Synonyms

Artificial habitats; Artificial reef structures; Man-made habitats; Man-made reefs

Definition

“An artificial reef is one or more objects of natural or human origin deployed purposefully on the seafloor to influence physical, biological, or socioeconomic processes related to living marine resources” (Seaman and Jensen, 2000, 5). While this definition is directed toward the marine environment, a few modifications to include “estuarine and freshwater” resources make it applicable to the estuarine environment as well. Also, it should be noted that artificial reefs are not always “deployed purposefully” as “accidental” deployments of ships or other objects sunk by storms, acts of war, or other episodic events can also create submerged structure that function under the banner of artificial reefs. Additionally, a broader appreciation of the utility of artificial reefs would certainly make them useful to disciplines which are not mutually exclusive. Thus, artificial reefs can simultaneously influence physical, biological, and/or socioeconomic processes.

Introduction

Artificial reefs have likely been deployed for the purposes of enhancing fishing for millennia. Most probably, fish associated with naturally occurring debris in rivers and lakes were recognized early in human history as functioning as preferred structure for some fish species. It would have been intuitive to help Mother Nature along by deploying similar-looking, natural structures in aquatic habitats to further enhance fishing. The modern impetus for artificial reef seems to have come from the congregations of reef fishes observed around sunken ships and downed warplanes resulting from sea battles during the World War II in the Pacific Ocean. For example, Chuuk (Truk) Lagoon in Micronesia, where 32 merchant ships and 249 aircraft were sunk, has become a diver’s “paradise” since the early 1970s (Trumbull, 1972). The advent and popular use of scuba by the general populace since the late 1950s helped the general recognition that artificial reefs were a “good thing” when it came to fisheries, but there were few data and studies directed toward truly establishing the verifiable reasons for their deployment. Of late, artificial reefs have garnered attention by natural resource managers, aquatic scientists, and the fishing public to improve fishing and fisheries around the world. The popular mantra is that “more reefs will mean more fish” without regard for the effects of these structures on other natural processes. Lastly, the “proof” of artificial reefs as an effective management tool is wanting, largely owing to the lack of scientifically valid opportunities to test various hypotheses.

Kinds of artificial reefs

Artificial reefs are generally of two basic types: benthic (i.e., located on the majority of mid-water/surface Fig. 1). The majority of mid-water and surface reefs serves as fish-aggregating device (FADs) and is directed primarily at the pelagic and epipelagic game fishes in coastal areas. Benthic reefs have been the subject of much effort in design and planning. A broad variety of structure have been used in artificial reefs, but generally they can be considered as either structure of opportunity (i.e., made of refuse materials) or designed and engineered specifically to serve as artificial reefs. Structure of opportunity include (but are not limited to) ships, automobiles, and other vehicles (e.g., railroad cars and airplanes), derelict oil and gas platforms, bridge rubble, remnant construction materials (e.g., broken sewer pipe), scrap metal of various composition and gauges, white goods (refuse household appliances such as washing machines, stoves, and refrigerators), vehicle tires, fiberglass materials, porcelain, and any materials considered dense enough to sink and withstand some degree of wave action or current surge.

Artificial Reef, Figure 1
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Examples of various artificial reefs, including FADs or fish-aggregating device deployed chiefly to attract pelagic or epipelagic fish (From Seaman and Sprague, 1991).

Full size image

Engineered structures can run the gamut of all of the above but are generally composed of structures thought to have a longer life span (durability) and greater stability (density) than reefs composed of structures of opportunity. Engineered structures are most often composed of concrete, concrete and rock aggregate, and heavy gauge steel.

Generally, artificial reefs are deployed as modules or units of a size and shape readily transportable and deployable. These modules are then organized into sets of two or more modules, and the sets are often organized into groups. Lastly, the groups of sets and modules comprise the entire reef complex that may be of considerable extent, covering several kilometers (Grove et al., 1991).

Locations of artificial reefs

Artificial reefs have been deployed in virtually every aquatic ecosystem from freshwater streams, rivers, ponds, and lakes to estuaries, fjords, bays, and the open ocean (both near coastal and far offshore). The substrate type upon which the reefs are deployed is an important consideration, as some substrates are easily eroded or scoured, and the reef can become quickly covered so as to become nonfunctional as an artificial reef.

Position of the reef in proximity to other biota is often a deployment consideration to facilitate colonization by juveniles, adults, or prey items. Depending on the intended function of the reef, positioning the reef to facilitate (or prohibit) access by users is often a consideration as well.

Functions of artificial reefs

Artificial reefs can have many functions via the ecosystem services they provide. Often a chief function is to enhance fisheries for fishing opportunities for both the commercial and recreational fishing public. Environmentally, artificial reefs can serve to mitigate damage to natural areas, serve to enhance biotic community diversity, or fulfill other goals of resource managers. Artificial reefs have been used as objects to deter various fishing activities such as the protection of seagrass beds from trawling (Fabi and Spagnolo, 2011). Recreationally, artificial reefs can serve as scuba and snorkeling sites, especially to enhance areas void of “interesting substrate” or as alternative dive sites to protect natural areas from potential damage by divers. Artificial reefs can provide a substrate to allow settling of sessile (attached) organisms such as bivalve mollusks (Relini et al., 1994). Japanese researchers have investigated the deployment of artificial reefs as structures to help divert ocean currents to facilitate upwelling that brings nutrients nearer to the surface to enhance phytoplankton productivity and, in turn, increase coastal fish abundance and growth (Okano et al., 2011).

The complete use of artificial reefs has yet to be fully explored. Interestingly, at least one company (www.eternalreefs.com) makes use of artificial reefs as human burial sites.

Concepts

The general idea behind deploying artificial reefs to enhance fisheries is related to either one or both of the two assumptions long argued by fisheries scientists. These assumptions are that artificial reefs attract fishery resources to a site, or they increase the productivity of fisheries resources. Clearly both concepts are viable and each can have utility in fisheries management. To date, however, both these assumptions are recognized, but little scientific testing has been done to allow proper rejection of either hypothesis in the attraction versus production debate (Bortone, 2006; Bortone, 2011).

Bohnsack (1989) diagrammatically summarized the general perspective of the attraction and production perspectives based on observations and published research. His perspective was that attraction and production were either ends of a continuum with regard to several attributes. In summary, artificial reefs that functioned more for production than attraction had low reef availability and low fishing intensity. They were inhabited by species that were habitat limited (as opposed to recruitment limited), more reef dependent, and, behaviorally, more territorial, demersal, and with high site affinity (philopatry). Polovina (1991) further clarified the attraction/production argument relative to fisheries. He indicated that if artificial reefs merely served to concentrate fish in an area, then the same biomass could be caught with less effort. If the artificial reef attracted fish from other areas, then fishing yield could increase as long as fishing effort increased. Lastly, if artificial reefs increased the carrying capacity of an area, then both total biomass and exploitable biomass should occur.

The presumption by both Bohnsack (1989) and Polovina (1991) is that attraction is the opposite of production. Bortone (2008) proposed that attraction and production were both at play in the response species made to the presence of artificial reef structure. He reasoned that some species could be both attracted to structure and that, for some species, the carry capacity of an area could also allow an increase in biomass as well (e.g., octopus and spiny lobster). Oppositely, some species are neither attracted nor do their populations increase because of the presence of artificial reefs (e.g., species that show no affinity for reefs). Most certainly, a variety of species show varying degrees of attraction to artificial reefs with varying degrees of biomass facilitation because of the reef’s presence.

The conundrum might be resolved if resource managers could determine the features of target species that are enhanced by deploying artificial reefs. This means that some species and their associated fisheries may, indeed, benefit from the deployment of an artificial reef, in terms of both attraction and production, while others may not. This implies that, at least if fisheries enhancement is the goal of an artificial reef deployment, each reef should have an objective directed toward a particular species and its life history feature that can be enhanced because of the reef.

Special features of estuarine artificial reefs

While there has been some attention given to the deployment of artificial reefs in estuaries (e.g., Bortone et al., 1994; Chapman and Clynick, 2006), they have received only passing attention to date, although artificial reefs have been used as mitigation in estuaries (Foster et al., 1994). It should be noted that the principles applied to artificial reefs in other aquatic systems are probably no different when applied to estuaries. Generally, diversity issues are less important in estuaries, and changes in salinity, tidal flow, and turbidity add extra dimensions when considering the results of artificial reef deployments. Their more recent usage in estuaries has been via the deployment of oyster reefs (Coen and Luckenbach, 2000). This is a popular estuarine enhancement activity conducted by many resource managers. The extrapolation of deploying oyster reefs as consideration of an estuarine artificial reef should not be overlooked by artificial reef researchers.

Current investigations

Research on artificial reefs continues, but, as Bortone (2011) warned, unless clear objectives are included in these investigative efforts, the resolution of the attraction versus production argument will remain elusive. Particularly disconcerting is the feature that current research results are unable to answer many of the questions resource managers face. While artificial reefs have long been touted as offering a solution to many fisheries management issues, their lack of specific prescription in management will continue to exclude artificial reefs from the proverbial “managers toolbox” until these and many other issues associated with artificial reefs are programmatically resolved.

Gaps in current knowledge

Bohnsack and Sutherland (1985) and Bortone (2006, 2001) presented arguments for new directions in artificial reef research. Each of these reviews indicated the overriding gap in artificial reef research is the lack of application of artificial reefs as a reliable and predictable option for natural resource managers. Resolution of the attraction/production hypotheses plays a prominent role in resolving this issue. More importantly, however, is the need to determine the life history “bottlenecks” that are likely to be relieved by the deployment of a reef. One example of how the future might look with regard to artificial reefs in fisheries can be seen in the example off South Korea (Kim et al., 2011). In this study, artificial reefs were deployed to enhance spawning success in a bottom-dwelling fish species. The reef deployment, coupled with fishing restrictions, is part of a long-term plan to increase a depleted fish stock. Similar investigations may give purposeful direction to future artificial reef designs and deployments.

Summary

Objects of various materials, shapes, and dimensions have been deployed in nearly every aquatic environment in all parts of the world to serve as artificial reefs. Few artificial reefs have been deployed in estuaries. Nevertheless, oyster shell material, deployed to improve oyster settlement can be considered an artificial reef or at least an artificial substrate that behaves similar to other objects deployed as artificial reefs. Artificial reefs have received considerable attention among aquatic scientists and natural resource managers to facilitate fisheries. However, there remain several issues relative to the attraction versus production nature of artificial reefs that need to be resolved before they become part of a regularly prescribed option for estuarine resource management.

Cross-references