Targeted substrate design is required to improve post-settlement survival of shellfish

The strength of biotic stressors (i.e., predation) can determine the success of shellfish reef restoration projects. In recruitment limited systems where seeding is a common restoration strategy, ensuring the survival of seeded individuals is an ongoing challenge. Testing of novel materials that provide refuges is becoming increasingly common to combat the issue. Although this addition of complexity to restoration substrates can improve the settlement of pediveliger larvae, increasing the density of seeded individuals, how this complexity then alters post-settlement survival has been less studied. The aim of this study was to investigate post-settlement survival of juvenile oysters on concrete surfaces with added surface roughness and complexity in contrast to control tiles. Concrete tiles that had either a rough surface (via sandblasting) or a complex surface (via the addition of grooves and ridges) were seeded with Australian flat oysters (Ostrea angasi) and placed into the field for 15 months. Predator access was tested by measuring the consumption of dried mussel baits (Mytilus galloprovincialis) on complex tiles. The addition of sandblasting or complexity to concrete tiles did not alter the survival of Australian flat oysters and the consumption of blue mussel baits did not differ among complex tiles. Sandblasting and adding complexity to concrete surfaces has previously been shown to improve the settlement of shellfish but in this study, the type of complexity was not sufficient to improve post-settlement survival of oysters or reduce consumption. To create restoration substrates for shellfish or other sessile invertebrates, understanding the mechanisms that drive their settlement and post-settlement survival is vital. Here, substrates with rough or complex surfaces that encouraged larval settlement were not sufficient to increase post-settlement survival, highlighting the importance of designing substrates that can achieve positive outcomes for target species at multiple life-stages.