A well known coral reef ecosystem

Apra Harbor is one of the most well-known coral reef ecosystems found in Guam. The Harbor is adjacent to Guam’s US Navy base on the West coast of the island. During World War II, it was mainly used as a coal and petroleum fueling station, as well as a repair station for Japanese warships and submarines. During World War II, Guam was recaptured by the US from the Japanese and Apra Harbor has since been used by the US Navy to service and refuel ships.

Even though it is an active military and commercial harbor, it hosts an unusually robust and resilient coral reef community with many well-known dive sites. These sites are most known for hosting a wide variety of coral species and other marine invertebrates.

Reef locations

There are 12 dive sites found throughout Apra Harbor, shown in Figure 1. At the apex of Apra Harbor lie four shoals: Jade Shoals, closest to shore, followed by Middle Shoals, Big Blue Reef (south of Middle Shoals), and Western Shoals, which lies furthest seaward. A shoal is a submerged ridge that has risen near the surface of a body of water.

Apra Harbor’s shoals are coral reefs, whereas most other shoals are sandbars. These shoals can be very dangerous when navigating watercraft in harsh conditions. Ships use nautical charts and other bathymetric maps to avoid colliding into these shoals and damaging corals. Further West from the shoals, along the southern rim of the Harbor, lie San Luis Beach, Gab Gab I, and Gab Gab II. Finger Reef is also on the southern rim of the Harbor but closer the mouth.

Two shipwrecks, the Tokai Maru and the Cormoran II, shown in Figure 2, lie side by side in 120 feet of water, directly across from San Luis Beach near the northern rim of the harbor. The Cormoran II was sunk in the harbor during World War I and the Tokai Maru was sunk 26 years later, during World War II. A diver can, therefore, stand between these two wrecks and touch ships from both WWI and WWII at the same time.

Further west on the northern rim and closer to the mouth of the harbor lie Harley Reef, Val Bomber and American Tanker, which lies close to the mouth of the harbor. Many of these sites have unique world war landmarks and marine life that attract thousands of divers annually to Apra Harbor. For example, Harley Reef is characterized by a World War II Harley Davidson motorcycle overgrown by coral and algae, shown in Figure 3. Val Bomber is a World War II Japanese Aichi D3A bomber that lies at approximately 80 feet depth. American Tanker is highlighted by another iconic World War II artifact; a large tanker that transported fuel from the US to Guam and was sunk after it was no longer needed.

Reef biodiversity

Porites rus, shown in Figure 4, is by far the most common coral species in the Harbor. It lives in a variety of depths, ranging from five to 90 feet. Porites rus and other corals species, such as Porites lutea and Porites lobata, host a variety of marine fauna such as the green sea turtle (Haggan), shown in Figure 5. Green sea turtles are one of the most sought-after creatures among scuba divers in the Harbor. They can be found resting in crevices and in the shade under Porites rus.

Figure 6 presents photos of some of the common animals associated with the coral reefs of Apra Harbor. Sea cucumbers, called balate in CHamoru, such as the black sea cucumber (Holothuria atra) and the leopard sea cucumber (Bohadschia argus) are also very common in these reefs and can be found at about the same depths as the corals. Sea cucumbers feed on debris in the sand, and in doing so, help clean it. Their feeding pattern is similar to how earthworms feed on, and rework, soil. Sea urchins, such as the burrowing sea urchin (Echinometra mathaei) are found living in crevices between coral colonies and graze on macroalgae that can overgrow corals.

Reef fish like the Forsten’s parrotfish (Chlorurus sordidus) are found feeding on coral and swim in schools throughout the reef. Moray eels, another sought-after creature that scuba divers like to photograph, are also usually found hiding in crevices under corals. For more pictures of Guam’s corals and marine life, visit guamreeflife.com.

Uniqueness of Apra Harbor’s reefs

The reefs of Apra Harbor have persisted through two world wars, as well as ongoing anthropogenic activity. Since the 1940s, the Harbor has undergone extensive alteration, such as the construction of the Glass Breakwater, dredging, new piers and wharves, and the construction of artificial shorelines along the northeastern and southeastern boundaries of the harbor.

Such human activity would normally harm nearby reefs, leading to coral smothering, crushing and mortality. This is because of sediment plumes and turbidity caused by construction that prevents the corals’ zooxanthellae from photosynthesizing.

Despite all the human activity within and near the harbor, Apra Harbor’s reefs are thriving in certain places.

About the author

Karim Primov was a graduate student in 2018 at the University of Guam Marine Lab. He graduated in 2020. His thesis was “The Phylogenetics and Population Genomics of Cryptic Lineages of Massive Porites on Guam.”

He is studying the population genetic structure and levels of genetic diversity of the shallow-reef massive coral Porites lutea around Guam. His major research interests include population genetics, benthic ecology, and invertebrate zoology.

For further reading

Burdick, David. “Regional Guide to the Health of Guam’s Reefs.” Guam Reef Life. Last modified 1 January 2017.

Haw, Jim. “Dreading the Dredging: Military Buildup on Guam and Implications for Marine Biodiversity in Apra Harbor.” Scientific American, 30 May 2013.

Lewis, Mark. “The Cormoran II And The Tokai Maru.” Forbes, 11 February 2009.

Pacific Wrecks. “Apra Harbor, Territory of Guam, United States of America (USA).” Last modified 14 May 2021.