Create grooves and small protrusions, ridges or ledges (1–50 mm) on intertidal artificial structures

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

COMMUNITY RESPONSE (14 STUDIES)

  • Overall community composition (9 studies): Three of five replicated, randomized, controlled studies (including one paired sites, before-and-after study) in Singapore and Israel found that creating groove habitats and small ridges/ledges on intertidal artificial structures, along with holes and environmentally-sensitive material in one, altered the combined macroalgae and invertebrate community composition on structure surfaces. Two studies found that creating grooves and small ridges, along with pits in one, had mixed effects on the community composition depending on the site, the presence of water-retaining and light-shading covers, and the size and arrangement of grooves and ridges. In contrast, one of the studies found that varying the size and arrangement had no significant effect. One of the studies, along with four other replicated, randomized, controlled studies in Singapore, Hong Kong and Australia, reported that groove habitats and small ridges/ledges, along with pits or holes and environmentally-sensitive material in two studies, supported species that were absent from structure surfaces without grooves and ridges/ledges.
  • Overall richness/diversity (11 studies): Six of 11 replicated, randomized, controlled studies (including one paired sites, before-and-after study) in Singapore, the USA, Israel, the UK, Hong Kong, Australia and worldwide found that creating groove habitats and small ridges/ledges on intertidal artificial structures, along with pits or holes and environmentally-sensitive material in two studies, increased the combined macroalgae and invertebrate species diversity and/or richness on structure surfaces. Five studies found that creating grooves and small protrusions/ridges/ledges, along with large ledges or using environmentally-sensitive material in two, had mixed effects on species diversity and/or richness, depending on the depth/height of grooves and ridges, the presence of large ledges on structure surfaces, the shore level, species group and site. One of the studies found that varying the size and arrangement of grooves and ridges increased the species richness, while one found that effects depended on the shore level. One of the studies found that partially-covering grooves and ridges with water-retaining and light-shading covers increased the species richness.
  • Algal richness/diversity (2 studies): One of two replicated, randomized, controlled studies in Singapore and worldwide found that creating groove habitats and small ridges on intertidal artificial structures had mixed effects on the macroalgal species richness on structure surfaces, depending on the size of grooves and ridges and the location. One study found that creating grooves and ridges, along with pits, increased the species richness, regardless of their size and arrangement.
  • Invertebrate richness/diversity (2 studies): One of two replicated, randomized, controlled studies in Australia and worldwide found that creating groove habitats and small ridges on intertidal artificial structures had mixed effects on the mobile and non-mobile invertebrate species richness on structure surfaces, depending on the size of grooves and ridges and the location. One study found that creating grooves and small ledges increased the mobile invertebrate species richness.
  • Fish richness/diversity (3 studies): Two of three replicated, randomized, controlled studies in Australia found that creating groove habitats and small ridges/ledges on intertidal artificial structures did not increase the fish species richness on and around structure surfaces. One study found that creating grooves and ridges had mixed effects on fish species richness depending on the site.

POPULATION RESPONSE (13 STUDIES)

  • Overall abundance (6 studies): Two of six replicated, randomized, controlled studies in Singapore, the USA and Australia found that creating groove habitats and small ridges/ledges on intertidal artificial structures did not increase the combined macroalgae and invertebrate abundance on structure surfaces. Two studies found that creating grooves and small protrusions/ridges, along with large ledges in one, and when partially-covered with water-retaining and light-shading covers in the other, did increase abundance. Two found that creating grooves and small ridges/ledges, along with pits in one, had mixed effects on abundance depending on the size and arrangement of grooves and ridges/ledges, the shore level and/or the site.
  • Algal abundance (3 studies): Two of three replicated, randomized, controlled studies in Singapore, the USA and worldwide found that creating groove habitats and small protrusions/ridges on intertidal artificial structures, along with large ledges in one, had mixed effects on rockweed or combined macroalgal abundance, depending on the presence of large ledges on structure surfaces, the depth/height of grooves and ridges, the shore level and/or the site. One study found that creating grooves and small ridges, along with pits, did not increase the macroalgal abundance, regardless of the size and arrangement of grooves and ridges.
  • Invertebrate abundance (7 studies): Five of seven replicated, randomized, controlled studies in the USA, Singapore, the UK, Hong Kong, Australia and worldwide found that creating groove habitats and small protrusions/ridges/ledges on intertidal artificial structures, along with large ledges or using environmentally-sensitive material in two, had mixed effects on mobile invertebrate, non-mobile invertebrate, limpet, mussel, juvenile oyster and/or barnacle abundances, depending on the depth/height of grooves and ridges, the presence of large ledges or water-retaining and light-shading covers, the shore level, and/or the site. Two studies found that creating grooves and small ridges/ledges increased oyster but not mobile invertebrate abundance on structure surfaces.
  • Fish abundance (4 studies): Three replicated, randomized, controlled studies and one before-and-after study in Australia and the USA found that creating groove habitats and small ridges/ledges on intertidal artificial structures, along with large ledges in one study, did not increase combined fish or juvenile salmon abundances on and around structure surfaces.

BEHAVIOUR (3 STUDIES)

Use (1 study): One replicated study in Spain reported that grooves and small protrusions created on an intertidal artificial structure were colonized by a number of microalgal species.

Fish behaviour change (2 studies): One replicated, randomized, controlled study in Australia found that creating groove habitats and small ledges on intertidal artificial structures increased the time benthic fishes spent interacting with structure surfaces but decreased the number of bites they took and did not change pelagic fish behaviour. One before-and-after study in the USA reported that creating grooves and small protrusions, along with large ledges, increased juvenile salmon feeding activity around the structure.

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

  1. A replicated, randomized, controlled study in 2011–2012 on two intertidal seawalls on island coastlines in the Singapore Strait, Singapore (Loke et al. 2016; same experimental set-up as Loke et al. 2017) reported that concrete settlement plates with groove habitats and small ridges, along with pits, supported higher macroalgal species richness but similar abundances compared with granite plates without added habitats. After 12 months, settlement plates with grooves, ridges and pits supported a total of five macroalgal species groups, while plates without supported three (data not statistically tested). Abundances of three species groups were statistically similar on plates with added habitats (18–41% cover) and without (5–61%) in five of six comparisons, while one group was more abundant on plates with added habitats (22–27 vs 5%) at one site. Abundances were similar on plates with variable (1–34%) and regular (3–41%) habitats. It is not clear whether these effects were the direct result of creating grooves, ridges or pits. Settlement plates (400 × 400 mm) were moulded with groove habitats and small ridges, with pits, and with neither. Plates with grooves, ridges and pits were concrete with four-to-five concentric circular grooves and ridges/plate or 36 pits/plate. Grooves, ridges and pits had either regular (32 mm width, depth/height and spacing) or variable (8–56 mm) arrangement. Plates without grooves, ridges or pits were granite fragments set in cement. Granite may be considered an environmentally-sensitive material compared with concrete (see “Use environmentally-sensitive material on intertidal artificial structures”). Five of each design were randomly arranged at lowshore on each of two granite boulder seawalls in July 2011. Macroalgae on plates were counted from photographs after 12 months.

    Study and other actions tested
  2.  A replicated, randomized, controlled study in 2009–2010 on two intertidal seawalls on island coastlines in the Singapore Strait, Singapore (Loke & Todd 2016a; same experimental set-up as Loke & Todd 2016b) found that concrete settlement plates with groove habitats and small ridges supported different macroalgae and invertebrate community composition with higher species richness but similar abundances compared with granite plates without grooves and ridges. After 13 months, macroalgae and invertebrate species richness was higher on settlement plates with grooves and ridges (8 species/plate) than without (3/plate), while abundances were statistically similar (126 vs 178 individuals/plate). Community composition differed on plates with and without grooves and ridges (data reported as statistical model results). Settlement plates (200 × 200 mm) were moulded with and without groove habitats and small ridges. Plates with grooves and ridges were concrete with four-to-five concentric circular grooves and ridges/plate with either regular (16 mm width, depth/height and spacing) or variable (4–28 mm) arrangement. Plates without grooves and ridges were granite fragments set in cement. Granite may be considered an environmentally-sensitive material compared with concrete (see “Use environmentally-sensitive material on intertidal artificial structures”). Eight of each design were randomly arranged at both lowshore and highshore on each of two granite boulder seawalls in November–December 2009. Macroalgae on plates were counted from photographs and invertebrates in the laboratory after 13 months.

    Study and other actions tested
  3.  A replicated, randomized, controlled study in 2009–2010 on two intertidal seawalls on island coastlines in the Singapore Strait, Singapore (Loke & Todd 2016b; same experimental set-up as Loke & Todd 2016a) found that concrete settlement plates with groove habitats and small ridges, along with small protrusions and pits, supported higher macroalgae and invertebrate species richness than granite plates without added habitats, while abundances and community composition varied depending on the habitat arrangement, shore level and site. After 13 months, macroalgae and invertebrate species richness was higher on settlement plates with grooves, ridges, protrusions and pits than without at lowshore (13–23 vs 6–10 species/plate) and highshore (5–9 vs 2–3/plate). Richness was higher on plates with variable habitats than regular ones at lowshore (22–23 vs 13–16/plate), but not highshore (6–9 vs 5–6/plate). Abundances were higher on plates with added habitats than without in four of eight comparisons (9–833 vs 3–208 individuals/plate), while community composition differed in three of four comparisons (data reported as statistical model results). In all other comparisons, results were similar (abundances: 104–1,957 vs 49–1,162/plate). It is not clear whether these effects were the direct result of creating grooves and ridges, protrusions or pits. However, plate quarters with grooves and ridges had similar richness (8 species/quarter) and abundances (126 individuals/quarter) to quarters with the other habitat types (6–11 species and 88–231 individuals/quarter). Settlement plates (400 × 400 mm) were moulded with and without groove habitats and small ridges, along with small protrusions and pits. Plates with added habitats were concrete. Each 200 × 200 mm quarter contained either four-to-five concentric circular grooves and ridges, 36 protrusions, 12 ridges or 36 pits. All habitats had either regular (16 mm width, depth/height and spacing) or variable (4–28 mm) arrangement. Plates without added habitats were granite fragments set in cement. Granite may be considered an environmentally-sensitive material compared with concrete (see “Use environmentally-sensitive material on intertidal artificial structures”). Eight of each design were randomly arranged at both lowshore and highshore on each of two granite boulder seawalls in November–December 2009. Macroalgae on plates were counted from photographs and invertebrates in the laboratory after 13 months.

    Study and other actions tested
  4. A replicated, randomized, controlled study in 2008–2011 on three intertidal seawalls in Puget Sound estuary, USA (Cordell et al. 2017) reported that seawall panels with grooves and small protrusions, along with large ledges, supported higher macroalgae, microalgae and invertebrate species diversity and live cover, with more rockweed Fucus distichus and mussels Mytilus spp., than seawall surfaces without added habitats, but that flat panels (i.e. without large ledges) with grooves and protrusions did not. After 42 months, ledged seawall panels with grooves and small protrusions supported higher macroalgae, microalgae and invertebrate species diversity (data reported as Evenness index, not statistically tested), rockweed abundance (5–13% cover) and mussel abundance (6–12%) than seawall surfaces without added habitats (rockweed/mussels: both 1%), but flat panels with grooves and potrusions did not (rockweed: 0%; mussels: 6%). Total live cover was 83–84% on ledged panels with grooves and protrusions, 81% on flat panels with grooves and protrusions, and 74% on seawall surfaces (data not statistically tested). Abundances of six other species groups were not statistically tested (see paper for results). It is not clear whether these effects were the direct result of creating grooves and protrusions or ledges. Concrete seawall panels (height: 2.3 m; width: 1.5 m; thickness: ~150 mm) were moulded with and without groove habitats and small protrusions (dimensions not reported) using a cobble-effect formliner. Panels had three long or six short horizontal large ledges, or were flat. One panel of each surface-ledge combination was randomly arranged, spanning high–lowshore on each of three vertical concrete seawalls in January 2008. Seawall surfaces were intertidal areas of seawall cleared of organisms (dimensions/spacing not reported). Macroalgae, microalgae and invertebrates were counted on panels (excluding downward-facing surfaces) and seawall surfaces during low tide after 42 months.

    Study and other actions tested
  5. A replicated, randomized, controlled study in 2011–2012 on two intertidal seawalls on island coastlines in the Singapore Strait, Singapore (Loke et al. 2017; same experimental set-up as Loke et al. 2016) found that concrete settlement plates with groove habitats and small ridges supported higher macroalgae and invertebrate species richness and different community composition compared with granite plates without grooves or ridges. After 12 months, settlement plates with variable grooves and ridges supported a total of 49 macroalgae and invertebrate species, while plates with regular grooves and ridges supported 35 species and plates without grooves and ridges supported 22 (data not statistically tested). Average richness was higher on plates with variable grooves and ridges (18 species/plate) than regular ones (13/plate), and higher on both than on plates without (7/plate). Community composition was similar on plates with variable and regular grooves and ridges, but both differed to plates without (data reported as statistical model results). Settlement plates (400 × 400 mm) were moulded with and without groove habitats and small ridges. Plates with grooves and ridges were concrete with four-to-five concentric circular grooves and ridges/plate with either regular (32 mm width, depth/height and spacing) or variable (8–56 mm) arrangement. Plates without grooves and ridges were granite fragments set in cement. Granite may be considered an environmentally-sensitive material compared with concrete (see “Use environmentally-sensitive material on intertidal artificial structures”). Five of each design were randomly arranged at lowshore on each of two granite boulder seawalls in July 2011. Macroalgae and invertebrates on plates were counted in the laboratory after 12 months.

    Study and other actions tested
  6. A replicated, randomized, paired sites, controlled, before-and-after study in 2014–2016 on an intertidal seawall in a marina in the Mediterranean Sea, Israel (Perkol-Finkel et al. 2018) found that groove habitats and small ledges created on seawall panels, along with holes and environmentally-sensitive material, supported higher macroalgae and invertebrate species diversity and richness and different community composition compared with standard-concrete seawall surfaces without added habitats. After 22 months, macroalgae and invertebrate species diversity (data reported as Shannon index) and richness was higher on panels with added habitats (8 species/quadrat) than on seawall surfaces without (3/quadrat), and compared with seawall surfaces before habitats were added (2/quadrat). Community composition differed between panels with added habitats and seawall surfaces without (data reported as statistical model results). Five species groups (1 macroalgae, 4 non-mobile invertebrates) recorded on panels were absent from surfaces without. It is not clear whether these effects were the direct result of creating grooves and ledges, holes, or using environmentally-sensitive material. Groove habitats and small ledges were created on seawall panels (height: 1.5 m; width: 0.9 m; thickness: 130 mm) using a formliner. Each panel had multiple interlocking grooves and ledges (length: 50–150 mm; width/depth/height: 10–50 mm) amongst multiple holes. Panels were made from patented ECOncreteTM material. Four panels were attached to a vertical concrete seawall in November 2014. The top 0.3 m were intertidal. Panels were compared with standard-concrete seawall surfaces cleared of organisms (height: 0.3 m; width: 0.9 m) adjacent to each panel. Macroalgae and invertebrates were counted in one 300 × 300 mm randomly-placed quadrat on each panel and seawall surface during high tide over 22 months.

    Study and other actions tested
  7. A replicated study (year not reported) on an intertidal seawall in Ceuta Port in the Alboran Sea, Spain (Sempere-Valverde et al. 2018) reported that settlement plates with groove habitats and small protrusions had chlorophyll-a and 15 diatom species on their surfaces. After two months, chlorophyll-a density on plates with grooves and protrusions ranged from 3–18 μg/cm2. Total abundances of 15 diatom species ranged from 1–752 individuals across all plates. Settlement plates (170 × 170 mm) were cut to create a regular grid of six groove habitats (length: 170 mm; width/depth: ~7 mm) between 16 square protrusions (length/width: 30 mm; height: ~7 mm) on their surfaces. Plates were either sandstone, limestone, gabbro, slate or concrete. One of each material was randomly arranged, horizontally, on each of five midshore boulders along a limestone boulder seawall (month/year not reported). Microalgae and chlorophyll-a on plates were measured using a scanning microscope and spectrophotometer, respectively, after two months.

    Study and other actions tested
  8. A replicated, randomized, controlled study in 2015 on two intertidal seawalls in Sydney Harbour estuary, Australia (Strain et al. 2018) found that creating groove habitats and small ridges on settlement plates increased the species richness of fish on and around plates at one of two sites, but did not increase fish abundances. After one month, at one site, fish species richness was higher on and around settlement plates with grooves and ridges (7 species/plate) than without (4/plate), while at the second site, there was no difference (both 3/plate). Maximum fish abundance was similar on and around plates with and without grooves and ridges (2–5 vs 3–4 individuals/plate) at both sites. Concrete settlement plates (250 × 250 mm) were moulded with and without groove habitats and small ridges. Plates with grooves and ridges had four vertical grooves (length: 250 mm; width: 15–50 mm; depth: 50 mm) between five ridges (length: 250 mm; width: 17–65 mm; height: 50 mm). Five plates with grooves and ridges and five without were randomly arranged at midshore on each of two vertical sandstone seawalls in November 2015. Plates had textured surfaces and 52 juvenile oysters attached. Fishes were counted on and around plates from time-lapsed photographs during two high tides after one month.

    Study and other actions tested
  9. A replicated, randomized, controlled study in 2010–2011 on two intertidal seawalls on island coastlines in the Singapore Strait, Singapore (Loke et al. 2019) found that concrete settlement plates with groove habitats and small ridges supported higher macroalgae and invertebrate species richness and abundance than granite plates without grooves or ridges, but that community composition and limpet Siphonaria guamensis abundance varied depending on the site and whether grooves and ridges were partially-covered. After eight months, macroalgae and invertebrate species richness and abundance were higher on settlement plates with partially-covered grooves and ridges (20 species/plate, 89 individuals/plate) than uncovered grooves and ridges (14 species/plate, 43 individuals/plate) and plates without grooves and ridges (10 species/plate, 40 individuals/plate). Community composition differed on plates with and without grooves and ridges in three of four comparisons (data reported as statistical model results). At one of two sites, there were 250 limpets/plate with partially-covered grooves and ridges, 420/plate with uncovered grooves and ridges and 225/plate without grooves and ridges (data not statistically tested). At the second site, limpet abundance was 0/plate for each. Settlement plates (200 × 200 mm) were moulded with and without groove habitats and small ridges. Plates with grooves and ridges were concrete with five concentric circular grooves and ridges/plate (8–56 mm width, depth/height and spacing). Some were partially-covered with water-retaining and light-shading plates. Plates without grooves or ridges were granite fragments set in cement. Granite may be considered an environmentally-sensitive material compared with concrete (see “Use environmentally-sensitive material on intertidal artificial structures”). Ten of each design were randomly arranged at lowshore on each of two granite boulder seawalls in August 2010. Macroalgae and invertebrates on plates were counted in the laboratory after eight months. Seven plates with grooves and ridges and four without were missing and no longer provided habitat.

    Study and other actions tested
  10. A replicated, randomized, controlled study in 2016–2017 on two intertidal seawalls on open coastline in the English Channel and in the Forth estuary, UK (MacArthur et al. 2019) found that creating groove habitats and small ridges on settlement plates, along with using environmentally-sensitive material, increased the macroalgae and invertebrate species richness and invertebrate abundance on plates at one of two sites. After 18 months, at one of two sites, macroalgae and mobile invertebrate species richness was higher on plates with grooves and ridges (2 species/plate) than without (1/plate). The same was true for mobile invertebrate abundance (9 vs 1 individuals/plate) and barnacle (Cirripedia) cover (48 vs 34%). At the second site, plates with and without grooves and ridges supported similar richness (both 1 species/plate), mobile invertebrate abundance (1 vs 3 individuals/plate) and barnacle cover (84 vs 83%). It is not clear whether these effects were the direct result of creating grooves and ridges or using environmentally-sensitive material. Settlement plates (150 × 150 mm) were moulded with and without groove habitats and small ridges. Plates with grooves and ridges had six chevron-shaped grooves between seven ridges with variable dimensions (maximum depth/height: 30 mm). Eight limestone-cement (environmentally-sensitive material) plates with grooves and ridges and eight concrete plates without were randomly arranged at upper-midshore on each of two vertical concrete seawalls in April–May 2016. Macroalgae and invertebrates on plates were counted from photographs over 18 months.

    Study and other actions tested
  11. A replicated, randomized, controlled study in 2016–2017 on three intertidal seawalls in Sydney Harbour estuary, Australia (Ushiama et al. 2019) found that creating groove habitats and small ledges on settlement plates did not increase fish species richness or abundance or alter pelagic fish behaviour, but altered benthic fish behaviour on and around plates. After 8–12 months, fish species richness and abundance were similar on and around settlement plates with and without grooves and ledges (data not reported). Benthic fishes spent longer interacting with plates with grooves and ledges (30 minutes/60-minute survey) than without (17 minutes/survey), but took fewer bites from their surfaces (2 vs 8 bites/survey). There were no significant differences for pelagic fishes (2 vs 1 minutes/survey, 8 vs 13 bites/survey). Concrete settlement plates (250 × 250 mm) were moulded with and without groove habitats and small ledges. Plates with grooves and ledges had four horizontal grooves (length: 250 mm; width: 15–50 mm; depth: 50 mm) between five ledges (length: 250 mm; width: 17–65 mm; height: 50 mm). Plates had textured surfaces with or without juvenile oysters, coralline algae, or both attached. Nine plates of each grooves/ledges-transplant combination were randomly arranged at mid-lowshore on each of three vertical sandstone seawalls in March 2016. Fishes were counted on and around one of each plate design from 60-minute videos during each of three high tides after 8–12 months. The time fishes spent within 50 mm of plates and the number of bites they took was recorded.

    Study and other actions tested
  12. A replicated, randomized, controlled study in 2016–2017 on two intertidal seawalls in the Pearl River estuary, Hong Kong (Bradford et al. 2020) found that creating groove habitats and small ridges on the seawalls had mixed effects on macroalgae and invertebrate species richness, and invertebrate abundances and biomass, depending on the depth/height of grooves and ridges and the site. After 12 months, in two of four comparisons, settlement plates with grooves and ridges supported higher macroalgae and invertebrate species richness (12–13 species/plate) and non-mobile invertebrate abundance (38–42% cover) than plates without (9 species/plate; 17% cover). In three of four comparisons, plates with grooves and ridges supported higher mobile invertebrate abundance (45–81 individuals/plate) and barnacle (Cirripedia) and oyster (Saccostrea cuccullata) recruit biomass (5–14 g/plate) than plates without (15–19 individuals/plate; 1 g/plate). In all other comparisons, plates with and without grooves and ridges were similar (richness: 11–12 vs 9 species/plate; non-mobiles: 14–27 vs 13% cover; mobiles: 31 vs 19 individuals/plate; barnacles/oysters: 2 vs 1 g/plate). Two mobile invertebrate species recorded on plates with grooves and ridges were absent from those without. Concrete settlement plates (250 × 250 mm) were moulded with and without groove habitats and small ridges. Plates with grooves and ridges had four vertical grooves (length: 250 mm; width: 15–50 mm) between five ridges (length: 250 mm; width: 17–65 mm). Grooves and ridges were either deep/tall (depth/height: 50 mm) or shallow/short (25 mm). Five of each were randomly arranged at midshore on each of two vertical concrete seawalls in November 2016 (month/year: M. Perkins pers. comms.). Plates had textured surfaces. Macroalgae and invertebrates on plates were counted from photographs and in the laboratory, and barnacle and oyster recruit biomass (dry weight) was measured after 12 months. One plate with deep/tall grooves and ridges was missing and no longer provided habitat.

    Study and other actions tested
  13. A before-and-after study in 2012–2018 on an intertidal seawall in Puget Sound estuary, USA (Sawyer et al. 2020) reported that creating grooves and small protrusions, along with large ledges, on the seawall did not increase juvenile salmon Oncorhynchus spp. abundance around the wall, but increased their feeding activity. Data were not statistically tested. Juvenile salmon abundances were lower after grooves and small protrusions were created during seawall reconstruction (5–151 individuals/m2) compared with before (47–431/100m2), but the frequency of their feeding behaviour increased by 6–27%. It is not clear whether these effects were the direct result of creating grooves and protrusions, ledges, increased light levels or reduced water depth in front of the wall. Groove habitats and small protrusions (dimensions not reported) were created on concrete seawall panels using a cobble-effect formliner. Panels also had one large ledge on their surfaces. Panels were attached to a vertical concrete seawall during reconstruction in 2017 (numbers/month not reported). Light-penetrating panels were also installed to increase light around the wall, and the seabed was raised in front. Juvenile salmon within 10 m of the wall were surveyed from 20–minute snorkels at high and low tide during March–August at three sites along the wall before reconstruction in 2012 (35 surveys), and at three different sites along the wall after reconstruction in 2018 (42 surveys).

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  14. A replicated, randomized, controlled study in 2015–2016 on two intertidal seawalls in Sydney Harbour estuary, Australia (Strain et al. 2020) found that creating groove habitats and small ledges on the seawalls increased the macroalgae and invertebrate species richness and oyster Saccostrea glomerata abundance on seawall surfaces, but did not increase abundances of macroalgae and other invertebrates, or the species richness and abundance of fishes. After 12 months, species richness was higher on settlement plates with grooves and ledges than without for macroalgae and non-mobile invertebrates (6 vs 2 species/plate) and mobile invertebrates (7 vs 4/plate), while there was no difference for fishes (both 2/plate). Oyster abundance was higher on plates with grooves and ledges (34% cover) than without (8%) but there were no significant differences in the abundances of macroalgae and other non-mobile invertebrates (46 vs 31% cover), mobile invertebrates (20 vs 16 individuals/plate) or fishes (both 1/plate). Eighteen species (5 macroalgae, 3 non-mobile invertebrates, 9 mobile invertebrates, 1 fish) recorded on plates with grooves and ledges were absent from those without. Concrete settlement plates (250 × 250 mm) were moulded with and without groove habitats and small ledges. Plates with grooves and ledges had four horizontal grooves (length: 250 mm; width: 15–50 mm; depth: 50 mm) between five ledges (length: 250 mm; width: 17–65 mm; height: 50 mm). Five plates with grooves and ledges and five without were randomly arranged at midshore on each of two vertical sandstone seawalls in November 2015. Plates had textured surfaces. Macroalgae and invertebrates were counted on plates during low tide, from photographs and in the laboratory after 12 months. Fishes were counted on and around plates from time-lapsed photographs during two high tides.

    Study and other actions tested
  15. A replicated, randomized, controlled study in 2015–2017 on 27 intertidal seawalls and breakwaters in 14 estuaries and bays worldwide (Strain et al. 2021) found that creating groove habitats and small ridges on settlement plates had mixed effects on the macroalgae and invertebrate species richness and abundance on plates, depending on the depth/height of grooves and ridges, the location, shore level and species group. After 12 months, plates with deep/tall grooves and ridges supported higher macroalgae and invertebrate species richness (4–28 species/plate) than plates without grooves and ridges (2–12/plate) in 11 of 14 locations, while in three locations there was no significant difference (2–6 vs 2–8/plate). Plates with shallow/short grooves and ridges supported higher richness (6–19/plate) than plates without (3–12/plate) in seven of 14 locations, while in seven locations there was no significant difference (both 2–8/plate). Out of 28 comparisons each time, plates with grooves and ridges supported higher macroalgal richness than plates without in two comparisons, higher macroalgal abundance in four, higher non-mobile invertebrate richness in 16, higher non-mobile invertebrate abundance in 11, and higher mobile invertebrate richness and abundance in 13 comparisons each. In all other comparisons, plates with and without grooves and ridges were similar (data not reported). [Significance results reported from Tables S5a,b in original paper]. Concrete settlement plates (250 × 250 mm) were moulded with and without groove habitats and small ridges. Plates with grooves and ridges had four vertical grooves (length: 250 mm; width: 15–50 mm) between five ridges (length: 250 mm; width: 17–65 mm). Grooves and ridges were either deep/tall (depth/height: 50 mm) or shallow/short (25 mm). Five of each were randomly arranged at highshore, midshore or lowshore on each of two vertical seawalls/breakwaters in each of 14 estuaries/bays worldwide between November 2015–2016. Plates had textured surfaces. Macroalgae and invertebrates on plates were counted in the laboratory after 12 months.

    Study and other actions tested
  16. A replicated, randomized, controlled study in 2015–2016 on two intertidal seawalls in Sydney Harbour estuary, Australia (Vozzo et al. 2021) found that creating groove habitats and small ridges or ledges on the seawalls had mixed effects on macroalgae and invertebrate species richness, diversity and abundances, depending on the depth/height of grooves and ridges, the species group and site. After 12 months, the macroalgae and invertebrate species richness was higher on settlement plates with deep/tall grooves and ridges or ledges (15 species/plate) than plates with shallow/short ones (10/plate) and plates without (8/plate), which were similar. At one site, the same was true for species diversity (data reported as Shannon index) and macroalgae and non-mobile invertebrate abundance (deep/tall: 77–99% cover; shallow/short: 30%; none: 31%). At the second site, no significant differences were found (deep/tall: 116–120%; shallow/short: 102%; none: 87%). Oyster (Ostreidae) abundance was higher on plates with grooves and ridges/ledges (52–91 individuals/plate) than without (15/plate), while mobile invertebrate abundance did not significantly differ (23–49 vs 11/plate). Twenty-three species (4 macroalgae, 14 mobile invertebrates, 5 non-mobile invertebrates) recorded on plates with grooves and ridges/ledges were absent from those without. The orientation of grooves and ridges or ledges had no clear effect on results. See paper for full results. Concrete settlement plates (250 × 250 mm) were moulded with and without groove habitats and small ridges or ledges. Plates with grooves and ridges or ledges had four grooves (length: 250 mm; width: 15–50 mm) between five vertical ridges or horizontal ledges (length: 250 mm; width: 17–65 mm). Grooves, ridges and ledges were either deep/tall (depth/height: 50 mm) or shallow/short (25 mm). Five of each were randomly arranged at midshore on each of two vertical sandstone seawalls in November 2015. Plates had textured surfaces. Macroalgae and invertebrates on plates were counted in the laboratory after 12 months.

    Study and other actions tested
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.

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