Artificial Rocky Reefs
Banner Photo Credit and Description: Eiko Jones; Black Rockfish, Sebastes melanops, Quatsino Narrows
The following section gives an overview of design, management, and monitoring recommendations for artificial rocky reef projects.
Rocky reefs are ecologically and economically valuable habitats for plant, invertebrate, and fish species, many of which contribute to commercial, recreational, and First Nations fisheries. Natural reef formations can occur in all water depths, and can extend through the water surface or terminate in subtidal waters. Rocky habitats (piled rocks, boulders, and rock cliffs) thus can be influenced by benthic, deep, mid-water, intertidal, and surface community dynamics. They provide ecologically valuable foraging, refuge, spawning, recruitment, and rearing habitat, and tend to be areas of high species abundance and diversity in the shore zone.
Artificial rocky reefs (ARs) are often constructed to compensate for rocky habitats that are lost, degraded, or destroyed through human activities such as coastal infrastructure development. The goal of AR habitat development is for marine community productivity to equal or exceed productivity levels of the habitat that was destroyed or displaced through human activities.
Productivity is Maximized When Artificial Reefs
- Facilitate algal and invertebrate colonization and thus food provision for higher trophic levels;
- Provide critical habitat for larval and juvenile recruitment and settlement; and
- Are effectively designed with respect to long-term structural integrity and managed with respect to harvesting pressures that may increase around the reef.
This section discusses critical design and management considerations for ARs, focusing on site selection, module design, and monitoring strategies.
Aggregation versus Productivity
Habitat availability is often the limiting factor for reef fish and crustacean abundance, and can be a demographic bottleneck to survival. ARs that are strategically positioned and that provide structures that natural reefs lack, such as body-sized crevices and holes, can potentially increase a region’s environmental carrying capacity and productivity levels. Achieving a net increase in secondary biomass production (biomass generated by animals instead of plants) requires increased foraging opportunities provided by benthic organism colonization (invertebrates and algae), and increased recruitment and survival of juvenile and adult life stages. ARs contribute to this by providing critical habitat for benthic and juvenile organisms. ARs can also increase net productivity by supporting communities (invertebrates, macrophytes, epiphytes, algae) that recycle nutrients, which otherwise would be trapped in sediments, effectively shortening the food web.
If, however, the reef is improperly sited, designed and/or managed, it may only attract adult individuals from surrounding natural habitats, who then experience disproportionate harvesting pressures by recreational or commercial fishermen. This is a dangerous instance where aggregation may be mistaken for increased productivity. Accompanying AR implementation with population monitoring and fishing regulation will help reduce overharvesting pressure (Fig. 12).
Artificial Reef Site Selection Considerations
Biological Processes: Target Species’ Habitat Use and Spatial and Temporal Distributions
Of paramount importance to an AR’s success at increasing productive capacity is identifying the target species and age-groups, and integrating their respective habitat requirements into the habitat design. These considerations include recruitment, spawning and colonization behaviour, habitat preference, food web interactions, and life-history stage survival. For example, juvenile rockfishes tend to settle in habitats with the greatest biogenic structural complexity and highest refuge potential – that is, sites with a high abundance of macroalgae, body-sized crevices and holes. So long as body-sized crevices were supplied, juvenile rockfishes were found to remain on ARs throughout the winter, even when macrophyte density declined. They did not exhibit this behaviour on natural reefs that lacked crevices.
When surveying potential sites for AR construction, understand and integrate temporal and spatial fish distribution patterns, such as diel and seasonal fish migration. Seasonal abundance of supporting species, such as algae, are also important to consider. Macroalgae presence influences fish abundance according to an ontogenic trend: juvenile rockfishes and greenlings seek refuge and prey in macrophytes, which are at peak density in the summer. Such trends influence migration and fish home ranges during each season. It is of critical importance to understand that seasonal and behavioural changes may not be conserved across sites when conducting and interpreting pre-construction surveys, since variability is high among local populations.
Placement within the Local Environment
Site placement within the local environment requires careful consideration and an acute understanding of the local physical and ecological context (see Chapter 2), and relevant human uses and practices. For example, Puget Sound is shallow and sandy with relatively few rocky reefs, and high accessibility to recreational boating and fishing. Just north, the Strait of Georgia has extensive deep rocky ridges and reefs. The reef depth and exposure make fishing more dangerous, therefore fishing activities tend to become more focused. Urban ARs in the Strait of Georgia can constrain public access to concentrated fish populations by constructing them in commercial hubs, such as the ARs in Roberts Bank.
Another important consideration is understanding the long-term effects of ARs on the surrounding biota, natural habitat, and oceanic conditions. Species that move from natural to artificial reef habitats will compromise the original natural reef’s community structure. If population surveys do not account for this trend, and if fishing pressures are inadequately monitored, the population may be overharvested, resulting in significant loss in economic returns and biodiversity.
The principle design parameter involved in the interaction between habitats is distance between the AR and surrounding natural habitats, since this determines colonization rates for algae and invertebrates. Placing ARs close to natural reefs can effectively extend the natural habitat and will facilitate juvenile recruitment. However, the Washington DFW recommends that ARs are placed at least 180 m away from other rock-habitats in order to reduce the probability of aquatic invasive species infestations. Spacing them apart in this way will also promote the formation of independent populations.
In order to maintain the dynamic between reef and benthic or infaunal species, an appropriate reef-to-bottom surface area ratio must consider community composition and benthic foraging rates. For example, the Washington DFW prohibits AR material from covering more than 50 percent of the natural substrate within their designated area, and individual rock piles from covering more than 10 percent of the total permitted area. Note that the designation and permitting process in Washington differs from BC, as do recommended ratios. This maintains the survival of benthic and infaunal communities, which can be important components to reef community food web dynamics. Each project’s unique site characteristics and AR purpose will contribute to the determination of the appropriate distance from surrounding natural reef and habitat features and reef-to-bottom surface area ratio.
Physical Processes and their Biological Implications
The primary physical considerations for AR site selection include available water depths, water quality considerations, ocean current and wave exposure, seabed geomorphology, substrate characteristics and local sediment dynamics. Site depth determines the extent of colonization by algal and invertebrate communities, the early successional species in reef community recruitment and structure. Algae and invertebrates depend upon light penetration for growth and reproduction; therefore water depth directly impacts colonization success as the available light decreases with increasing water depth. An additional consideration for post-construction monitoring purposes is the level of diver accessibility. Many ARs are placed between water depths of 12 and 30 m.
The site water quality must also be within acceptable limits for the desired species. Water quality parameters of concern include salinity, temperature, dissolved oxygen, pH, nutrients and suspended sediment concentrations.
Ocean currents and surface wave action impact the delivery of nutrients, respiratory gases, and food particles to benthic habitats, affect the transport and settling of larval populations, facilitate the dispersal of waste materials, and act to prevent the burial of egg masses through sedimentation. If ocean currents or wave motions are too strong, feeding activities may cease and larval settlement may be inhibited. During extreme storm events, detachment of sessile species and anchored macrophytes from the underlying substrate may occur. In areas where forage fish utilize strong current flows as migratory pathways, AR placement should consider the potential for increased prey exposure for the target reef species in regions of high current flow.
The sediment dynamics at the reef site are a function of the sediment supply, sediment characteristics, wave and current conditions and the site geomorphology. If suspended sediment concentrations at the proposed AR site are high, light penetration will be reduced and reef colonization may be inhibited.
Smothering of the reef structure and reduction in substrate suitability for sessile species may also occur if sedimentation is ongoing and the wave and current regimes are insufficient for sediment resuspension.
It should be noted that the placement of an artificial reef structure will alter the ambient hydrodynamic and sediment transport processes, with consequences that may extend beyond the limits of the reef to the adjacent areas. The interactions between currents, waves, sediments and reef morphology should be assessed as part of the AR design process
Artificial Reef Module Considerations
The main goal of AR projects is to provide reef habitat features that meet the life history requirements of target species, and to maintain the structure’s integrity over the design life of the AR (Fig. 13). ARs constructed along British Columbia’s shore zone must be suitable for algal and invertebrate colonization, recruitment and survival of juvenile rockfish and greenlings, and foraging and spawning for sub-adult and adult rockfish and greenlings.
Many sub-adult and adult reef and pelagic fish are attracted to high profile reef substrates: vertical or near-vertical structures. Fish use areas of high vertical relief for refuge, resting, or foraging. High relief reefs are more effective than low-profile reefs in deep water (> 100m), and are where adult rockfishes tend to aggregate. Juveniles tend to occupy shallower water (< 15 m), where low-relief high-complexity habitats with a large number of small-sized crevices support their survival needs.
In order to provide shelter for juveniles and shellfish, ARs must contain crevices and internal spaces that correlate with the size of animal that seeks protection. Juveniles seek refuge in crevices with openings that are directly proportional to their body sizes. If only large cavities are present, large piscivorous fishes will congregate and increase the predation pressure on juveniles. Along with size, the number of chamber openings is important: compartments with more than one exit are preferable to those with a single exit since they provide more light, water flow, and escape opportunity.
Designing complex ARs that provide small holes for juveniles will significantly help local population growth, since the greatest mortality of reef fishes occurs within the first two weeks of juvenile settlement and recruitment. ARs that have a highly variable surface layer with a high proportion of small holes will sustain a higher carrying capacity for the number of juvenile fishes that can find shelter from predation.
Available evidence indicates that heterogeneous substrate composition and sizing provides biologically valuable habitat and refuge for soft-bodied animals through the provision of a variety of crevice sizes.Substrate size and texture also influence the primary colonizers (benthic invertebrates and algae), who’s biomass tends to increase on uneven surfaces. Although rough surfaces are favoured by primary colonizers, soft-bodied animals such as fishes do not appear to favour sharp edges. The desired surface roughness characteristics, angularity of the structural materials, and crevice size distributions for the AR should be identified during the development of the project basis of design document (see Chapter 3).
It has been suggested that a favourable configuration may consist of large boulders placed on top of smaller boulders and cobbles (e.g. Fig.14). This arrangement creates caves for larger species, such as lingcod, to use as refuge. However, it should be noted that such a configuration may not provide long-term structural stability. The sizes of materials used as structural elements is determined in the engineering design process through analysis of site-specific factors including seabed morphology, hydrodynamic loading, acceptable risk of failure, foundation conditions, etc. The inclusion of non-structural elements in the AR design is one option for providing the desired habitat diversity; other options should be explored during the conceptual design phase of project development (Chapter 3).
Material selection is a key component of the engineering design process and typically includes considerations such as material strength, durability, suitability for use in the marine environment, constructability and cost. For AR projects, additional parameters related to habitat value such as geochemical influences on primary colonizers should be identified and added to the project design criteria