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

Increasingly, urbanised coastlines are being armoured by shoreline protection structures, such as seawalls. Seawalls typically lack the complex microhabitats and protective spaces of natural shorelines and consequently organisms that settle on them may be particularly susceptible to predation. We tested whether the addition of complex microhabitats to seawalls enhances the survivorship of oysters, key habitat-forming species on intertidal shores, by reducing the intensity of predation. At two sites in Sydney Harbour, we compared the magnitude and sources of mortality of juvenile oysters among (1) flat tiles, without crevices or ridges; (2) complex tiles, with 2.5 cm high ridges, separated by crevices; and (3) complex tiles, with 5 cm high ridges, separated by crevices. We also compared predatory fish visitation and feeding among sites and treatments using GoPro® cameras. The abundance and feeding of predatory fish was much greater at one of the study sites than the other, but at neither site differed among treatments. At the site with greater predatory fish abundances, survivorship of juvenile oysters was 50% greater on the 5 cm complex tiles than flat tiles, and on complex tiles approximately 300% greater in crevices than on ridges. Of the dead oysters, almost all were cracked, indicative of fish predation. In contrast, at the site with fewer predatory fish, there were no detectable differences in oyster survivorship between treatments. These results suggest that the addition of complex habitat to seawalls could be an effective strategy in reducing fish predation pressure on juvenile oysters at sites with abundant predatory fish. A greater understanding of the site-specific pressures is required to enhance the abundances of desirable species and functions on seawalls.