recommendations for further deployment.
Background
Hong Kong's fisheries are heavily exploited. The Fisheries Resources and Fishing Operations Study recently completed by ERM-HK Ltd. has confirmed that Hong Kong's fisheries are over-fished. As one of the measures to restore local fisheries the Hong Kong Government of the SAR has provided HK$100 million to finance a five-year artificial reef (AR) programme which will enhance fisheries resources and promote biodiversity in Hong Kong's marine environment.
As part of the first phase, ARs are being deployed in the existing marine parks. Twenty vessels have been prepared and sunk in Hoi Ha Wan and Yan Chau Tong Marine Parks during 1998. Between July and September 1998 some 200 tyre-AR units were deployed at eight sites in Hoi Ha Wan and Yan Chau Tong Marine Parks. Each tyre unit consists of 42 tyres strapped together to form a pyramidal-shaped structure, nearly 3 metres high and 20 m3 in volume. Plans are also in hand to deploy concrete and quarry rock AR units at Yan Chau Tong and Sha Chau and Lung Kwu Chau Marine Parks and the Chek Lap Kok Marine Exclusion Zone Airport during 1999. AFD has brokered a no fishing agreement with the permitted fishers on the ARs currently deployed in the marine parks. This agreement was made possible by deploying the ARs in habitat limited areas of open mud, away from the traditional fishing grounds of the small-scale fishers.
The second phase of AR deployment will take place outside the existing marine parks. A one-year AR consultancy study, conducted by ERM-HK Ltd., is nearing completion. As part of this study extensive liaison has taken place with the community in two stages to identify, by consensus, sites for future deployment.
Attraction-Production Issue
Over a decade ago fishery scientists challenged the notion that ARs benefit fish stocks, They questioned whether ARs actually produced more fish or merely aggregated them. This has become known as the aggregation-production question. Fishery scientists Bohnsack and Sutherland (1985) reasoned that if fishing mortality has reduced stocks to levels below the optimum that natural reef habitat would support, then, with stocks below the carrying capacity, the amount of hard-bottom habitat could not be limiting. If reef fish and invertebrate populations are recruitment-limited then ARs are unlikely to enhance productivity The theory of the 'recruitment limiting' argument, applied to Hong Kong, is that since fishing pressure has decreased reef fish populations there is plenty of habitat for the reduced number of recruits to colonise. Consequently there is no point in constructing ARs as the habitat is not in short supply.
The 'aggregation-production' and 'recruitment limiting' arguments in heavily fished areas are only of concern if ARs are also over-exploited. If there is no fishing on ARs then any aggregated fish would be protected resulting in stock enhancement. In my opinion ARs initially aggregate fish but in time, they support a large biomass of invertebrates and associated 'bait' fish, which in turn support a large numbers of commercial predatory fish. Determining just how many fish are aggregated' and how many fish are 'produced' is extremely difficult and quite frankly a waste of time and resources. Provided ARs are protected from over-fishing it doesn't matter whether fish are 'aggregated' or 'produced'. Managed ARs will serve as fish sanctuaries supporting balanced fish populations.
Marine Protected Areas
Many fishery scientists will argue that improvement in fish stocks should be achieved by establishment of marine protected areas (MPAs) on natural rocky shore areas rather than through AR programmes. Is this a practical solution? In Hong Kong MPA programmes are already in progress with the establishment of three marine parks and one marine reserve. I hasten to add that fishing is still permitted in these marine parks for dependent fishers. There are 20,000 fishermen based in Hong Kong and more than 700 permits have been issued to dependent fishers for Yan Chau Tong and Hoi Ha Wan Marine Parks. The fishermen's ex-gratia payment issues for the extremely small Cape d'Aguilar Marine Reserve, where fishing is prohibited, are still unresolved. The area occupied by Cape d'Aguilar is a small fraction of 1% of Hong Kong waters. To have measurable impacts on Hong Kong fisheries it is obvious that significant areas of Hong Kong waters need to be protected.
A carefully planned AR programme has the potential to overcome many of the problems, which beset the establishment of no fishing areas, or 'no-take' areas in traditional small-scale fishing grounds. The advantages of linking an AR programme with MPAs and managing them as fish sanctuaries were detailed in Wilson and Cook (1998). ARs can be deployed in areas of open water away from the heavily fished natural rocky shores. These ARs will form substitute rocky shore fisheries in areas with limited hard-bottom habitat. The physical presence of ARs will also prevent bottom trawling which is a non-selective method of fishing. In the long run trawlers will also benefit from AR deployment since catches in waters adjacent to AR areas will be enhanced.
AR Recommendations
The AR consultancy study has proposed five potential areas, totalling some 10% of Hong Kong waters, for second phase deployment. It is recommended that these areas should be designated as 'Marine Special Areas and managed under the Fisheries Protection Ordinance, (Chapter 171). The study has also recommended that most of the ARs deployed in Hong Kong should be protected from high fishing pressure and located in 'no-take' zones where fishing will not be permitted. The key factor is effective management of ARs deployed.
Colonisation: the results so far
Even during the short period ARs have been established in Hoi Ha Wan and Yan Chau Tong Marine Parks there is ample evidence that they are beginning to achieve their objectives. Juveniles of many high value reef fish have already begun to establish impressive populations. In addition a few sizeable grouper adults have also taken up residence on several of the ARs.
Between June and July large numbers of John's Snapper (Lutjanus johnii), colonised many of the vessel ARs. This is a highly commercial and valuable fish, which used to be common in local trawl catches. During the seventies juveniles were widely caught locally and reared in Hong Kong mariculture zones. Today adults are rare and fry so scarce that they are no longer collected for the mariculture. The ARs now support and protect these fish from over-exploitation. It will be interesting to obtain data from these fish at maturity, when spawning events occur.
The Red Snapper (Lutjanus malabarius) is another high value fish, which is rarely encounter in local waters. In the late seventies trawl catches of this snapper used to outweigh the catches of all the other snappers added together. A small number of juveniles of this fish have colonised several of the boat ARs and they are growing rapidly. The ARs are also supporting thousands of bream. This year's fry have grown so rapidly that many now exceed 15 cm. SCUBA surveys, using underwater video and camera, have catalogued more high value juvenile fish such as Yellow Grouper (Epinephelus awoara), Brown-striped Red Snapper (Lutjanus vitta), Pinjalo Snapper (Pinjalo pinjalo), Three-banded Sweetlip (Plectorhynchus cinctus), Painted Sweetlip (Diagramma pictum), Red Pargo (Pagrus major), Gold-lined Seabream (Rhabdosargus sarba), Black Seabream (Acanthopagrus schlegeli) and Yellow-finned Seabream (Acanthopagrus latus). Also, adult Mangrove Snapper (Lutjanus argentimaculatus) and Chicken Grunt (Parapristipoma trilineatus).
The early indications are that the ARs deployed in Hong Kong are capable of supporting large numbers of high value fish. Two contracts have recently been awarded, which will monitor the progress and development of macroinvertebrates and fish populations. These monitoring programmes will catalogue the colonisation progress of ARs and quantitatively determine their ability to support a high biomass of reef fish.
It may be non-scientific, but it is my observation, after completing many hundreds of SCUBA dives in Hong Kong waters on ARs, rocky shores and wrecks, that ARs are much more attractive to commercial reef fish than natural reef areas.
Multi-species Ecosystem Modelling
Development of Hong Kong's AR proposals has involved the use of multi-species ecosystem models developed and applied by the Fisheries Unit of the University of British Colombia. These models have assessed the cost benefits of the proposed AR programme. The models indicate that the AR proposals will reverse the current trend of deterioration in the value of the Hong Kong fishery. Even after consideration of the capital cost of construction and deployment, and the full cost of management, there will be a predicted increase of about 28% in the value of the fishery after 15 years and 52% after 30 years. The models predict that as long as ARs are well managed they are an effective tool for rebuilding damaged fisheries.
References
Bohnsack, J., and D. Sutherland, 1985. Artificial reef research a review with recommendations for future priorities. Bull. Mar. Sci. 37: 11-39.
Wilson, K.D.P. and D.C. Cook, 1998. Artificial reef development: a marine protected area approach. Marine Biology of the South China Sea III (ed. B. Morton). Proceedings of the Third international Conference on the Marine Biology of the South China Sea, Hong Kong 1996. Hong Kong. Hong Kong University Press, pp. 529-539.
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