4. Siting, design and construction of artificial reefs

4.1. Site evaluation for artificial reef deployment

This chapter describes the environmental and socio?economic aspects to be taken into consideration when selecting a site for artificial reef construction.

From an environmental point of view, the proper location of an artificial reef is essential to optimize its ecological features and can strongly influence the effects expected from its establishment.
Physical and chemical variables as well as ecological features should be taken into account when identifying the location of an artificial reef. Physical and chemical variables include sediment type, depth/bathymetry, currents, waves, sedimentation rate, water turbidity, salinity and nutrients.
The stability of a reef is related to its structural characteristics (i.e. weight, density and modules design) as well as to the sediment type, current intensity, and wave motion. On muddy bottoms, strong currents and wave action can cause sediment movement leading to sinking and scouring, with consequences such as the destruction or displacement of the artificial structures. Waves and currents can also cause sliding, toppling and displacement due to excessive lateral forces, as well as the redistribution of sediments and mud on the horizontal surfaces of the substrates.

This mud can be subsequently removed by current and wave action, with consequential loss of recently settled sessile organisms. Concomitantly, areas characterized by a strong sedimentation (such as the areas close to river mouths and coastal areas with limited water flow) should be avoided. These effects are more severe at shallower depth close to the shore.
Depth and turbidity affect the light penetration into the water, thus influencing the colonization of artificial substrates by algae and other photophylous organisms. This can affect in turn the fish assemblage that will inhabit the reef. Moreover, water temperature is often related to depth, as warm waters tend to stratify above the colder waters, creating a thermocline that can represent a barrier to some organisms.
Nutrient concentration can deeply affect the composition of the community settled on the substrates. In oligotrophic water with a low sedimentation rate, it is well known that the temporal evolution and the structure of the sessile community mainly depend on the gradient of light attenuation, hence on the depth. Hydrozoans, serpulids and bryozoans are usually the main pioneer organisms just after the immersion of artificial structures but, later, algae tend to become dominant. Mussels are usually absent while oysters may constitute a relevant component of the benthic assemblage. In eutrophic waters instead, light is less important. In this case too, pioneer organisms are represented by hydrozoans, serpulids and hydrozoans but, after a short time, the benthic community becomes largely dominated by filter?feeders such as mussels and interstitial organisms associated to them which find a suitable habitat in the mussel byssum (e.g. errant polychaetes and amphipods). The proximity of the deployment site to sources of pollution may lead to an accumulation of contaminants in the organisms inhabiting the artificial reef.
The biological variables to be used to determine the right position of an artificial reef are the following: habitats existing at the reef site and in the surroundings, life history of target species and connectivity.
In general, artificial reefs should not be deployed on rocky substrates, existing coral reefs or inside seagrass meadows, unless the reef is not designed to restore an existing damaged habitat. When an artificial reef is deployed close to hard?bottom habitats or other sensitive habitats, a buffer of sufficient size should be placed around the natural habitat to protect it from unintentional deviations from the planned deployment (Lindberg and Seaman, 2011). The typology of surrounding habitats can affect the benthic community and fish assemblage at the artificial reef in terms of recruitment, composition and abundance.
Usually, the proximity of seagrass meadows and natural reefs is associated with the recruitment rate at the artificial reef by fish and larvae of benthic organisms (Bombace et al., 1994). On the contrary, the level of isolation of artificial reefs has been linked to top?down predator control of the community structure with a higher predation pressure on larger reefs or reefs close to natural reefs with respect to small isolated reefs (Shulman, 1985; Connell, 1998; Belmaker et al., 2005). Hence, it is expected that the same structures will be colonized by different assemblages and at different rates when placed at various distances from similar habitats.
Moreover, it is important to take into account the life history, the role of some environmental physical and chemical parameters in the different life stages, migratory routes and linkages between adults and juveniles of the target species as well as known migratory routes of threatened and protected species (such as cetaceans), especially in the case of restoration and production artificial reefs.
Therefore, pre?deployment assessments should be conducted at the reef site to determine the sediment type, grain size and thickness (which may be used as an indicator of wave action and water movement); water depth; the occurrence, buffer and cover of natural hard substrates and/or seagrass meadow as well as any identified existing critical habitats adjacent to the proposed site; the intensity and direction of currents and waves (including maximum wave heights for extreme weather events such as 1/100 RI storm events). In addition, any valuable information on the biology and ecology of target species should be collected. This information will help refine the reef site selection and identify the most suitable materials and modules to ensure the stability and effectiveness of the reef over time.
Siting an artificial reef must also take into consideration the purposes of the project and the expected end users. Proximity to ports and other facilities is important if an artificial reef is constructed to enhance local artisanal fisheries, recreational fisheries or diving opportunities, as the reef might not be fully used if it is placed too far from safe harbours and mooring sites. On the contrary, if fisheries or diving enhancement are not one of the primary goals of the reef, its distance from land may not be relevant.
Depth and currents should also be considered when the goal of the artificial reef is to create new areas for diving, as high depth and strong currents might make the area not desirable and potentially dangerous for divers.
Finally, in order to avoid conflicts among users, placement decisions regarding artificial reefs should account for already existing or planned activities in the area. These activities include navigation, recreation, fishing, aquaculture as well as those related to MPAs. This consideration is particularly important in the case of large?scale artificial reefs. A useful tool in the initial selection of suitable reef sites is the use of constraints mapping. This is where all known barriers to reef construction are layered and mapped concurrently. These may include (but not be limited to) existing adjacent MPAs, natural reefs and other natural habitats such as seagrass meadows, known critical habitats, designated commercial fishing areas (such as trawl shots), cable exclusion zones, military exclusion zones, water depth and distance from safe harbour and moorings. This will ultimately narrow down the area that remains suitable and help better define and justify areas of greater interest which need further detailed investigation. In general, prior to artificial reef deployment, the different users of the area and potential stakeholders should be adequately informed through direct and indirect consultation about the reef project, and their viewpoints should be considered in the selection of the reef site.