Seeded Acropora digitifera corals survive best on wave-exposed reefs with grazing from small fishes

In a 16-month field study, we deployed aquaria-reared Acropora digitifera spat on engineered coral seeding devices across 10 sites spanning a wave-energy gradient at Moore Reef (Great Barrier Reef, Australia). Two devices were used to investigate the role of grazing: a fish-exclusion device and a featureless control.Mature gravid Acorpora digitifera (Dana 1846) colonies were collected from Moore Reef ahead of coral spawning in February 2022. The corals were transported back to the Australian Institute of Marine Science National Sea Simulator (SeaSim) facility and kept in temperature controlled outdoor aquaria. Gamete bundles were collected, separated, washed and fertilized, then the embryos were transferred to indoor larval rearing tanks where they remained until they reached settlement competency after 5-6 days. Conditioned concrete tabs (14 × 14 mm2; developed under the Reef Restoration and Adaptation Program) were used as settlement substrates. The plugs contained a mixed community of crustose coralline algae and bacteria biofilms that are known to induce settlement of Acorpora corals; these were conditioned in aquaria for 2-months prior to settlement and the corals were held in aquaria (temperature control, flow through, no light, mixed feed of microalage and enriched rotifers) for 2-months after settlement. Corals were seeded to 10 sites on Moore Reef in May 2022. The field deployment followed a hierarchical design including location (n=1), sites (n=10), experimental plots (n=25, 2 devices per plot), device type (n=2, featureless control and a fish exclusion device), contrete settlement tabs (n=3 per device), and coral spat (n=> 5 per tab) . Devices were deployed to the reef crest (3-5 m depth, high tide) and fixed in experimental plots with a cable tie in an upright, horizontal position. Devices and their fixings were removed after 16 months.Data were collected at six timepoints and included quantitative and qualitative assessments to determine the influence of biological (fish abundance and grazing, benthic composition) and environmental (wave energy) drivers on coral growth and survival. Assessments were categorised and analysed by spatial scale. Bayesian hierarchical logistic mixed-effects regression models, principal component and redundancy analyses were used to interogate the data.