The global decline of oyster reefs from anthropogenic sources has driven the need for innovative and effective restoration methods. This study investigates how substrate complexity and colour influence recruitment and survival of Sydney rock oysters (Saccostrea glomerata) on urban seawalls. Using eco-engineered panels mimicking traits of natural reefs, the experiments tested oyster recruitment on panels varying in colour (green, yellow, red, grey) and complexity (flat, complex), as well as survival under predation.
Results showed structural complexity greatly boosted recruitment, with complex panels supporting over five times more oysters than flat ones. Most recruits settled in crevices that provided protection from predators and desiccation. Colour effects were stronger on complex panels. Yellow panels, which are spectrally similar to live oysters, also showed higher recruitment. Predation by fish and crabs was significant, with larger oysters (>40 mm) exhibiting better survival due to size refuge. Complexity also improved survival, with survivors found in crevices, but colour had no effect on predation outcomes.
Overall, complexity strongly enhanced oyster recruitment and survival, while colour effects on recruitment was subtle. Once colour effects are untangled, eco-engineered complex substates tailored to species and location has the potential to improve oyster restoration success across urban environments.
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