Increasing microhabitat complexity on seawalls can reduce fish predation on native oysters

Published source details Strain E.M.A., Morris R.L., Coleman R.A., Figueira W.F., Steinberg P.D., Johnston E.L. & Bishop M.J. (2018) Increasing microhabitat complexity on seawalls can reduce fish predation on native oysters. Ecological Engineering, 120, 637-644.
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This study is summarised as evidence for the following.

	Action	Category
	Transplant or seed organisms onto intertidal artificial structures Action Link
	*Create grooves and* small protrusions, ridges or ledges (1–50 mm) on intertidal artificial structures** Action Link

Transplant or seed organisms onto intertidal artificial structures

A replicated, randomized study in 2015–2016 on two intertidal seawalls in Sydney Harbour estuary, Australia (Strain et al. 2018) found that 28–94% of oysters Saccostrea glomerata transplanted onto settlement plates survived, and that oyster survival and fish species richness around plates, but not fish abundance, varied depending on the presence and size of grooves and small ridges on plates and the site. Over six months, at one of two sites, transplanted oyster survival was higher on settlement plates with deep/tall grooves and ridges (52%) than on plates without (28%), and both were similar to plates with shallow/short grooves and ridges (43%). Survival was higher in grooves (85–95%) than on ridges (30–35%). Fish species richness was higher on and around oyster plates with deep/tall grooves and ridges (7 species/plate) than without (4/plate), while maximum fish abundance was similar (5 vs 4 individuals/plate) (not reported for shallow/short grooves and ridges). At the second site, no significant differences were found for oyster survival (deep/tall grooves and ridges: 94%; shallow/short: 80%; none: 91%; grooves: 96–98%; ridges: 95–98%), fish species richness (3 species/plate with and without grooves and ridges) or fish abundance (2 vs 3 individuals/plate). Dead oysters were either cracked (0–60%), intact (0–5%) or missing (0–8%). Hatchery-reared juvenile oysters were attached to concrete settlement plates (250 × 250 mm) using epoxy glue and transplanted onto vertical sandstone seawalls. There were 52 oysters/plate (24 mm average length) in patches of 4–5 individuals covering ~220 cm². Plates had textured surfaces with or without deep/tall (50 mm) or shallow/short (25 mm) grooves and small ridges. Five of each were randomly arranged at midshore on each of two seawalls in November 2015. Transplanted oysters were monitored during low tide over six months. Fishes were counted on and around plates from time-lapsed photographs during two high tides after one month.

(Summarised by: Ally Evans)
Create grooves and small protrusions, ridges or ledges (1–50 mm) on intertidal artificial structures

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.

(Summarised by: Ally Evans)