The Role of Outplant Density on Coral Survivorship, Growth, Predation Impacts, and Disease Spread

In February 2023, Restoration partners from The University of Miami, Nova Southeastern University, Mote Marine Laboratory, Biscayne National Park, NOAA Southeast Fisheries
Center, and the Florida Aquarium collaborated to evaluate the role of coral outplant density on the survivorship and growth of corals with branching (Acropora palmata) and massive
(Colpophyllia natans, Diploria labyrinthiformis, and Orbicella faveolata) colony morphologies. This project was extended to allow us to continue monitoring corals up to 1 year after outplanting.

Fish predation was highest during initial surveys and was the main driver of tissue losses for massive corals. No evidence of disease was observed on these coral outplants. Predation impacts
varied between sites, by coral species and density treatment (for one species, C. natans). Predation on C. natans was significantly lower when outplanted in high-density arrays (12 corals
per m-2) compared to corals outplanted in low- (3 corals m-2) and medium- (9 corals per m-2) density arrays, suggesting that predator dilution can limit predation impacts on a per-coral basis
and that predation-susceptible corals may benefit from high-density deployments on reefs with high fish predation. In contrast, outplant density did not influence the impacts of fish predation
on O. faveolata and D. labyrinthiformis. Fish predation was highest on C. natans, followed by O. faveolata, and D. labyrinthiformis. As in prior studies, fish-predation impacts were concentrated
during the first week after outplanting and declined over time.

A total of 4,140 A. palmata fragments from 50 parent genotypes were provided by Mote Marine Laboratory for this project. These fragments were deployed onto 5 sites throughout Florida’s
Coral Reef in arrangements of 3, 9, and 12 corals per cluster/base. These outplanted corals were monitored visually and photographically for one year after outplanting. The surviviorship of the
A. palmata clusters was high through August 2024 (6 months after outplanting), ranging from 86.3% (NSU) – 100% (NOAA). These high survivorship values clearly indicate that A. palmata
corals grown from parents from the Florida Keys can be transferred to the northern counties (Miami-Dade and Broward) as part of an assisted gene flow project. The Summer 2023 bleaching event caused a significant amount of bleaching and mortality across sites with clear regional patterns. Coral outplants in the Lower Florida Keys, which experienced 24.8 Degree Heating Weeks (DHW), had 100% mortality, followed by corals in Key Largo (~ 50% mortality), Biscayne National Park (96% mortality), Miami (< 3 % mortality), and Broward (16% mortality caused by predation and not bleaching). The symbiont community structure was assessed at the time of outplanting and after 6 months (prior to the onse of the bleaching) and while the corals outplanted had 8% abundance of Durusdinium (Symbiodinium was present at > 72% average abundance), the abundance of Durusdinium declined to non-detectable levels before the onset of bleaching and thus did not provide any protective benefits during the thermal anomaly. Productivity was significantly higher in the low-density clusters compared to the high density treatments (p<0.05), suggesting that outplanting A. palmata in lower densities (e.g. 3 corals per base) is a preferred method to maximize growth in this species.