Response of fish communities to eco-engineered seawalls in Sydney Harbour.

Urbanisation of coasts has replaced large amounts of complex near-shore habitat with relatively featureless structures such as piers, pontoons and seawalls, resulting in losses of marine biodiversity. Eco-engineering seeks to mitigate this ecological impact, by co-designing coastal structures for biodiversity. The “Living Seawalls” project is ecologically enhancing seawalls by retrofitting them with complex habitat panels. The panels have positive effects on invertebrate and algal communities, but their effects on fish communities are unknown. This study tested whether (1) fish communities differ between seawalls and a natural analogue, rocky shores; (2) adding complex habitat panels to seawalls results in fish communities more similar to rocky shores; and (3) at the patch-scale, fish communities vary with type of complexity added. Seawalls, at times, supported fewer observations of fewer species of fish than rocky shores, though this effect was temporally variable. When seawalls were eco-engineered with Living Seawalls panels, fish assemblages became more similar to rocky shores. At the patch-scale, however, fish utilization of panels did not significantly vary with the type of complexity provided. This study provides important information on how eco-engineering can mitigate effects of coastal urbanisation on fish communities.