Create long flexible habitats (>50 mm) on subtidal artificial structures

Overall effectiveness category Awaiting assessment
Number of studies: 5
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Background information and definitions

Definition: ‘Long flexible habitats’ are flexible protruding materials such as rope, ribbon or twine >50 mm in length (modified from “Soft structures” in Strain et al. 2018).

Long flexible habitats, such as macroalgal canopies and soft-bodied invertebrates, provide other organisms three-dimensional habitat space and refuge from predation in subtidal rocky habitats (Levin & Hay 1996; Smale et al. 2020). The size, density and material properties of flexible habitats are likely to affect the size, abundance and variety of organisms that can use them and the spaces they create.

Some organisms that form flexible habitats tend to be absent or sparse on artificial subtidal structures (Wilhelmsson & Malm 2008), although some readily colonize in suitable conditions. Artificial flexible habitats such as ropes or nets can be present on some structures, but are likely to be temporary and regularly disturbed (e.g. removed and replaced) when present. Long flexible habitats can be created on subtidal artificial structures by adding material, either during construction or retrospectively. Material choice is important for creating flexible habitats, since some flexible materials are unlikely to persist in the marine environment, while those that do may become entanglement hazards or contribute to pollution if dislodged. Studies that investigate the effects of transplanting live soft-bodied organisms onto structures are not included here, but are considered under the action “Transplant or seed organisms onto subtidal artificial structures”.

There are bodies of literature describing the use of artificial flexible habitats to investigate the effects of structural complexity on ecological interactions in subtidal rocky habitats (e.g. Shelamoff et al. 2020), for artificial reefs or fish aggregation devices (e.g. Kellison & Sedberry 1998; Vega Fernández et al. 2009), and for bivalve or seaweed cultivation (e.g. Peteiro et al. 2007; Walls et al. 2019). There are also laboratory-based studies investigating species preferences for different flexible habitats (e.g. Hellyer et al. 2011). 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.

^{Hellyer C.B., Harasti D. & Poore A.G.B. (2011) Manipulating artificial habitats to benefit seahorses in Sydney Harbour, Australia. Aquatic Conservation: Marine and Freshwater Ecosystems, 21, 582–589.}

^{Kellison G.T. & Sedberry G.R. (1998) The effects of artificial reef vertical profile and hole diameter on fishes off South Carolina. Bulletin of Marine Science, 62, 763–780.}

^{Levin P.S. & Hay M.E. (1996) Responses of temperate reef fishes to alterations in algal structure and species composition. Marine Ecology Progress Series, 134, 37–47.}

^{Peteiro L.G., Filgueira R., Labarta U. & Fernández-Reiriz M.J. (2007) Effect of submerged time of collector ropes on the settlement capacity of Mytilus galloprovincialis L. Aquaculture Research, 38, 1679–1681.}

^{Shelamoff V., Layton C., Tatsumi M., Cameron M.J., Edgar G.J., Wright J.T. & Johnson C.R. (2020) Kelp patch size and density influence secondary productivity and diversity of epifauna. Oikos, 129, 331–345.}

^{Smale D.A., Epstein G., Hughes E., Mogg A.O.M. & Moore P.J. (2020) Patterns and drivers of understory macroalgal assemblage structure within subtidal kelp forests. Biodiversity and Conservation, 29, 4173–4192.}

^{Strain E.M.A., Olabarria C., Mayer-Pinto M., Cumbo V., Morris R.L., Bugnot A.B., Dafforn K.A., Heery E., Firth L.B., Brooks P.R. & Bishop M.J. (2018) Eco-engineering urban infrastructure for marine and coastal biodiversity: which interventions have the greatest ecological benefit? Journal of Applied Ecology, 55, 426–441.}

^{Vega Fernández T., D’Anna G., Badalamenti F. & Pérez-Ruzafa A. (2009) Effect of simulated macroalgae on the fish assemblage associated with a temperate reef system. Journal of Experimental Marine Biology and Ecology, 376, 7–16.}

^{Walls A.M., Edwards M.D., Firth L.B. & Johnson M.P. (2019) Ecological priming of artificial aquaculture structures: kelp farms as an example. Journal of the Marine Biological Association of the United Kingdom, 99, 729–740.}

^{Wilhelmsson D. & Malm T. (2008) Fouling assemblages on offshore wind power plants and adjacent substrata. Estuarine, Coastal and Shelf Science, 79, 459–466.}

Study locations

Key messages

Five studies examined the effects of creating long flexible habitats on subtidal artificial structures on the biodiversity of those structures. Three studies were in estuaries in southeast Australia and two were in a port in the Netherlands.

COMMUNITY RESPONSE (2 STUDIES)

Overall community composition (2 studies): Two replicated, controlled studies (including one randomized study) in Australia and the Netherlands reported that long flexible habitats created on subtidal artificial structures supported macroalgae and non-mobile invertebrate or fish species that were absent from on and around structure surfaces without flexible habitats.
Invertebrate community composition (1 study): One replicated, controlled study in the Netherlands reported that creating long flexible habitats on subtidal artificial structures altered the non-mobile invertebrate community composition on structure surfaces.
Fish richness/diversity (1 study): One replicated, randomized, controlled study in Australia found that creating long flexible habitats on subtidal artificial structures had mixed effects on the fish species richness around structures, depending on fish presence when flexible habitats were created.

POPULATION RESPONSE (4 STUDIES)

Overall abundance (1 study): One replicated, controlled study in the Netherlands reported that long flexible habitats created on subtidal artificial structures supported higher combined macroalgae and invertebrate (mostly mussels) biomass than structure surfaces without flexible habitats, and found that deeper flexible habitats supported higher biomass than shallower ones.
Invertebrate abundance (3 studies): Two of three studies (including two replicated, two controlled and one randomized study) in Australia and the Netherlands found that creating long flexible habitats on subtidal artificial structures had mixed effects on the mobile and/or non-mobile invertebrate abundance on and around structure surfaces, depending on the species group and survey week, or the flexible habitat length and density. One study reported that creating flexible habitats decreased the mussel abundance on structure surfaces but that the flexible habitats themselves supported higher biomass (mostly mussels) than the structure surfaces.
Fish abundance (2 studies): Two randomized, controlled studies (including one replicated study) in Australia found that creating long flexible habitats on subtidal artificial structures had mixed effects on the abundance of fishes or seahorses on and around structures, depending on the species and fish presence when flexible habitats were created, or the survey week.

BEHAVIOUR (1 STUDY)

Use (1 study): One replicated study in Australia reported that long flexible habitats created on subtidal artificial structures were used by seahorses.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

One replicated, randomized, controlled study in 1989 on 36 subtidal pontoons in Port Hacking estuary, Australia (Hair & Bell 1992) found that creating long flexible habitats (artificial seagrass units, ASUs) on pontoons did not increase the fish species richness or abundance under pontoons in one trial, but did in a second trial in which pontoons had been cleared of fishes initially. In the first trial, after six weeks, fish species richness and abundance (excluding blennies Parablennius sp.) was similar under pontoons with ASUs (3–4 species/pontoon, 5–7 individuals/pontoon) and those without (1–2 species/pontoon, 2–4 individuals/pontoon). In the second trial, six weeks after clearing fishes from beneath pontoons, species richness and abundance was higher under pontoons with ASUs (4–5 species/pontoon, 6–11 individuals/pontoon) than without (0–1 species and individuals/pontoon). Blenny abundance was similar under pontoons with and without ASUs (0–17 vs 0–22 individuals/pontoon) in both trials. Three species recorded under pontoons with ASUs were absent from those without. Long flexible habitats (ASUs) were created by suspending steel mesh sheets (7 m²) with buoyant plastic fragments (length: 280 mm; density: 800/m²) under pontoons. One ASU was attached at 0.3 m depth under each of six randomly-selected pontoons in each of three sites within an estuary in September 1989. Fishes under pontoons with ASUs and under six without were netted (1 mm mesh size) and counted after six weeks. The trial was repeated in October after clearing fishes from under pontoons. Five ASUs were dislodged and no longer provided habitat.
Study and other actions tested
Referenced paper
Hair C.A. & Bell J.D. (1992) Effects of enhancing pontoons on abundance of fish: initial experiments in estuaries. Bulletin of Marine Science, 51, 30-36.
A replicated study in 2003–2004 on two subtidal jetties in Sydney Harbour estuary, Australia (Clynick 2008) reported that long flexible habitats (nets) created on jetty pilings were used by two species of seahorse. Over 10 months, between one and three White’s seahorses Hippocampus whitei were seen on nets attached to jetty pilings during three of five surveys at each of two sites. One big-belly seahorse Hippocampus abdominalis was seen during three of the surveys at one site. Two juvenile seahorses were seen on nets. Long flexible habitats were created by attaching five nets (length: 5 m; height: 3 m; material not reported) to wooden jetty pilings at each of two sites in May 2003. Nets were in contact with the seabed (depth not reported). Seahorses on nets were counted over 10 months.
Study and other actions tested
Referenced paper
Clynick B.G. (2008) Harbour swimming nets: a novel habitat for seahorses. Aquatic Conservation: Marine and Freshwater Ecosystems, 18, 483-492.
A randomized, controlled study in 2008 on two subtidal swimming-enclosure nets in Sydney Harbour estuary, Australia (Hellyer et al. 2011) found that creating long flexible habitats (double-netting) on enclosure-net panels had mixed effects on seahorse Hippocampus whitei and mobile invertebrate abundances, depending on the survey week and invertebrate species group. Over two months, net panels with double-netting had higher seahorse abundance (1/panel) than panels without (0/panel) during two of seven surveys, but similar abundance in the other five (both 0–1/panel). Mobile invertebrate abundances on panels with and without double-netting varied depending on the species group and survey week (see paper for results). Long flexible habitats were created on polyethylene rope swimming-enclosure nets (100 mm mesh size) in March 2008 by attaching a second layer of enclosure netting (‘double-netting’). Three net panels (length: 0.3 m, height: 1 m) with double-netting and three panels without were randomly arranged along each of two enclosure nets (depth not reported). In May 2008, sixty-three seahorses were released onto the nets. Seahorses were counted on panels with and without flexible habitats over two months and mobile invertebrates (seahorse prey) were surveyed using a suction-pump over three months.
Study and other actions tested
Referenced paper
Hellyer C.B., Harasti D. & Poore A.G.B. (2011) Manipulating artificial habitats to benefit seahorses in Sydney Harbour, Australia. Aquatic Conservation: Marine and Freshwater Ecosystems, 21, 582-589.
One replicated, controlled study in 2009 on seven subtidal jetty pilings in the Port of Rotterdam, the Netherlands (Paalvast et al. 2012a) reported that creating long flexible habitats (‘hulas’) on pilings altered the non-mobile invertebrate community composition and reduced mussel Mytilus edulis cover on piling surfaces, but that hulas supported higher macroalgae and invertebrate biomass (mostly mussels) than piling surfaces without flexible habitats. Data were not statistically tested unless stated. After eight months, hula ropes supported mussels (60% cover), nine macroalgae and other non-mobile invertebrate species (0–2% cover), and five mobile invertebrate species groups (1–10 to >100 individuals/rope). Piling surfaces under hulas had 50% barnacle cover (Amphibalanus improvisus), while pilings without flexible habitats had 50% mussel and 14% barnacle cover. At least eight species (2 macroalgae, 6 non-mobile invertebrates) recorded on hulas were absent from piling surfaces without. Biomass was 44–113 kg/m² on hulas and 10 kg/m² on surfaces without. Biomass was statistically similar on ropes at 0.5 m depth (6 g/cm) and 1 m (7 g/cm), and higher on both than those at 0 m (3 g/cm). Long flexible habitats were created by attaching nylon rope skirts (‘hulas’) around pilings in March 2009. Three overlapping hulas with 167 ropes/hula (rope diameter: 6 mm; length: 550 mm; density: 167/m) were attached around each of five wooden and two steel pilings, cleared of organisms, with one hula at each of 0, 0.5 and 1 m depths. Hulas were compared with subtidal surfaces (200 × 200 mm) on seven additional wooden/steel pilings without hulas, cleared of organisms. Macroalgae and invertebrates on hula ropes and piling surfaces were counted and biomass (wet weight) measured in the laboratory over eight months.
Study and other actions tested
Referenced paper
Paalvast P., van Wesenbeeck B.K., van der Velde G. & de Vries M.B. (2012) Pole and pontoon hulas: an effective way of ecological engineering to increase productivity and biodiversity in the hard-substrate environment of the port of Rotterdam. Ecological Engineering, 44, 199-209.
One replicated study in 2009 on five subtidal pontoons in the Port of Rotterdam, the Netherlands (Paalvast et al. 2012b) found that long flexible habitats (‘hulas’) created under pontoons supported different invertebrate biomass depending on the rope length and density. After eight months, around hula edges, biomass of mussels Mytilus edulis, and other mobile and non-mobile invertebrates was higher on hulas with long ropes (17–19 g/cm) than mixed-length ropes (14–16 g/cm). In hula centres, biomass was similar on both designs (long ropes: 9–13 g/cm; mixed: 10–15 g/cm). Biomass was higher on hulas with low-density ropes (15 g/cm) than medium-density (12 g/cm), and higher on both than those with high-density (9 g/cm). Long flexible habitats were created by suspending plastic frames with nylon rope skirts (‘hulas’, 12 mm rope diameter) beneath five pontoons in March 2009. Two hulas (2.0 × 1.6 m, 208 ropes/hula) had different rope lengths (long: 1.5 m; mixed: 0.3–1.5 m), while three hulas (2.3 × 0.9 m, 1.5 m rope length) had different rope densities (high: 64 ropes/m²; medium: 32/m²; low: 16/m²). Invertebrates on hula ropes were counted and biomass (wet weight) measured in the laboratory over eight months.
Study and other actions tested
Referenced paper
Paalvast P., van Wesenbeeck B.K., van der Velde G. & de Vries M.B. (2012) Pole and pontoon hulas: an effective way of ecological engineering to increase productivity and biodiversity in the hard-substrate environment of the port of Rotterdam. Ecological Engineering, 44, 199-209.

Please cite as:

Evans, A.J., Moore, P.J., Firth, L.B., Smith, R.K., and Sutherland, W.J. (2021) Enhancing the Biodiversity of Marine Artificial Structures: Global Evidence for the Effects of Interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

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This Action forms part of the Action Synopsis:

Biodiversity of Marine Artificial Structures

Related Actions

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Create large adjoining cavities or ‘swimthrough’ habitats (>100 mm) on subtidal artificial structures Action Link	Awaiting assessment	2	Synopsis Link
*Create grooves and* small protrusions, ridges or ledges (1–50 mm) on subtidal artificial structures** Action Link	Awaiting assessment	3	Synopsis Link
Transplant or seed organisms onto subtidal artificial structures Action Link	Awaiting assessment	11	Synopsis Link