Create small protrusions (1–50 mm) on subtidal artificial structures
Overall effectiveness category Awaiting assessment
Number of studies: 1
Background information and definitions
Definition: ‘Small protrusions’ are elevations with a length to width ratio ≤3:1 that protrude 1–50 mm from the substratum (modified from “Small elevations” in Strain et al. 2018).
Small protrusions create vertical or horizontal (i.e. overhangs) relief in subtidal rocky habitats. They can provide organisms refuge from predation or grazing (Wahl & Hoppe 2002) and have positive effects on fish populations (Morris et al. 2018). Some species preferentially recruit to habitats with high vertical or horizontal relief (Andrews & Anderson 2004). The size and density of protrusions 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 and limit their growth. Large habitats can be used by larger-bodied organisms but may not provide sufficient refuge from predators for smaller organisms. By default, horizontal protrusions (overhangs) create shaded and downward-facing surfaces, which can be associated with the presence of non-native species (Dafforn 2017).
Protrusions are sometimes present on artificial structures such as cable mattresses (Lacey & Hayes 2020) or quarried boulders (MacArthur et al. 2020), but are often absent from other types of structures. Small protrusions can be created on subtidal artificial structures by adding material, either during construction or retrospectively.
There is a body of literature investigating the effects of creating protrusions on artificial reefs (e.g. Gratwicke & Speight 2005; Morris et al. 2018). 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 textured surfaces (≤1 mm) on subtidal artificial structures; Create natural rocky reef topography on subtidal artificial structures; Create large protrusions (>50 mm) on subtidal artificial structures; Create small ridges or ledges (1–50 mm) on subtidal artificial structures; Create large ridges or ledges (>50 mm) on subtidal artificial structures; Create groove habitats and small protrusions, ridges or ledges (1–50 mm) on subtidal artificial structures.
Andrews K.S. & Anderson T.W. (2004) Habitat-dependent recruitment of two temperate reef fishes at multiple spatial scales. Marine Ecology Progress Series, 277, 231–244.
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.
Gratwicke B. & Speight M.R. (2005) Effects of habitat complexity on Caribbean marine fish assemblages. Marine Ecology Progress Series, 292, 301–310.
Lacey N.C. & Hayes P. (2020) Epifauna associated with subsea pipelines in the North Sea. ICES Journal of Marine Science, 77, 1137–1147.
MacArthur M., Naylor L.A., Hansom J.D. & Burrows M.T. (2020) Ecological enhancement of coastal engineering structures: passive enhancement techniques. Science of the Total Environment, 740, 139981.
Morris R.L., Porter A.G., Figueira W.F., Coleman R.A., Fobert E.K. & Ferrari R. (2018) Fish-smart seawalls: a decision tool for adaptive management of marine infrastructure. Frontiers in Ecology and the Environment, 16, 278–287.
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.
Wahl M. & Hoppe K. (2002) Interactions between substratum rugosity, colonization density and periwinkle grazing efficiency. Marine Ecology Progress Series, 225, 239–249.
Supporting evidence from individual studies
A controlled study in 1985–1989 on a subtidal breakwater block on open coastline in Toyama Bay, Japan (Watanuki & Yamamoto 1990) reported that small protrusions created on the block supported more kelp Ecklonia stolonifera but less canopy algae Sargassum spp. than a block surface without protrusions. Data were not statistically tested. After 42 months, there were 58 kelp individuals on the surface with small protrusions (wet weight: 1.09 kg) and 20 on the surface without (0.31 kg). There were 2–3 individuals of each of three other canopy algae species on the surface with protrusions (0.01–0.09 kg) and 3–18 of each on the surface without (0.05–0.17 kg). Small protrusions were created on a concrete breakwater block (2.3 × 2.3 × 0.8 m) by attaching 45 pebbles (diameter/height: 35–45 mm), evenly-spaced on a 644 × 529 mm horizontal surface. One adjacent surface had no protrusions. The block was placed on sandy seabed at 9 m depth in November 1985. Macroalgae on surfaces with and without small protrusions were counted and weighed (wet weight) after 42 months.Study and other actions tested