Large-scale Outplanting Designed to Assess Coral Restoration Feasibility in Response to Stony Coral Tissue Loss Disease

The survival rate of outplanted coral over the two-year study was 77%. Survival rates ranged from 71.4% at lower Keys sites to 85.9% at middle Keys sites. Species-specific survival rates were ~ 86% for Montastraea cavernosa, ~ 78% for Orbicella faveolata, and ~ 68% for Pseudodiploria clivosa.

The mean SCTLD rate of living colonies across all survey periods was 1.4%. A total of 194 (16.6%) of the outplanted colonies were recorded as exhibiting signs of SCTLD infection during at least one survey period. Of those colonies, 76 (39%) had died by the end of the survey, which represents 6.5% of the total outplanted colonies.

Species-specific probabilistic modeling did not identify statistically significant differences in the probability of colonies exhibiting external signs of SCTLD infection across survey regions, reef strata, or colony source. Moreover, the prevalence of SCTLD on natural coral communities at the outplant sites was not different than the control sites and consequently yielded no support for the hypothesis that outplanting SCTLD-susceptible coral species will increase the prevalence of the disease in the natural coral community in the immediate area. Therefore, we recommend that Florida’s resource managers continue efforts to develop a coral reef restoration strategy that includes outplanting SCTLD susceptible coral species in SCTLD endemic regions in Florida.

Neither host genetic lineages nor algal symbiont types of the genotypes tested significantly affected SCTLD susceptibility, negating the hypothesis of SCTLD-resistant coral lineages. Future research should target specific gene regions known to correspond to coral immunity to further the chances of identifying genomic variations that may affect SCTLD resistance or survival. Furthermore, based on the results from this study and previous work, research into the drivers of SCTLD should combine genomic approaches with microbial, transcriptomic, metabolomic, or environmental conditions.

Probabilistic modeling identified differences in each of the three species across the survey area. In general, survival of all three species was higher on sites located comparatively closer to shore than those on the offshore sites. Coral abundance of the natural coral communities along the FCR is typically comparatively higher on nearshore habitats to those offshore, so it is reasonable to suspect that there were additional stressors acting on corals outplanted at the offshore sites compared to those outplanted closer to shore. We also note that P. clivosa is typically confined to shallower habitats than M. cavernosa or O. faveolata, so lower survival at offshore sites is in part likely depth related. P. clivosa was included in this project as it was, apart from M. cavernosa and O. faveolata, the only other SCTLD-susceptible species being maintained in coral propagation facilities in sufficient numbers to accommodate this study’s experimental design and is the only one of the three species considered by the SCTLD case definition as being highly susceptible to SCTLD.

Colony source was also a predictor in the probability of survival to varying degrees among the three species and was a particularly important predictor of P. clivosa survival. Differences in colony source, coupled with clear geographic differences in survival, underscore the need for a coral reef restoration strategy that continues to prioritize the use of genetically diverse coral assemblages in its coral outplanting efforts. We recommend that genotyping should be used within nurseries and before coral outplanting projects specifically to maximize genetic variation to increase coral reef resilience in response to the stressors threatening coral reefs.

A protocol using ImageJ software was developed and used to examine species-specific growth information. As with colony survival, the net growth of the outplanted colonies that survived to the end of the study varied by species and across geographic regions and colony sources.

As with earlier experimental outplant efforts, predation by corallivorous finfish was common on all three of the outplanted species during the first-month post-outplant then declined. Predation was particularly intensive at sites located off Miami-Dade and Broward Counties. The probability of whole colony survival decreased as predation intensity – measured as the proportion of coral tissue damaged or removed – increased. However, the survival rate of less severely damaged colonies was similar to colonies that had not exhibited evidence of finfish predation. Yet, we note that we cannot discount that many of the coral fragments that went missing early in the study before they fused with one another may have been the result of unrecorded finfish predation. We further note that there was evidence that net growth of M. cavernosa colonies measured at the end of the study was negatively affected by finfish predation. We had hoped to confirm the role of coral lipid content as a driver of predation on newly-outplanted colonies and had opportunistically retained some coral fragments to test that hypothesis. However, lipid data were ultimately quite limited, and we did not find an association between predation rates of coral genotypes and their pre-outplant lipid content.