Create small adjoining cavities or ‘swimthrough’ habitats (≤100 mm) on subtidal artificial structures
Overall effectiveness category Awaiting assessment
Number of studies: 4
Background information and definitions
Definition: ‘Small adjoining cavities or ‘swimthrough’ habitats’ are adjoining internal cavities sheltered from, but with access to/from, outside the structure. Dimensions can vary but are ≤100 mm in any direction.
Small adjoining cavities or ‘swimthrough’ habitats are not well-studied in subtidal rocky habitats. They may form through weathering of softer rocks, amongst loosely-consolidated cobbles/boulders, or within three-dimensional structures created by living organisms. They likely provide organisms refuge from predation, in the same way crevice and hole habitats do (Mercader et al. 2019; Nelson & Vance 1979). They could also serve as corridors, connecting adjacent refuge habitats. The size and density of cavities or swimthroughs is likely to affect the size, abundance and variety of organisms that can use them. Small habitats can provide refuge for small-bodied organisms but may exclude larger organisms, limit their growth and get rapidly filled-up (Firth et al. 2020). Large habitats can be used by larger-bodied organisms but may not provide sufficient refuge from predators for smaller organisms. By default, cavities and swimthroughs contain shaded surfaces, which can be associated with the presence of non-native species (Dafforn 2017).
Cavities/swimthroughs are sometimes present on marine artificial structures made of consolidated boulders or blocks (Sherrard et al. 2016) or gabion baskets (Firth et al. 2014), but are absent from many other structures. Small adjoining cavities or ‘swimthrough’ habitats can be created on subtidal artificial structures by adding or removing material, either during construction or retrospectively.
There is a body of literature investigating the effects of creating swimthrough habitats on artificial reefs (e.g. Brotto et al. 2006; Hylkema et al. 2020; Noordin Raja Omar et al. 1994). These studies are not included in this synopsis, which focusses on in situ conservation actions to enhance the biodiversity of structures that are engineered to fulfil a primary function other than providing artificial habitats.
See also: Create hole habitats (>50 mm) on subtidal artificial structures; Create crevice habitats (>50 mm) on subtidal artificial structures; Create large adjoining cavities or ‘swimthrough’ habitats (>100 mm) on subtidal artificial structures.
Brotto D.S., Krohling W., Brum S. & Zalmon I.R. (2006) Usage patterns of an artificial reef by the fish community on the northern coast of Rio de Janeiro – Brazil. Journal of Coastal Research, 39, 1276–1280.
Dafforn K.A. (2017) Eco-engineering and management strategies for marine infrastructures to reduce establishment and dispersal of non-indigenous species. Management of Biological Invasions, 8, 153–161.
Firth L.B., Airoldi L., Bulleri F., Challinor S., Chee S.-Y., Evans A.J., Hanley M.E., Knights A.M., O’Shaughnessy K., Thompson R.C. & Hawkins S.J. (2020) Greening of grey infrastructure should not be used as a Trojan horse to facilitate coastal development. Journal of Applied Ecology, 57, 1762–1768.
Firth L.B., Thompson R.C., Bohn K., Abbiati M., Airoldi L., Bouma T.J., Bozzeda F., Ceccherelli V.U., Colangelo M.A., Evans A., Ferrario F., Hanley M.E., Hinz H., Hoggart S.P.G., Jackson J.E., Moore P., Morgan E.H., Perkol-Finkel S., Skov M.W., Strain E.M., van Belzen J. & Hawkins S.J. (2014) Between a rock and a hard place: environmental and engineering considerations when designing coastal defence structures. Coastal Engineering, 87, 122–135.
Hylkema A., Debrot A.O., Osinga R., Bron P.S., Heesink D.B., Izioka A.K., Reid C.B., Rippen J.C., Treibitz T., Yuval M. & Murk A.J. (2020) Fish assemblages of three common artificial reef designs during early colonization. Ecological Engineering, 157, 105994.
Mercader M., Blazy C., Di Pane J., Devissi C., Mercière A., Cheminée A., Thiriet P., Pastor J., Crec’hriou R., Verdoit-Jarraya M. & Lenfant P. (2019) Is artificial habitat diversity a key to restoring nurseries for juvenile coastal fish? Ex situ experiments on habitat selection and survival of juvenile seabreams. Restoration Ecology, 27, 1155–1165.
Nelson B.V. & Vance R.R. (1979) Diel foraging patterns of the sea urchin Centrostephanus coronatus as a predator avoidance strategy. Marine Biology, 51, 251–258.
Noordin Raja Omar R.M., Eng Kean C., Wagiman S., Mutalib Mat Hassan A., Hussein M., Bidin Raja Hassan R. & Omar Mat Hussin C. (1994) Design and construction of artificial reefs in Malaysia. Bulletin of Marine Science, 55, 1050–1061.
Sherrard T.R.W., Hawkins S.J., Barfield P., Kitou M., Bray S. & Osborne P.E. (2016) Hidden biodiversity in cryptic habitats provided by porous coastal defence structures. Coastal Engineering, 118, 12–20.
Supporting evidence from individual studies
A study in 2009–2010 on a subtidal pipeline in a lagoon in the Mozambique Channel, Mayotte (Pioch et al. 2011) reported that small swimthrough habitats created on pipeline anchor-weights, along with large swimthroughs and environmentally-sensitive material, were used by juvenile spiny lobster Panulirus versicolor, juvenile blue-and-yellow grouper Epinephelus flavocaeruleus, sea firs (Hydrozoa) and adult fishes from five families. After one month, juvenile spiny lobsters and blue-and-yellow groupers, sea firs, and adult damselfish/clownfish (Pomacentridae), wrasse (Labridae), butterflyfish (Chaetodontidae), squirrelfish/soldierfish (Holocentridae) and surgeonfish (Acanthuridae) were recorded on and around anchor-weights with swimthrough habitats and environmentally-sensitive material. Small swimthrough habitats were created by attaching basalt rocks or semi-cylindrical tiles to the horizontal surfaces of concrete anchor-weights placed over a seabed pipeline (400 mm diameter). Basalt may be considered an environmentally-sensitive material compared with concrete. Large swimthrough habitats were also created between the anchor-weights and pipeline. Habitat dimensions/numbers were not reported. A total of 260 anchor-weights were placed with one every 10 m along the pipeline at 0–26 m depth during December 2009–March 2010. Fishes were counted on and around the pipeline from videos after 1 month.Study and other actions tested
A replicated, paired sites, controlled study in 2013–2014 on subtidal seawalls and pontoons in five marinas in the Mediterranean Sea, France (Bouchoucha et al. 2016) found that creating small swimthrough habitats on seawalls and pontoons had mixed effects on juvenile seabream Diplodus spp. abundance and habitat usage on and around the structures, depending on the species, juvenile development stage, site and survey month. Over 17 months, juvenile seabream (four species) used swimthrough habitats created on seawalls as frequently as those created under pontoons, and in three of six comparisons, they used both more than seawall and pontoon surfaces without swimthroughs, but in the other three comparisons no significant difference was found (data reported as habitat preference index). Abundances on and around swimthroughs and seawall and pontoon surfaces varied depending on the species, development stage, site and survey month (swimthroughs: 0–6 individuals/survey for any one species; seawall and pontoon: both 0–2/survey; see paper for results). Small swimthrough habitats were created by attaching steel cages containing oyster shells (Biohuts: height: 0.8 m; length: 0.5 m; width: 0.3 m; mesh size: 25–50 mm) to seawalls and pontoons in March 2013. Eight Biohuts were attached to each of three vertical seawalls, and three were suspended under each of three pontoons, in each of five marinas (depth not reported). Biohuts were compared with seawall (height: 0.8 m; length: 5 m) and pontoon (4 m2) surfaces without swimthroughs. Juvenile seabreams were counted on and around Biohuts and seawall/pontoon surfaces over 17 months.Study and other actions tested
A replicated, paired sites, controlled study in 2014 on three subtidal seawalls in a port in the Mediterranean Sea, France (Mercader et al. 2017) found that creating small swimthrough habitats on seawalls had mixed effects on juvenile fish species richness, abundance and community composition on and around the walls, depending on the site, survey month and species. Over four months, at two of three sites, juvenile fish species richness and total abundance was higher on and around seawall surfaces with swimthrough habitats (3–4 species and 13–18 individuals/10 m seawall) than those without (0–1 species and 3–12 individuals/10 m). At the third site, there were no significant differences (1 species and 3 individuals/10 m seawall with and without swimthroughs). Community composition (data reported as statistical model results) and individual species abundances varied on and around seawall surfaces with and without swimthroughs, depending on the site, survey month and species (see paper for results). Six species recorded on and around swimthroughs were absent from seawall surfaces without. Small swimthrough habitats were created in May 2014 by attaching steel cages containing oyster shells (Biohuts) to seawall surfaces (30 m long). Thirty-five Biohuts (height: 0.8 m; length: 0.5 m; width: 0.3 m; mesh size: 25–50 mm) were attached at 1 m depth on each of three vertical seawalls. Biohuts were compared with adjacent seawall surfaces (30 m long) on each wall. Juvenile fishes were counted on and around seawall surfaces with and without Biohuts over four months.Study and other actions tested
Referenced paperMercader M., Mercière A., Saragoni G., Cheminée A., Crec’hriou R., Pastor J., Rider M., Dubas R., Lecaillon G., Boissery P. & Lenfant P. (2017) Small artificial habitats to enhance the nursery function for juvenile fish in a large commercial port of the Mediterranean. Ecological Engineering, 105, 78-86.
A replicated study in 2014–2015 on subtidal pontoons in a marina in the Alboran Sea, Morocco (Selfati et al. 2018) found that small swimthrough habitats created under pontoons were used by seven species of juvenile fishes. After 12 months, 34 juvenile mottled groupers (Mycteroperca rubra) and 28 juvenile dusky groupers (Epinephelus marginatus) were recorded on and around swimthrough habitats (Biohuts). Juveniles of three seabream species (Diplodus sargus, Diplodus cervinus, Sarpa salpa), European bass (Dicentrarchus labrax) and mullet (Mugilidae) were also recorded on and around swimthroughs. On average, there were 3 juveniles/Biohut. Small swimthrough habitats were created in June 2014 by attaching steel cages containing oyster shells (Biohuts) beneath pontoons. Fifty Biohuts (height: 0.8 m; length: 0.5 m; width: 0.3 m; mesh size: 25–50 mm) were attached at 1 m depth beneath pontoons (arrangement not reported). Juvenile fishes were counted on and around Biohuts after 12 months.Study and other actions tested