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Restoring the Caribbean's endangered coral reefs

In a world hidden by water, staghorn corals can be found in the national parks that touch the Caribbean Sea and the Straits of Florida.

The staghorn coral is a branching, stony coral with cylindrical branches ranging from a few centimetres to over two metres in length and height.

Due to their bush-like features, staghorn coral reefs provide complex habitat for fish and other coral reef organisms.

In regard to human importance, abundant coral reefs, such as those made up of the staghorn coral, provide shoreline protection from storms and large waves as well as millions of pounds in recreation and tourism.

Despite their significance to both the human and natural world, more than 95% per cent of these distinctive corals have died since the 1970s. It is reasonable to be fearful that the staghorn coral will soon vanish from our oceans due to rapidly worsening environmental conditions that threaten their existence.

Image credit: Bulb

The primary threat to staghorn corals and causing their unprecedented decline over the past 40 years is white-band disease. This syndrome completely destroys the coral tissue and is particularly harmful to Caribbean acroporid corals, of which staghorn corals are.

Much research suggests that white-band disease has been strongly coupled with thermal stresses associated with climate change, which has contributed to the regional decline of these once-dominant corals.

According to the National Oceanic and Atmospheric Administration (NOAA), populations of staghorn corals appear to consist mostly of isolated colonies or small groups of colonies compared to the vast thickets (dense groups of coral) once prominent.

Thickets, which are found in very shallow waters, now exist at a handful of locations and with successful reproduction being very rare, it is hard for the staghorn coral populations to increase.

Today, staghorn coral populations are listed as "threatened" under the Endangered Species Act.

Coral reef restoration projects have been in place in various locations around the world for a number of years now. In terms of the staghorn corals, new research indicates that a vigorous transplanting initiative could help recover the dwindling numbers of the species found in the Caribbean and Straits of Florida.

The research is based on reef restoration projects in the Florida Keys National Marine Sanctuary that borders Dry Tortugas National Park.

In 2006, NOAA initiated a recovery plan. A central part of this plan is 'outplanting', in which corals are cultivated in a protected area before being transplanted to the restoration site.

While outplanting initiatives have been in place for many years, only recently has enough time passed to analyse their long-term potential.

Through the use of techniques such as photographic monitoring and in-person measurements to assess coral colonies, a team from Florida State University have been researching 2,419 coral colonies outplanted to 20 different sites in the Florida Keys between 2007 and 2012 by the Coral Restoration Project.

Image credit: The Nature Conservancy

The analysis revealed that survivorship, i.e. the percentage of colonies containing living tissue, was high for the first two years after outplanting, but declined in subsequent years.

Within 7 years of outplanting, 0-10 percent of colonies are thought to survive.

The results indicate that large numbers of colonies would need to be outplanted to start with, allowing ecologically relevant numbers to survive longer-term.

Despite limitations in the analysis, including a lack of comparison to natural populations at outplant sites, differences in colony numbers and outplant strategies among sites, and low sample sizes for some years, the findings suggest that outplanting could help restore staghorn coral populations.

Mitigating stressors that harm and kill corals such as disease and bleaching - which are both related to global warming - are necessary in order to achieve full, long-term recovery.

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