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Guam's Coral Reefs

Polyps of a soft coral in Apra Harbor
Polyps of a soft coral in Apra Harbor (Photo: D. Burdick)

The inshore coastal waters around Guam are characterized by a variety of interrelated habitats including coral reefs, seagrass beds, mangrove forests, and lagoons. Fringing reefs, the predominant type of coral reef, extend around most of the island, but there are also patch reefs, submerged reefs, barrier reefs, and offshore banks. There are two barrier reef lagoons on Guam: Cocos Lagoon on the southern tip and Apra Harbor on the west coast.

Coral Reef Area

Recent estimates for the combined area of coral reef and lagoon is approximately 42 square miles (108 km2) in nearshore waters (within three nautical miles from shore) at depths between 0-18 feet (0-5.5 meters) and an additional 42.5 square miles (110 km2) in federal waters at a distance greater than three nautical miles offshore. NOAA scientists estimate that of the 105 square miles (273 km2) of potential coral reef habitat up to a depth of 600 feet (183 meters) within the Exclusive Economic Zone (including offshore banks), 78 square miles (203 km2) of reef are directly associated with the island of Guam. There are also well-developed lagoons, the best-known of which are Cocos Island Lagoon, Ipan Beach Lagoon, the Ylig River Estuary, and Tumon Bay (Blue Lagoon).


Mangrove forests are poorly represented on Guam and are now restricted to two coastal areas. Much of the original mangrove forest was destroyed by dredging years ago. Where they do occur, only a few of the typical western Pacific mangrove species are represented. Among these are: Avicennia marina var. alba, Bruguiera gymnorrhiza, Heritiera littoralis, Hibiscus tiliaceus, Lumnitzera littorea, Nypa fruticans, Rhizophora mucronata, and Xylocarpus moluccensis. Apra Harbor has the largest and most developed mangrove forest in Guam, as well as in the entire Mariana Archipelago. Achang Bay is the only sizeable area of mangrove forest in the southern tip of Guam.


Only three species of seagrasses occur in Guam waters: Enhalus acoroides, Halophila minor, and Halodule uninervis. The largest species, Enhalus acoroides, inhabits the sandy-silt areas near the mouths of rivers in the southern half of Guam. Halodule uninervis is abundant in Cocos Lagoon; a few patches can also be found on the shallow sandy reef flats near shore in the southern bays. Halophila minor can be found in shallow sandy reef flats and deeper lagoon environments.


Guam is located close to the Indo-Pacific center of coral reef biodiversity and has a species-rich marine ecosystem. Over 5,100 species of marine organisms have been identified from Guam’s coastal waters, including more than 1,000 species of fishes and near 500 species of hard corals. Major threats to Guam’s biodiversity include illegal fires, harmful water recreation practices, and land-based sources of pollution from adjacent watersheds.

Guam is home to more than 300 species of marine macroalgae, approximately 400 known species of hard corals, 77 species of soft corals, 128 sponges, 295 species of Foraminifera, 53 flatworm species, 1,673 mollusks, 104 marine polychaete worms, 840 arthropods, 194 echinoderms, 117 species of sea squirts (Ascidiacea), three sea turtles, and 13 species of marine mammals. Nearly 100 families of fishes, containing more than 880 species, occur on Guam’s coral reefs, half of which are contained within 10 families. These include the gobies (Gobiidae), wrasses (Labridae), damselfish (Pomacentridae), snappers (Lutjanidae), blennies (Blenniidae), cardinalfish (Apogonidae), butterflyfish (Chaetodontidae), surgeonfish (Acanthuridae), moray eels (Muraenidae), and pipefish (Syngnathidae). The species-rich area of the southern Marianas contains more than 1,000 species of epipelagic and demersal fishes to 656 feet (200 meters).

The Guam green sea turtle
The Guam green sea turtle. (Photo: David Burdick)

Three of the seven species of the world's marine turtles have been reported from the coral reefs of the Mariana Archipelago: the green turtle (Chelonia mydas), hawksbill turtle (Eretmochelys imbricate), and leatherback turtle (Dermochelys coriacea). Harvesting of sea turtles for food was legal on Guam until 1976, when numbers were so low that the government chose to ban fishing of turtles. Yet, even with a ban in place, poaching remains a significant problem in Guam and continues to contribute to the decline of turtle populations in this region.

Crown-of-thorns Starfish

Raccoon butterflyfish
Raccoon butterflyfish (Chaetodon lunula) at Gun Beach, in the Tumon Bay Marine Preserve, Guam (Photo: David Burdick)

Macroinvertebrate surveys (towed divers’ field observations of invertebrates that can be seen without the aid of a microscope) in 2005 and 2007 revealed that macroinvertebrates, with the exception of sea urchins and crown-of-thorns starfishes (COTS), were in relatively low abundance around Guam’s coral reefs. The current abundance of COTS (Acanthaster planci) in Guam is a direct result of the outbreak of tens of thousands of COTS in the late 1960s and early 70s.  This outbreak devastated corals in the northwest quarter of Guam. Live coral cover in some areas was reduced from 50-60% to less than one percent. Most corals recovered to some degree (>60%) by the early 1980s. However, continued coral resiliency has been affected by ongoing degradation of water quality, low abundance of target fish species, additional COTS outbreaks, and reduced rates of coral recruitment.


Of the fisheries catch in the coastal waters, crustaceans make up a large portion of non-finfish catch. There are several hundred species of crustaceans on Guam's coral reefs, but only about nine species of crab are targeted, including land and marine crabs. Carpilius maculatus (the “7-11 crab”) and Etisus splendidus (the splendid pebble crab) are well fished. Spiny lobsters (Panulirus pencillatus and other species) and slipper lobsters (Scyllarides squamosus and Parribacus antarcticus) catches are also highly prized. Mantis shrimp and freshwater shrimp (Macrobrachium rosenbergii) are also harvested.

Echinoderms harvested include two species of sea urchins, the priest-hat urchin or hairy pincushion urchin (Tripneustes gratilla) and the Rock boring or math sea urchin (Echinometra mathaei), as well as two species of sea cucumbers, the warty Selenka's sea cucumber (Stichopus horrens) and the black sea cucumber or lolly fish(Holothuria atra). The introduced marine gastropod, Trochus or top shell (Trochus niloticus), is one of the larger edible shellfish that can be found on Guam's fringing reefs and reef flats. Species of octopus, including the common reef octopus (Octopus cyanea) and the Hawaiian night octopus (Octopus ornatus) are also popular mollusk food items.

Shore-based finfish harvesting is by cast nets, surround nets, spear-fishing, hook and line, hooks and gaffs, and gill netting. The principal fishes caught by these methods are surgeonfishes, jacks, rabbitfishes, goatfishes, snappers, emperors, and rudderfshes. Boat-based fishing adds barracudas and mackerels to this list.

According to The State of Coral Reef Ecosystems of Guam (Burdick et al. 2008), fish abundance and diversity is generally low in Guam’s inshore waters. Species diversity and abundance are higher around the northern and eastern shores where the reefs are more rugose and live coral cover is greater, providing additional available habitat for colonization by benthic sessile organisms and shelter and foraging area for mobile organisms. 

The splendid pebble crab, Etisus splendidus
The splendid pebble crab, Etisus splendidus (Photo: University of Florida)
7-11 crab (Carpilius maculatus) possesses a very distinctive color pattern
The 7-11 crab (Carpilius maculatus) possesses a very distinctive color pattern - light brown with a few large red spots. (Photo: Dr. Dwayne Meadows, NOAA/NMFS)