Scientists learning from death of staghorn outplants
May 20, 2020
KEY LARGO — Nursery-bred staghorn corals off the Upper Keys have about a 10% chance of surviving past seven years, according to a newly published scientific paper.
Outplanting nursery-raised coral is one of the more recent strategies for preserving coral reef ecosystems, according to biologists.
The study’s findings may prompt further discussion on what’s needed for more viable coral restoration efforts in the future, according to Matthew Ware, Ph.D., lead author of “Survivorship and growth in staghorn coral outplanting projects in the Florida Keys National Marine Sanctuary.”
Over the last 40 years, healthy coral cover in the Florida Keys reefs has declined more than 90%.
“We had hoped to see a better survival rate [of outplanted coral], but it doesn’t appear to be the case. This leads us to ask why aren’t they surviving long-term and has opened up a whole new series of questions. There’s the human management decisions and the biology of corals itself,” Ware said.
“In the ‘70s coral was plagued by disease. In the ‘80s, it was bleaching. Now we are trying to figure out how to help them survive. We need to take what works from the surviving outplants and figure out how to transfer that knowledge to help the other reefs survive. Hopefully, this will drive the action of us all helping in multiple aspects.”
Ware’s research on Coral Restoration Foundation outplants were studied over a two-year period from 2013-15, primarily at Molasses Reef, Carysfort, and Conch Reef and a smaller sites between Dry Rocks and White Bank. CRF did their first ouplant project at Molasses Reef in 2007.
Ware and his team physically documented the on-site health and growth of 2,419 staghorn outplant colonies and looked at more than 3,000 photos to fill in the timeline to measure survivorship and growth.
What they found was that survivorship was initially high but generally decreased after two years.
The nursery coral restoration approach didn’t begin until early 2000s, according to Ware.
“Coral restoration is starting to be old enough now that we can take a look at historical data. It’s still an early approach to restoring reefs and some of the bigger projects around the world were looked at after the first year or two,” he said.
The research found that staghorn coral outplant losses were caused by a combination of coral disease and bleaching, with possible contributions from other stressors, including pollution and predation.
“Survivorship among projects based on colony counts ranged from 4% to 89% for seven cohorts monitored at least five years. Survival models were used to estimate survivorship beyond the duration of the projects and ranged from approximately 0% to over 35% after five years and 0% to 10% after seven years,” the paper says.
“Growth rate averaged 10 centimeters/year during the first two years then plateaued in subsequent years. After four years, approximately one-third of surviving colonies were [equal to or less than] 50 cm in maximum diameter. Projects used three to 16 different genotypes and significant differences did not occur in survivorship, condition or growth.”
The paper concludes that reducing stressors is required before significant population growth and recovery will occur through outplanting. Until then, outplanting can help to protect against local extinction and to maintain genetic diversity in the wild.
Jessica Levy, Coral Reef Foundation’s reef restoration program manager who co-published the paper, said the research has ignited discussions.
“It’s been viewed over 2,500 times within the first week, which is almost unheard of,” she said. “It’s a very honest paper. It clearly points out that the outside stressors have to be reduced.”
CRF has already adapted its outplanting program since the research on initial outplant projects concluded in 2015.
“We’ve already moved our program to include repeat site visits and we concentrate our efforts by consolidating projects onto a few critical sites — meaning, we outplant a large number of diverse corals on a few sites rather than a small amount of coral across many sites,” Levy said.
“We have a large investment in ensuring that the outplants are genetically diverse from each other so some may be more temperature tolerant while others may be more disease resistant.”
Diversifying coral means CRF isn’t putting all the eggs into one basket.
“We outplant four coral species with a lot of genetic diversity. Overall, we have more than 300 different genotypes over 11 different species of corals,” Levy said.
This is an effort of 265 individual volunteers who logged more than 15,000 hours of volunteer dive time with CRF last year.
“Just as corals in the wild population are in decline from all stressors that everyone knows about, outplanted corals face the same stressors,” said Steven Miller, who serves as a director on CRF’s board.
“Some projects had high survivorship after five years, almost 90% in one project, 78% in another and 33% in a third. But there’s high variability in results with some projects doing poorly for reasons that are not always clear but are related to things like coral bleaching, coral disease, the 2010 cold-water event and storms. So, related to the 10% number, this is a theoretical result based on models that we did to learn if there were survivorship differences based on outplanting at different times, different reefs and in different habitat types. It averages everything out, and because a number of projects did not do well, the models are negatively influenced in the long-term.”
Even with high mortality, significant numbers of corals survive on reefs where they were previously absent, Miller said. For example, if over a period of 10 years 10,000 corals are outplanted to a reef, the result is 1,000 corals survive with the highest mortality rates that were modeled.
“That’s a number that is ecologically relevant because the corals attract fish and other animals, they potentially reproduce and they add aesthetic value to the reefs because of their beauty,” Miller said.
Outplanting is just one of a multi-prong approach for reef conservation.
“Outplanting is our expert field but to complement that, we have an education program geared toward reef education and outreach. We try to engage the local and visiting communities to reduce carbon emission, reduce single-use plastics and more. Whatever you do wherever you are, it flows downstream and affects us directly in the Keys.”
CRF is also working to put in place policies and legislation that support reef conservation.
As part of CRF’s annual “Coralpalooza” event, which is being held virtually this year due to the COVID-19 pandemic, a panel of the research authors will convene and interact with the public. For more information, visit coralrestoration.org/coralpalooza-digital-2020.
“Survivorship and growth in staghorn coral outplanting projects in the Florida Keys National Marine Sanctuary” can be read at https://tinyurl.com/ya9w8kop.