On Wednesday 6 May, PLoS One released my long-awaited article “Survivorship and growth in staghorn coral (Acropora cervicornis) outplanting projects in the Florida Keys National Marine Sanctuary” (https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0231817)!
Staghorn corals are critical reef-builders throughout the Caribbean and Florida; however, their populations experienced a drastic decline in the 1970s and 1980s due to disease, coral bleaching, and other disturbances. Where once they grew into large, dense thickets, most populations of the coral now in Florida represent small, isolated colonies. This decline has severely threatened reef complexity, biological diversity, and aesthetic quality. In 2006, both staghorn coral and their genetic cousin, elkhorn coral (Acropora palmata), were placed on the U.S. Endangered Species List.
In an effort to restore these populations, coral nurseries have been established to rear large numbers of colonies for outplanting back to the reef – similar to terrestrial nurseries supplying trees for forest restoration. As these efforts have been around for little more than a decade, the growth and survival of outplanted colonies and the broader efficacy of such restoration efforts is currently being evaluated.
The Coral Restoration Foundation (https://www.coralrestoration.org/) started its outplanting efforts in 2007 on Molasses Reef off Key Largo, Florida and to date have grown tens of thousands of staghorn colonies in the Tavernier nursery. The growth and survival of 2,419 of these colonies outplanted from 2007 to 2013 were evaluated using photogrammetry and in-situ measurements on SCUBA.
Survivorship of outplanted colonies was generally high for the first 2 years but variable over longer timeframes (4% to 89%). Weibull survival models projected 0% to 10% survival over 7 years – the longest project record available. Colony condition (i.e., the percentage of live tissue covering on a given colony) also remained high during the first 2 years – greater than 85% coverage. Unfortunately, like survival, these metric of coral health also declined with time.
Coincident with decline in survivorship after 2 years was an apparent plateau in coral growth. By this time, most corals did not exceed 40cm in diameter. By the end of the study, the average maximum diameter approached 50cm. However, 16.5% of the surviving colonies reached 1 meter across or more, indicating that there is still potential for colonies to grow to their former size. The apparent plateau may represent a functional size limit as a combination of colony fragmentation from wave action and the energy balance between axial extension, branching, tissue repair following predation, and reproduction.
Given these growth and survival observations under present-day conditions, it may take hundreds to thousands of years to meet the NOAA Recovery Plan’s criteria for staghorn coral restoration in the Florida Keys. Increasing the number of coral outplants may help restore some of the lost ecosystem function but will likely not alter the percentage of colonies lost through time. If the restoration times are to be significantly reduced through increased colony survival, regional and global stressors including climate change and marine pollution must be addressed. Though this may sound daunting, this article represents just an early step in a much larger coral restoration story. Understanding how genetics and adaptation (e.g., heat tolerance, calcification rate, growth rate, reproduction through fragmentation vs. spawning, coral microbiome and Symbiodinium symbiosis), environmental conditions (e.g., temperature, nutrients, carbonate chemistry, waves/storms), ecological conditions (e.g., community structure, herbivore density), and management actions (e.g., outplanting in optimal locations, increased outplant capacity with improved bulk attachment techniques, selective breeding, marine protected areas, socioeconomic investment) all interact will prove crucial in our attempts to restore these beautiful ecosystems.
Check out the article at PLoS One: