Elkhorn and Staghorn Coral Surveys

Florida Keys National Marine Sanctuary

Scientists Study Threatened Corals

Coral researchers from Nova Southeastern University (NSU) Oceanographic 

Center (formerly with University of North Carolina Wilmington) have been 

conducting annual surveys of corals and other reef organisms in the Florida 

Keys National Marine Sanctuary since 1999. In more recent years, surveys 

have focused on staghorn coral (Acropora cervicornis) and elkhorn coral (A. 

palmata). In 2006, these two important reef-building species were listed as 

threatened under the Endangered Species Act due to significant declines in 

their populations throughout the wider Caribbean, including the Florida Keys.

Declines and lack of recovery have most likely resulted from a combination of 

stressors that has also affected other marine life in the region. Widespread 

coral bleaching (the loss of beneficial algae living in their tissues) and disease, 

especially white band disease, have caused significant losses in Acropora

species in the past and still threaten their recovery. Destructive storms and 

predation by coral-eating snails, damselfish and fire worms have also contributed to tissue loss and mortality. 

Surveys Collect Information on Habitats and Populations

Surveys of benthic (bottom) reef habitat have provided sanctuary managers with critical information on habitat distribution, 

sizes, and condition of staghorn and elkhorn corals in Keys’ waters. Data have also been used to estimate the population, or total 

number of coral colonies, of each species. Surveys were conducted in collaboration with 

state and federal partners and encompassed a variety of coral reef habitats including 

hard-bottom, nearshore reefs and deeper fore-reefs to a depth of 89 feet. Sites were 

located throughout the Keys and both inside and outside of sanctuary protected zones 

(Sanctuary Preservation Areas, Research-only Areas, and Ecological Reserves).

At each survey location, science divers recorded the number of Acropora coral colonies, 

sizes of colonies, live tissue area and general condition of corals found witihin 3.3 

feet on either side of a 50 foot tape measure, or transect line. During the surveys, 

scientists also recorded the presence and condition of other corals, long-spined urchins 

(Diadema antillarum) and marine debris. Since 1999, over 2,100 sites in the Florida 

Keys have been assessed and data are available for each species by location, habitat 

type, colony size and zone type (inside vs. outside of a sanctuary protected zone).

Most Staghorn Corals are Located Outside of Sanctuary Zones 

In the 2012 Keys-wide surveys that included 600 locations from northern Biscayne 

National Park to Key West, staghorn coral colonies were found in nearly every reef 

habitat type. Mid-channel and offshore patch reefs, as well as shallow hard-bottom and 

inner reef tract habitats, had the greatest colony densities (measured as the number of 

corals per square meter). Colony sizes were also greatest in these habitats. Historically, 

large thickets of staghorn also existed on some deeper fore-reef areas, but such 

thickets have not been documented in the past decade. Population estimates indicate 

that approximately 98% of the existing staghorn corals in the Florida Keys, excluding the 

Dry Tortugas, are located outside of sanctuary protected zones. 

http://floridakeys.noaa.gov

Over the years sanctuary 

biologists have developed 

techniques to restore coral 

reefs damaged by vessels. 

Today, restoration efforts 

are also focusing on reefs 

that until the recent past had 

dense stands of staghorn 

coral. In a project coordinated 

by The Nature Conservancy 

and funded by a NOAA grant, 

thousands of nursery-grown 

staghorn corals are being 

transplanted to reef sites in 

the sanctuary where they once 

grew. Corals are being grown 

in underwater “nurseries” 

maintained by The Coral 

Restoration Foundation, Mote 

Marine Laboratory and Florida 

Fish and Wildlife Conservation 

Commission. 

Nursery-grown corals thrive after being transplanted. 

Photo: Coral Restoration Foundation
Staghorn Colonies Tend to Be Small in Size, but Number in Millions

Measurements of staghorn colonies show a predominance of smaller-sized colonies, with about 65% of the colonies measuring 

less than 12 inches in diameter. This result has been evident for over a decade and suggests that while staghorn corals are broadly 

distributed in the Florida Keys, they are not attaining very large sizes or developing into larger thickets. Only 4% of the staghorn 

corals had a maximum diameter of 27 inches or greater. In addition, predation by snails, damselfishes and fire worms continues 

to be observed on relatively small colonies. Despite these continued threats and relatively small size of colonies, the population of 

staghorn corals in the Florida Keys has been stable since the study began in 1999. NSU scientists estimate that staghorn colonies 

number in the millions, thereby providing ample seed stock for recovery. Additionally, successful restoration results with staghorn 

corals in offshore nurseries suggest that environmental conditions support recovery of this species.

Most Elkhorn Corals are Found on Spur and Groove Reefs

Elkhorn coral is much less abundant than staghorn coral and NSU scientists estimate that the remaining Keys’ population is 

several hundred thousand colonies. Surveys of elkhorn coral indicate that it is also less broadly distributed than staghorn coral. 

Although most of the spur and groove reefs in the Florida Keys were 

assessed in 2012, elkhorn corals were present at only 20% of the 

sites visited. Surveys from the late 1990s documented a few elkhorn 

coral colonies on offshore patch reefs, back-reef rubble, and shallow 

hard-bottom habitats. However, in 2012, nearly 80% of the elkhorn 

colonies were found in only two habitat types, high-relief spur and 

groove reefs and offshore patch reefs. Prior to the die-off that began 

in the late 1970s, elkhorn coral was found in major thickets at 

perhaps two dozen locations throughout the Florida Keys, reflecting 

the named reefs found on nautical charts. Today, elkhorn is found at 

less than half the sites where it previously thrived in recent history. 

Elkhorn corals exhibited a mixture of size classes with both large 

and small colonies detected. A few spur and groove reefs in the 

Florida Keys support stands or thickets of elkhorn corals, not just 

isolated colonies. For example, South Carysfort Reef, Horseshoe 

Reef, Grecian Rocks and French Reef have thickets that range in size 

from 49 to 65 feet in diameter. Most, but not all, thickets are located 

inside Sanctuary Preservation Areas (SPAs). When compared with 

nearby sites outside of protected zones, the density, average colony size, and occurrence of elkhorn corals were all greater inside 

the protected SPAs. Approximately 52% of the elkhorn coral population in the Florida Keys, excluding the Dry Tortugas region, is 

found within sanctuary protected zones. This result is not surprising since many protected zones are located at popular diving sites 

in high relief spur and groove reef habitat where elkhorn corals were historically abundant.

Study Highlights Habitat Differences of Acropora Corals

Results from the surveys identify which habitats currently support elkhorn and staghorn corals and therefore could be important 

for species’ recovery. Elkhorn coral is found primarily on spur and groove formations located on the fore-reef, even though it 

once formed abundant thickets on many reef flats. Staghorn coral is found in a variety of habitats, but is especially prevalent in 

patch reefs and shallow hard-bottom areas where smaller colonies predominate. Until relatively recently, deeper fore-reefs also 

supported staghorn thickets. 

No-trap Zones Help Prevent Damage to Corals

For years, sanctuary anchoring regulations and mooring buoys have helped prevent anchor damage to corals. In 2012, NOAA 

Fisheries Service, the agency charged with managing threatened marine species, implemented 60 relatively small no-trap zones 

around known stands of staghorn and elkhorn corals in federal waters within the Florida Keys National Marine Sanctuary. These 

zones, which prohibit the use of commercial lobster traps, are expected to reduce injuries to corals from traps that move during 

storms and damage corals. Coral survey data collected by NSU scientists were used by fisheries managers to identify coral stands 

that were candidates for the no-trap designation. While many elkhorn corals are located within SPAs where fishing is not allowed, 

no-trap zones will offer additional protection to corals outside these areas. Scientists will continue to monitor these two threatened 

species to provide critical information to ecosystem managers.

Principal Investigators: Drs. Steven Miller and Mark Chiappone, Nova Southeastern University Oceanographic Science Center 

Primary Funding Sources: NOAA Coral Reef Conservation Program, Endangered Species Acts Project, FKNMS

Written by: Nancy Diersing, Florida Keys National Marine Sanctuary

Publication Date: April 2013

Large elkhorn thickets are found on some fore reefs.