Substrate-mediated alterations to hydrodynamic conditions enhances shellfish larval settlement: Implications for artificial reef restoration

Shellfish reef restoration in systems with limited larval supply has generally relied on seeding reefs with hatchery raised juveniles. Although the growth and survival of seeded individuals is sufficient in some systems, to speed up the process of reef formation we need to create substrates that also maximise natural larval settlement. This can be achieved through mimicking the emergent traits of shellfish reefs by creating complex substrates that create desirable flow conditions for settlement. To test shellfish settlement under altered hydrodynamics, we performed larval settlement experiments on tiles with either enhanced surface roughness (sandblasted concrete) or surface complexity (the addition of different configurations of ridges and grooves) relative to control (smooth and flat) tiles. We used particle image velocimetry (PIV) to understand how the hydrodynamics were altered by each tile design, and computational fluid dynamics (CFD) particle modeling to determine if observed larval settlement patterns to complex tiles differed from the retention of modeled passive particles. The addition of surface roughness increased larval settlement and reduced the surface mean flow velocities when oyster shell was used as aggregate. Surface complexity created drastically different hydrodynamic conditions to flat control tiles, which aligned with increases in larval settlement for oysters and passive particle retention. Overall, fine scale hydrodynamics were influenced by both rough and complex surfaces that substantially increase particle retention, and likely also recruitment success. This study highlights the importance of considering hydrodynamics when designing engineered substrates for shellfish reef restoration projects.