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Acanthurus olivaceusgrazing on algae on new reef material: demonstrates artificial reef contribution to food chain
Acanthurus olivaceusgrazing on algae on new reef material: demonstrates artificial reef contribution to food chain
Courtesy of Dr. James P. McVey/NOAA Sea Grant Program

Background

Artificial reefs are structures placed on the sea bottom to gather fish. They are either used alone or with Fish Aggregating Devices (FADs). Most artificial reefs are large, permanent structures set in rather shallow water. Generally they are made of modern synthetics and hardware (concrete, metal, plastic pipes) which are sometimes finished using local vegetal materials (bamboo, coconut, leaves, coir for the anchor rope, etc.). In some cases lighter structures made from local materials are installed temporarily to lure fish to a specific area, during a certain fishing season.

Artificial reefs are established for various purposes:

  • to enhance resources in coastal waters in order to facilitate exploitation;
  • to create a biological reserve; and,
  • to prevent the use of certain fishing gear, particularly bottom trawls, in a given area.

Construction and maintenance

Different structures can be used as artificial reefs, such as wrecks, offshore oil rigs and pipelines on the sea bottom, heaps of oyster shells, etc. as well as specifically designed modules.

Constructions covering a large area can be composed of individual modules, of which there are many models depending on the purpose.

For instance, to prevent trawling, a model might consist only of slabs of concrete with stakes. To enhance resources, that is to provide shelters for certain fishes and fish aggregation in general, modules should include holes (big enough to avoid being quickly sealed by marine organisms) where the material can facilitate the fixing of "fouling" organisms as food for aggregating fish.

The overall dimensions of the reef and numbers and sizes of the holes for shelter are critical factors, as well as waves around it (modules should not bury themselves in bottom sediment). The modules should be bulky enough to project from the ocean bottom and massive enough to remain in place (even during severe storms); it might be suitable to anchor the modules of the artificial reef, particularly in shallow waters.

Material should resist rapid corrosion and should not introduce harmful substances into the marine environment. Various elements can be used to create artificial reefs, including:

  • scrap material (car, old ships and barges, scrap concrete, household appliances, rock and rubble from excavations, etc.);
  • baled urban solid waste;
  • products such as sandbags and bamboo frames to construct temporary "reefs";
  • large concrete blocks, elements made from FRP and PVC; and,
  • offshore structures for oil or gas exploitation can also serve as artificial reefs and fish aggregating devices.

Building costs and installation factors (a very large number of artificial reefs must be set up) are also considerations in the choice of modules used. In shallower waters, setting FADs in conjunction with an artificial reef has proven profitable in several places. It has not been possible to identify a single model which could be universally recommended.

Aggregation process

An artificial reef plays the role of an offshore bank and protected area or park. The process of aggregation is often made in several steps: concretizing the blocks, then aggregating small fish and finally attracting larger fish.

The first few fish species arrive and eat organisms which have fixed on the artificial reefs which act as a food reservoir for demersal fish. Then, other species, such as predators of the former come. For certain species (rock fish, octopus, crustaceans) artificial reefs are shelters; for small fish/juveniles, reefs provide protection from predators. Artificial reefs are also spawning areas for certain species (e.g. cephalopods) which explains how artificial reefs can enhance resources. In shallow waters shoals of small pelagic fish have been observed to use artificial reefs as stopping grounds.

A typical school of the sparid, Montaxis grandoculis (Mu), circling an artificial reef. This fish primarily eats molluscs
A typical school of the sparid, Montaxis grandoculis (Mu), circling an artificial reef. This fish primarily eats molluscs
Courtesy of Dr. James P. McVey/NOAA Sea Grant Program

Impact of artificial reefs

Enhancement and recruitment

It takes weeks for a new artificial reef to attract fish and for the establishment of fish colonies/populations. Artificial reefs mainly concentrate on the remaining resource and the potential enhancement effect is, in general, considered quite low. In terms of habitat rehabilitation, artificial reefs have little, if any, success as they only concern a limited area.

It is worth observing that the assessment of some possible habitat rehabilitation and production enhancement would require a careful survey over a long period with regular data collection and careful monitoring of all fishing activities and in-depth knowledge of local ecosystems and ecological mechanisms. In most cases it is very difficult to evaluate fishing activity on artificial reefs. Therefore, regarding the potential socio-economic impact, there is a lack of thorough economic analyses of the costs and benefits of artificial reefs.

In general, the establishment of artificial reefs in coastal areas can facilitate smaller scale fishing activities with passive gears such as lines, pots and gillnets. In some places, recreational fishers also benefit from the creation of artificial reefs.

Biological

Regarding the biological impact of artificial reefs, two theories exist concerning the number of fish and fish species using reefs:

  • competition for limited resources, such as food and shelter is the chief factor; and,
  • fish mortality and recruitment are essential. The potential advantage from reefs could be a reduction in mortality (for instance by providing refuge from predators for small fish/juveniles) and/or recruitment stability.

Fish behaviour on artificial reefs differs from one species to another. This fact determines the different aggregating ability of the artificial reef for various species.

Legal Aspects and Regulation

Legal aspects related to artificial reefs include:

  • ownership of the artificial reef;
  • ownership of the fish on it;
  • limitation/prevention of certain fishing gear and therefore certain fisheries, resource utilization and management; and,
  • shipping and navigation.

A number of countries have set up strict regulations concerning the position of the artificial reefs, design and material used, proper marking, issue of a government authorization, amount of fishing gear authorized, regular reporting of fishing activities on artificial reefs, etc.

Any national regulation must take into consideration the provisions of UNCLOS (United Nations Convention on the Law of the Sea), conventions on navigational safety, shipping routes, conventions on dumping at sea, and decisions by fisheries management bodies.

When offshore structures for oil or gas exploitation act as a permanent fish aggregating device or artificial reef, they are covered by international standards and guidelines developed by the International Maritime Organization (IMO).

Conclusion

The creation of artificial reefs has changed the perception of fishery resources from a primarily competitive hunting attitude to a collaborative rearing and nursing one.

In addition to direct impact of local fisheries, performances and revenues, artificial reefs have side-impacts on other fisheries and related activities in terms of catches and incomes.

Where there is heavy or overexploitation, deployment of artificial reefs appears to become an alternative to more traditional development efforts of artisanal fisheries for effective near-shore fishery resources management. It is worth mentioning that the settlement of such structures introduces important changes in the management of fishing operations: e.g. fishing rights, working time allocation, relationship among fisher groups and between small and larger scale fisheries.

 
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