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A glance at the marine aquarium fish trade in Hong Kong
Restocking - an effective measures to restore the depleted fishery stocks in Hong Kong?
The case of the disappearing croaker, the Chinese bahaba, Bahaba taipingensis
Modification of local wire cage traps to reduce injuries to small mammals
Feral dogs and civet mortality on Kau sai Chau, Sai Kung
The changing bird community of Tai Po Kau
Is the Javan Mongoose native and does it matter?

Restocking - an effective measures to restore the depleted fishery stocks in Hong Kong?

by William Cheung Wai Lung

World Wide Fund For Nature Hong Kong

Overfishing is an undeniable fact in Hong Kong waters. Local waters are suffering from growth overfishing, recruitment overfishing, ecosystem overfishing, as well as economic overfishing (ERM, 1998; Pitcher et al., 1998; Sadovy, 1998; Cheung, 2001). In view of the depleted situation, ERM (1998) conducted a territory wide fishery stock assessment in 1996-97 and recommended a series of measures which aimed to restore the depleted fishery stocks in Hong Kong. One of the recommendations that the ERM (1998) ranked as high priority is a restocking programme in local waters1. In this connection, the Agriculture, Fisheries and Conservation Department (AFCD) carried out a series of restocking trials, the latest trial involved releasing 15,000 physically tagged green grouper (Epinephelus coioides) and red snapper (Lutjanus malabaricus) at artificial reef sites in Yan Chau Tong and Hoi Ha Wan Marine Parks in mid-October this year. However, the effectiveness and the risk associated with restocking hatchery-reared marine animals are still highly controversial. This article aims to discuss the validity of considering restocking as a high-priority fishery restoration measure in Hong Kong.

Restocking can be defined as stock released to compensate for depletion of a natural stock (Bannister, 1991). It involves seeding important habitats with captively grown juveniles of high value species and often associated with the use of artificial habitats and substrates such as artificial reefs (ARs) (Oshima, 1984). The ultimate aim of restocking should be a permanent increase in the sustainable population of that species, rather than a "put and take" fisheries where species are re-stocked for the purpose of being caught later (Agriculture, Fisheries and Forestry, Australia, 2001). Therefore, throughout this article, the measure of "success" of any restocking programme is judged by its demonstrated ability to re-establish sustainable populations of targeted species.

Although restocking may be seen as an attractive and easy way out of the overfishing problem and is often highly acceptable to fishing communities, clear-cut quantitative evidence of its success is lacking (Johannes, 2001). Despite modeling studies suggesting that restocking might potentially increase a stock’s yield and rate of recovery during the recovery period for selected species, e.g. Pacific ocean perch (Sebastes alutus) (Polovina, 1990), its effectiveness in terms of marine species, particularly those associated with tropical or sub-tropical reef systems, has not been empirically demonstrated. There have been several published trial restocking studies which provide evidence that stocking cultured marine organisms into coastal areas may result in recruitment of released individuals to fisheries, for instance cod (Gadus morhua) in Norway and Denmark (Svasand et al. 1990), Flounder (Paralichthys olivaceus) in Japan (Kitada et al., 1992), turbot (Scophthalmus maximus) in Spain (Iglesias & Rodriguez-Ojea, 1984), striped mullet (Mugil cephalus) in Hawaii (Leber and Lee, 1997). However, none of the studies conclusively demonstrated the long-term success of restocking (i.e. a permanent increase in the sustainable population of that species).

Indeed, restoration of depleted stocks through restocking is highly uncertain. For example, Liao (1997), in his review of the Taiwan restocking programme in the last two decades, could not conclude that there had been any success for the restocking programme. Instead, he noted that further studies would be required to demonstrate the significance of restocking, particularly regarding the efficiency and effectiveness of the released species. Also, results of a small scale Nassau grouper restocking experiment suggest that there are many unanswered questions as well as many concerns in restocking of species and that restocking should be an option only when simpler management methods for attaining wild stock recovery have failed (Roberts et al., 1995). Moreover, Polovina (1990) concluded that the knowledge gap in the post-release ecology, and the unknown behavioural and biological differences between hatchery-released juvenile and the natural stock, rendered effectiveness of restocking uncertain. The differences include less adaptability of hatchery-released juvenile in feeding and anti-predation at early stage of release, abnormal behaviour and undesirable morphological characters. The impacts of such differences to the survival potential of individuals after release, impacts on wild natural populations as well as how long effects may last, are still entirely unclear (Svasand, 1993).

Even worse, restocking may pose a potential threat to natural genetic diversity and have unknown effects on the ecosystem (Bannister, 1991). Introduction of juveniles from non-native populations may risk disrupting the genetic integrity of local populations through cross breeding2. There is also concern that over-production of some overseas hatchery provides incentive for them to sell/export fingerlings for restocking purpose at low price. Even if the hatchery-reared juveniles are from local native brood stock, the limited size of the brood stock which produces juveniles in a hatchery may limit the genetic variability of the juveniles. Also, the highly controlled environment in a hatchery may favour selection of certain genetic traits. Should such hatchery-produced juveniles be released in large numbers into the wild and breed with natural stock, especially when natural stock has been depleted to a very small size, natural genetic variability of future generations may be reduced. The reduced genetic diversity may result in reduced resilience of the natural stock to

changing environmental conditions. Besides, the effect on the marine ecosystem from a sudden increase in size of selected groups through restocking is also uncertain. In particular when the ecology of the restocked groups and its relation to other groups of the marine ecosystem is unclear. Therefore, relationships between stock enhancement and biodiversity must be clearly defined before mass restocking is conducted (Liao, 1997).

Yet even if the above concerns have been dealt with, restocking does not solve the root cause of stock depletion in local waters, which is overfishing. A basic principle of restocking is that the released juveniles can enhance recruitment of the natural population and facilitate stock recovery. However, restocking is of little use if the cause of the decline in the population is still present (Liao, 1997; Agriculture, Fisheries and Forestry, Australia, 2001). The major cause of stock depletions locally is excessive and unregulated fishing, which has resulted in growth and recruitment overfishing. Currently, fisheries are still largely unregulated, except that there is no fishing in the marine reserve. Even in the limited space of the marine parks (<2% of Hong Kong waters), fishing, including fish fry capture for grow-out in mariculture farms, is still allowed through a permit system. In addition, the highly ineffective fishery monitoring system in local waters renders evaluation of any mitigation measures impossible. Therefore, without proper protection, management and monitoring of the fisheries, the released juveniles are highly likely to end up in the fishermen’s nets before they can grow to a mature size and reproduce.

Restocking can also be costly. Although hatchery-reared juveniles may not be expensive nowadays and the actual release of the juvenile may incur only a small budget, it is an expensive task to carry out the background and precautionary studies necessary to ensure effective and ecologically safe restocking. Such studies may include genetic and ecology studies of the natural and brood stocks, stock assessment and monitoring before and after restocking trials etc. Therefore, it is vital to assess the cost-effectiveness before any large-scale restocking is pursued.

Considering the highly uncertain effectiveness of restocking, its various associated risks, its inability to address the root cause of stock depletions in Hong Kong, the lack of basic information necessary for a proper restocking programme, and the high cost of tax-payer money it may incur, I consider that restocking is not an effective measure to restore the depleted fishery resources in Hong Kong. The government must focus on those measures which address the root causes of local stock depletions more directly and effectively, such as speeding up the implementation of a licensing system to restrict and reduce local fishing effort, and improving the fishery monitoring system. Also, an ecosystem-based approach should be adopted, which restores fishery resources through rehabilitation of the whole marine ecosystem, rather than only particular targeted groups. Such an approach can restore depleted stocks and conserve marine biodiversity more effectively and holistically. It is also recommended that the IUCN/Re-introduction Specialist Group Guidelines for Reintroduction be consulted and applied ( In addition, all fishery management initiatives, including restocking, should be finalized through public consultation and should be scientifically based, while the results of any trials or monitoring studies should be readily available to the public. Restocking is too expensive and risky an experiment for taxpayers’ money.

1. However, a study carried out by the Fisheries Centre, University of British Columbia (Pitcher et al., 1998), the biological basis for the ERM consultation study regarding fishery stock assessment and management aspects, did not recommend restocking to address the overfishing problem!

2. At least one species, the high-fin grouper, Cromileptes altivelis, which was being released during the previous government restocking trails in Hong Kong in 2000, has been brought in from Bali, Indonesia.


Agriculture, Fisheries and Forestry, Australia. (2001). Guidelines for Fish Restocking.

Bannister, R.C.A. (1991). Stock enhancement (Workshop Report) ICES Mar. Sci. Symp. 192: 191-192.

Cheung, W.L. (2001). Changes in Hong Kong’s Capture Fisheries during the 20th Century and Reconstruction of the Marine Ecosystem of Local Inshore Waters in the 1950s. M.Phil. Thesis. The University of Hong Kong, Hong Kong. 186 p.

Environmental Resource Management-Hong Kong Ltd. (1998). Fisheries resources and fishing operations in Hong Kong Waters – Final Report. 7 March 1998. 255 p.

Iglesias, J. and Rodriguez-Ojea. (1994). Fitness of hatchery-reared turbot, Scophthalmus maximus L., for surval in the sea: first year results on feeding, growth and distribution. Aquaculture and Fisheries Management 26(Suppl.1): 179-188.

Kitada, S., Taga, Y. and Kishino, H. (1992). Effectiveness of a stock enhancement program evaluated by a two-stage sampling survey of commercial landings. Can. J. Fish. Aquat. Sci. 49, 1573-1582.

Johannes, R.E. (2001). Editor’s mutterings. SPC Live Reef Fish Information Bulletin no. 8, p.2.

Leber, K.M. and Lee, C.S. (1997). Marine stock-enhancement potential with striped mullet, Mugil cephalus, in Hawaii. Bull. Natl. Res. Inst. Aquacult. Suppl., 3: 117-134.

Liao, I.C. (1997). Status, problems and prospects of stock enhancement in Taiwan. Hydrobiologia 352: 167-180.

Pitcher, T.J., Watson, R., Courtney, A. and Pauly, D. (1998). Assessment of Hong Kong’s inshore fishery resources. Fisheries Centre Research Reports 6(1): 81p.

Polovina, J.J. (1990). Evaluation of hatchery releases of juveniles to enhance rockfish stocks, with application to Pacific Ocean Perch Sebastes alutus. Fishery Bull. U.S. 89: 129-136.

Roberts, C.M., Quinn, N., Tucker, J.W., Jr., Woodward, P.N. (1995). Introduction of hatchery-reared Nassau grouper to a coral reef environment. N. Am. J. Fish. Manage. 15(1): 159-164.

Savasand, T. (1993). Are reared juveniles fit for release into the wild? Presented at the ICES statutory meeting 1993(Session R).

Svasand, T., Jorstad, K.E. and Kristiansen, T.S. (1990). Enhancement studies of coastal cod in western Norway. Part 1. Recruitment of wild and reared cod to a local spawning stock. J. Cons. Int. Explor. Mer. 47, 5-12.





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