Background information and definitions
Artificial reefs are man-made structures intentionally introduced into the marine environment and aimed to act similarly to natural reefs. Originally used to improve biological resources, they may also increase other local biodiversity (Bohnsack & Sutherland 1985; Clark & Edwards 1999). Creating an artificial reef in an area can potentially help increase subtidal benthic invertebrate biodiversity by creating additional suitable habitat for them.
Evidence comparing artificial reefs of different typologies (material used and/or 3-D structures) is summarised under “Habitat restoration and creation – Create artificial reefs of different 3-D structure and material used”. When artificial reefs are created as an offset strategy, evidence has been summarised under “Habitat restoration and creation – Offset habitat loss from human activity by restoring or creating habitats elsewhere”.
Bohnsack J.A. & Sutherland D.L. (1985) Artificial reef research: a review with recommendations for future priorities. Bulletin of Marine Science, 37, 11–39.
Clark S. & Edwards A.J. (1999) An evaluation of artificial reef structures as tools for marine habitat rehabilitation in the Maldives. Aquatic Conservation: Marine and Freshwater Ecosystems, 9, 5–21.
Supporting evidence from individual studies
A replicated, site comparison study in 1995 of three artificial and two natural reefs in the Mediterranean Sea, off the coast of northwest Sicily, Italy (Badalamenti et al. 2002) found that artificial reefs developed similar molluscan abundance but not species richness or diversity to natural reefs after three years. Abundance was similar on artificial reefs (41–50 individuals/sample) and natural reefs (abundance: 19–42 individuals/sample). However, molluscan species richness and diversity (as diversity indices) were lower on artificial reefs (4–11 species/sample) compared to natural reefs (10–27 species/sample). Of the 166 species found in total across all reefs, only 29% were found on both artificial and natural reefs. In spring 1995, molluscs were surveyed on three artificial reefs made of concrete created three years earlier and two nearby natural reefs (0.5–4.5 km from the artificial reefs). A total of 28 samples (400 cm2 each) were manually collected at 16–22 m depth (4–8/artificial reef; 4/natural reef). All molluscs were identified and counted.Study and other actions tested
A replicated, site comparison study in 1997–1998 of sandy sediments surrounding two artificial reefs in the Mediterranean Sea, off the coast of Italy (Danovaro et al. 2002) found that the effects of creating artificial reefs on small invertebrate abundance varied with distance to the reefs. Abundance was lower at the artificial reef sites (87–180 individual/10 cm2) compared to nearby natural sites (146–265 individual/10 cm2). However, abundance was higher at sites adjacent to the artificial reefs 2–20 m away (135–332 individual/10 cm2). Authors suggested that the lower abundance at the artificial reef sites was linked with a higher silt-clay:sand ratio and changes in oxygen penetration. In winter 1997/1998 and summer 1998, small invertebrates were surveyed in the sediments surrounding two artificial reefs (groups of pyramids; material unspecified). One was created in 1987, and the other in 1992. Samples were taken with increasing distance from the reef: one at 0 m (artificial reef site), three at 2–20 m (affected adjacent sites), and one at 50 m (unaffected natural site). Sediment samples (3/station) were collected using a core (4.6 cm diameter, 10 cm depth), and invertebrates (37 µm–1 mm) identified and counted.Study and other actions tested
A site comparison study in 1997–1999 of sandy sediments surrounding an artificial reef and at a natural site in the Mediterranean Sea, off the coast of Italy (Fabi et al. 2002) found that invertebrate species richness and abundance tended to be similar in the sediments inside the artificial reef area and directly adjacent to it, but lower than at a nearby natural site. Data were not statistically tested. Total invertebrate species richness was 91–109 species/station inside the reef area, 79–88/station 2–20 m away, 92/station 50 m away, and 96/station at the natural site. Average invertebrate abundance was 930–1,000 individuals/m2 inside the reef area, 750–930/m2 2–20 m away, 1,010/m2 50 m away, and 2,060/m2 at the natural site. An artificial reef made of 29 concrete pyramids was created in 1987. Seasonally in 1997–1999, invertebrates were surveyed in the sediments at 17 stations: six within the reef area, eight 2–20 m from the edge of the reef, two 50 m from the reef, and one at a natural site (2.5 nm away). Invertebrates (>0.5 mm) were sampled using a suction sampler from 1,600 cm2 quadrats (3 quadrats/station/survey), identified and counted.Study and other actions tested
A site comparison study in 1990–1994 of an artificial reef and nearby sandy habitat in Delaware Bay, North Atlantic Ocean, USA (Steimle et al. 2002) found that invertebrate biomass and secondary production were higher on the artificial reef than in adjacent natural sediments over five years. Average invertebrate biomass was higher on the reef (8,000 g/m2) than in the nearby sediments (180 g/m2). Average estimated secondary production (measure of consumers biomass regeneration over time) was also higher from invertebrates growing on the reef (3,990–9,555 kcal/m2/year) compared to invertebrates in the sediments (215–249 kcal/m2/year). This corresponded to an increase in average secondary productivity by a factor of 19–38 on artificial reef habitat compared to natural sandy habitat. An artificial reef made of complex concrete panels was created in 1989 to mitigate the loss of mudflats elsewhere. Twice per summer in 1990–1994, sessile invertebrates (>0.05 mm) growing on the artificial reef and within nearby sediments were identified and their biomass measured. Biomass data were used to estimate annual secondary production.Study and other actions tested
A replicated, site comparison study in 2005 of three reefs in Malaya Bay, Hawai’i, North Pacific Ocean, USA (Fukunaga & Bailey-Brock 2008) found that overall invertebrate abundance was similar at one but lower at a second artificial reef, compared to a natural reef. Average invertebrate abundance was similar at the sunken vessel Sea Tiger (131 individuals/sample) and at the natural reef (115), but lower at the sunken vessel YO257 (47). In addition, polychaete worm (the dominant group at all sites) diversity (reported as a diversity index) and species richness were similar at Sea Tiger (16 species) and the natural reef (13), but were lower at YO257 (8.3). Polychaete community composition was similar between YO257 and the natural site, but significantly different at Sea Tiger thought to be due to the development of seagrass (data presented as graphical analysis and statistical model results). Two vessels were deployed as artificial reefs on sandy seabed 1.5–2 km off the coast at 35–38 m water depth: the YO257 in 1989 (along with some gravels) and the Sea Tiger in 1999. Two transect lines were surveyed at each artificial reef (one on each side), and one at a natural reef located 1.5 km off the coast at 32 m depth. Divers collected six sediment samples/transect by randomly placing corers (7.6 cm diameter, 6 cm depth). Invertebrates (>500 µm) were identified and counted.Study and other actions tested
A replicated, site comparison study in 2005–2006 of nine sites in the Firth (Lynn) of Lorn, west coast of Scotland, UK (Hunter & Sayer 2009) found that abundances of edible crab Cancer pagurus and velvet swimming crab Necora puber were typically higher on artificial than natural reefs, but varied with the complexity of the reefs and the season. For edible crabs, in summer and autumn abundances were similar at artificial and natural reefs and averaged 0–0.05/m2. In winter, abundance was higher at one of two types of artificial reefs (0.13/m2), compared to natural reefs (0.01/m2), but not in the other artificial reef type (0.04/m2). In spring, abundance was not significantly different at artificial and natural reefs and averaged 0.04–0.15/m2. For swimming crabs, in summer abundance was higher at artificial reefs (0.15–0.27/m2) than at natural reefs (0.08/m2). In all other seasons, abundance was higher at one of two types of artificial reefs (0.34–0.45/m2), than natural reefs (0.10–0.14/m2), but not in the other artificial reef type (0.05–0.18/m2). In 2003–2004, an artificial reef complex made of two types of modules (concrete blocks; perforated concrete blocks) was created. Nine sites were surveyed: six with artificial modules and three nearby natural reefs. Monthly in August 2005–June 2006, divers recorded edible and swimming crab abundance along two 9 m2 belt transect/site. Data were grouped by season.Study and other actions tested
A site comparison study in 2004–2009 in two areas off the coast of south Cornwall and Devon, English Channel, UK (Hiscock et al. 2010) found that the invertebrate and algae community found on an artificial reef was different to that of nearby natural reefs five years after its creation. Results were not statistically tested. After five years, 263 taxa were found on the artificial reef. The total number of taxa on natural reefs was not specified. Nine conspicuous species were only found on the artificial reef, and 39 conspicuous species were only found on the natural reefs (definition of “conspicuous” unspecified). The abundance of some species on the artificial reef was reported to be lower compared to natural reefs (see paper for details). An ex-Royal Navy boat was placed on the seabed for recreational purposes in March 2004 at 20 m depth. The occurrence and abundance of invertebrates and algae were recorded by divers opportunistically in 2004–2009 (approximately monthly in the first 18 months and then approximately every 10 weeks). Divers also took photographs. The invertebrate and algae community present end of summer 2008 was compared to that of nearby natural bedrock reefs previously surveyed (number of sites unspecified).Study and other actions tested
A replicated, site comparison study in 2006 of two artificial and two natural reefs in the Faro/Ancão reef system, off the southern coast of Portugal, North Atlantic Ocean (Carvalho et al. 2013) found that artificial reefs developed similar invertebrate abundance and biomass, but not similar invertebrate species richness, diversity and community composition to natural reefs after 16 years. Invertebrate abundance and biomass were similar on artificial reefs (abundance: 17,111–52,933 individual/m2; biomass: 18–40 g/m2) and natural reefs (abundance: 16,400–25,644 individual/m2; biomass: 27–262 g/m2). However, species diversity (as diversity index) and richness were lower on artificial reefs (162 species) compared to natural reefs (218 species). Invertebrate community composition was different on artificial reefs compared to natural reefs (data presented as graphical analyses and statistical model results). In August 2006, two artificial reefs made of concrete created in 1990 and two natural reefs (0.5–0.9 km away from the artificial reefs) were surveyed. Three 15 x 15 cm quadrats were placed at each reef on vertical surfaces 1 m from the seabed, scraped, and organisms collected. Invertebrates (>0.5 mm) and algae were identified, counted, and dry-weighed.Study and other actions tested
A replicated study in 1999-2004 of one artificial reef off southern California, North Pacific Ocean, USA (Schroeter et al. 2015) found that from one to five years after artificial reef modules were deployed there were changes in community composition, and an increase in species richness (by 49%) and abundance (by 86%) of invertebrates growing on the reef modules (sessile). Over time, artificial reef modules had increased invertebrate species richness (2000: 4 species/m2, 2004: 7 species/m2) and abundance (2000: 47% cover, 2004: 70%). The artificial reef was created to compensate for the loss of giant kelp forest. Low lying (<1 m tall) artificial reef modules (40 x 40 m) made of either granite boulders or concrete rubble were deployed in seven sites in 1999 (8 modules/site) at 13–16 m depth. Sessile invertebrate communities were sampled in summer one and five years after deployment. Invertebrate abundance was assessed for 42 of the 56 modules using six 1 m2 quadrats/modules.Study and other actions tested
A replicated, site comparison study in 2008–2009 of six sites in northwest Mobile Bay, Gulf of Mexico, Alabama, USA (Scyphers et al. 2015) found that artificial breakwaters had more small mobile animal species (invertebrates and fish combined, referred to as “nekton”), but similar overall nekton abundance compared to adjacent mudflats 1.5 years after deployment. Artificial breakwaters had more species of nekton (2.2–2.3 species/m2) compared to adjacent mudflats (1.3 species/m2). However, breakwaters did not have statistically higher nekton abundance (0.5 individual/m2) compared to mudflats (0.1 individual/m2). Four artificial breakwaters made of either bagged oyster shells or concrete domes, acting as artificial reefs, were deployed in May 2008 along an eroding shoreline in Mobile Bay (60 m from, and parallel to the shore; 0.75 m depth). Between May 2008 and November 2009, nekton was surveyed at each breakwater and at two adjacent natural mudflats. During each survey, a bag seine (6.25 mm mesh) was deployed over 12.5 m on each side of the breakwaters and twice in the mudflats. All individuals were identified and counted.Study and other actions tested
A study in 2009–2013 of two artificial reefs in the southern Bay of Biscay, North Atlantic Ocean, France (Castège et al. 2016) found that artificial reefs hosted at least five to seven species of large mobile invertebrates. Five species were recorded in Porto artificial reef, with two recorded during >75% of dives (edible crabs Cancer pagurus; velvet crabs Necora puber). Seven species were recorded in Capbreton artificial reef, with one recorded during >75% of dives (the common octopus Octopus vulgaris) and two recorded during 50–75% of dives (the common prawn Palaemon serratus; the velvet crab). Other large mobile invertebrate species recorded in lower frequencies included European spider crabs Maja brachydactyla, hermit crabs Pagurus bernhardus, and common cuttlefish Sepia officinallis. Porto artificial reef was created in 1994 and made more complex over time until 2004. Capbreton artificial reef was created in 1999 and made more complex in 2010. Both reefs were made of barges, concrete modules and pipes, and were located on sandy seabed at 12–25 m depth, 84 km and 20 km away from the nearest rocky shore, respectively. Anchoring, diving, and all types of fishing were prohibited. Annually between 2009–2012 (Porto) and 2010–2013 (Capbreton), 2–4 stations/artificial reef were surveyed during 2–5 dives/station. During each dive, two divers visually recorded and counted the number of large mobile invertebrate species in a 2 m radius circle for 3 min. Frequency of occurrence was calculated for each species as: (number of dives in which the species was counted/total number of dives) Х 100.Study and other actions tested
A replicated, site comparison study in 2012–2013 of 29 sites in three areas of Shandong province, Yellow Sea, China (Sun et al. 2017) found that creating artificial reefs had mixed effects on the abundances and sizes of mobile invertebrates. Of 17 species found at both artificial reefs and natural sites with no artificial reefs, abundances tended to be higher at artificial reef sites compared to natural sites for 10 species, lower for six, and unrecorded for one (see original paper for details). Individual sizes tended to be higher at artificial reef sites compared to natural sites for seven species, equal for one, lower for six, and unrecorded for three. Differences were not statistically tested. Artificial reefs made of various materials and structures (including natural rock, stones, concrete blocks, concrete pipes, concrete slaps, and wooden shipwrecks) were created in 2005–2010 to boost fisheries. Three areas were chosen, and 3–8 artificial reef sites selected/area. For comparison, 3–6 natural sites/area were also selected located 800 m from the artificial reefs. During five surveys between September 2012 and August 2013, mobile invertebrates were sampled at each site (but not directly on the artificial reefs) using nets (28 m long, 3 m high, 10 cm outer mesh, 4 cm inner mesh) soaked for 24h. Invertebrates were identified, counted, and measured.Study and other actions tested