Artificial Reefs and Marine Fishery Reserves as tools in the management of reef fisheries by Yvonne Sadovy
Two approaches to the management of reef fisheries are relatively new: Artificial Reefs (ARs) and Marine Fishery Reserves (MFRs). Hong Kong is using a hybrid of these approaches to manage her local reef fisheries (see last issue of Porcupine!). Both methods are subject to controversy in terms of their effectiveness in the recovery and enhancement of fishery stocks, especially in areas, like Hong Kong, that are already overfished. I briefly outline the theoretical underpinnings to these two management approaches and evaluate supporting scientific evidence in both cases. While the underlying principles differ between the two management measures, both require a firm commitment to controlling fishing effort.
The use of artificial reefs to enhance fishery resources through the provision of additional food and shelter, though popular and appealing, remains controversial. This controversy, running since the mid 1980s, hinges on the question of whether ARs actually enhance fishery production, or simply attract and concentrate fishes from nearby natural reefs, making them easier to catch and thereby exacerbating fishing pressure (Fig. 1). ARs are most likely to enhance fishery resources if these are habitat-limited i.e. there is insufficient substrate for newly settling or passing fishes or invertebrates to shelter and feed. Conversely, ARs are unlikely to enhance resources if these are recruitment-limited i.e. too few young are produced to colonize all naturally available habitat. Reef communities are generally considered to be recruitment-limited, and this is especially likely when the fishery is overexploited.
No studies unequivocally demonstrate enhancement of fish resources by ARs. However, two studies yielded increased production of invertebrates following AR deployment. In both cases the artificial reefs were specially designed, in terms of size and configuration, as shelter for the target species, one a lobster in the Caribbean, the other an octopus in Japan. The reef modules proved to be effective shelters for certain small size classes and increased their survival through particularly vulnerable early life history phases. Such 'custom -built' modules may be essential for effective stock enhancement using ARs but require detailed knowledge of the ecology of the target species; they are also likely to be expensive.
Fig. 1. Problems with artificial reefs that primarily attract fishes: before (above) and after (below) reef deployment. Source: Bohnsack, 1996.
Clearly, the required detailed studies for such tailored AR designs would be onerous to complete in a multi-species reef fishery and it would be particularly difficult to design artificial shelters aimed at enhancing a wide range of very different species. Because of such problems, as well as the paucity of evidence that ARs do enhance fisheries, AR programmes have been down-sized in a number of places pending further evaluation. For example, the AR programme in the Philippines was banned last year due to fears that the tendency to aggregate available fishes increased fishing pressure because fishing effort over ARs could not be controlled.
I believe that the jury is still out over the effectiveness of ARs to enhance production. While I also believe that this will prove a difficult issue to resolve scientifically, such studies must be attempted otherwise we will never know whether ARs are worth the effort and cost. It is possible, for example, that ARs may prove to be a prohibitively expensive approach, suitable only for more valuable species, and effective only when custombuilt. Moreover, if ARs don't enhance production, their application diverts money and effort away from potentially more effective measures, and failure to manage them further increases fishing pressure.
A more promising approach to the management of reef fisheries, and one that has received increasing attention since the early 1990s, is the Marine fishery Reserve, also known as the Marine Protected Area. MFRs are permanent 'no-take' areas where fishing and all other extractive activities are prohibited. MFRs could potentially deal with many fishery problems not effectively addressed by other, more traditional, management measures (such as size restrictions on catches, fear restrictions, etc.). Recent research is promising and suggests that there are both economic and biological benefits to MRFs.
The principle behind these reserves is to maintain a harvest refugium such that the reef fauna within, which are characteristically long-lived and sedentary, will grow bigger and have an opportunity to reproduce and send propagules and emigrants to surrounding unprotected areas to become available to fishers (Fig. 2). Natural community structure may also be maintained. It is important that the areas selected are large enough and crucial that they be protected from fishing. It is also important that MFRs are properly sited to incorporate habitats critical to different life history stages. For example, juveniles may require the shelter of shallow waters around mangroves or seagrass beds, whereas the adults move out to the cover of adjacent rocky or coral reefs. Since this is a common pattern in many reef species, these habitats should all be incorporated into MFRs.
Fig.2. Larval dispersal from MFRs: larvae generated by adults should disperse and recruit to harvested areas. Adults and juveniles may also emigrate from reserve areas into the fishery. Source: Plan Development Team (PDT), 1990.
There are encouraging data which suggest that MFRs not only significantly increase the abundance and size (and hence fecunity since this is size-related in females) of protected fishes and invertebrates but that the fishery close to reserve boundaries improves, compared to more distant unprotected areas, the latter is an important key to demonstrating the success of reserves in fishery management and winning public support. In the Florida Keys National Sanctuary, marked increases in abundance have been noted for spiny lobster and reeffish in protected areas which incorporate a range of critical habitats, There is little evidence, to date, however for significant restocking of fishing grounds from any study. Ongoing studies are evaluating the general effectiveness of MFRs.
Managing reef fisheries is a major challenge and both ARs and MFRs have been used with varying degrees of success, depending on the species and circumstances. The approaches differ, however. MFRs require the protection of a range of natural habitats relevant to the different species and life-history phases that live within the reserve. Ars, on the other hand, are not so constrained. They can be placed away from heavily fished natural rocky and coral reefs in areas where there is likely to be considerably less conflict from user groups. ARs are thus easier to implement that MFRs, but the expense involved in producing sufficiently large and properly configured ARs which enhance, rather than simply aggregate, resources, is likely to be prohibitive. Moreover, ARs are not intentded to ensure that critical natural habitats are protected, nor that natural community structure is maintained.
MFRs represent a promising approach to restoring, is not enhancing, reef fishery resources, and it may well make better economic and biological sense to develop a network of appropriately placed MFRs and do away with the expense and uncertainty of ARs. The penalities for failing to control fishing effort are also likely to be considerably less for MFRs than for ARs; at least with MFRs the fishery would be no worse than before implementation. While the introduction of MFRs woll initially meet public resistance, protected areas are increasingly acknowledged to be one of the few remaining opportunities, like protected areas on land, of maintaining diversity and abundance in an increaseingly impacted world.
Further reading:
Bohnsack, J. A. 1989. Are high densities of fishes at artificial reefs the result of habitiat limitation or behavioral preference? Bulletin of Marine Science 44: 631-645.
Bohnsack, J. A. 1996. Maintenance and recovery of reef fishery productivity. pp. 283-314 In: Reef Fisheries (NVC Polunin and CM Roberts eds.) Chapman and Hall. London
Bohnsack, J. A. 1998. Application of marine reserves to reef fisheries management. Australian Jouranl of Ecology 23: 298-304
PDT. 1990. The potential of marine fishery reserves for reef fish management in the U.S. Southern Atalntic. coastal Resources Division, Contribution No. CRD/89-90/04. National Marine Fisheries Services. USA
Roberts, C. M. and N. V. C. Polunin. 1993. Marine Reserves: simple solutions to managing complex fisheries? Ambio 22 (6): 63-366
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