NOAA Research Cruise Studies Little-Known Mesophotic Coral Reefs in the Waters of the Auau Channel Between Maui and Lanai

July 6, 2010

On July 7th 2010, eleven scientists from the NOAA Pacific Islands Fisheries Science Center, the University of Hawaii, and Bishop Museum departed Pearl Harbor for a twelve day cruise aboard the NOAA ship Oscar Elton Sette. Chief Scientist for the expedition is PIFSC researcher John Rooney, an employee of the University of Hawaii Joint Institute for Marine and Atmospheric Research.

The mission of the cruise is to study a series of coral reefs in the Auau Channel between the islands of Maui and Lanai in the main Hawaiian Islands. The reefs feature luxuriant expanses of stony, or scleractinian, corals as well as macroalgae, reef fish, and other coral reef organisms. Most reefs in Hawaii are found at depths of about 100 ft or shallower, and are most prolific at depths of around 40 ft. The reefs being studied on this cruise are unique in that they are most prolific at a depth of approximately 280 ft, and have been found to flourish even below 400 ft!

Stony corals typically get much of their nourishment from tiny cells of algae that live within the coral’s tissue. Called zooxanthellae, these algal cells use sunlight and photosynthesis to produce their own food, just as plants on land do. Some of that food is used by the algae's coral host. Scientists have long observed that, as a general rule, as the depth increases light levels decline and corals are less abundant. Because levels of available sunlight are reduced at deeper depths, less food is available from zooxanthellae to support coral growth. So finding fewer corals there made perfect sense. Occasional coral colonies or patches of reef encountered at deeper depths were generally considered to be little more than curiosities.

A rich fish assemblage on a mesophotic reef at 250 ft.
A rich fish assemblage on a mesophotic reef at 250 ft.

However, a few years ago scientists found a complex of stony coral reefs at Pulley Ridge on the West Florida Shelf at a depth of about 215 feet. Their speculations that this might be the deepest reef in U.S. waters garnered national attention and sparked the realization among researchers that reefs did in fact exist at such depths, and that they might be important components of coral reef ecosystems. The term "mesophotic coral ecosystem" (MCE) has been coined to describe these light-dependant communities of corals and other organisms found within the 100 ft to 500 ft depth range.

The Sette expedition is part of a multi-agency study launched after the 2003 discovery of stony corals at depths of at least 230 ft by the Hawaii Department of Land and Natural Resources, Division of Aquatic Resources (HDAR). During a search for communities of black corals in the Auau Channel, HDAR biologists found stony corals on reefs deeper than the reef complex at Pulley Ridge. The discovery of stony corals at these depths was made possible by advances in scuba equipment and methods enabling divers to work at depths well below 100 ft but shallower than waters suitable for research submersibles. Expanded use of the new scuba technology led to more discoveries of mesophotic coral reefs, including those in Hawaii, American Samoa, the Mariana Archipelago, and the Pacific Remote Island Areas. The surveys have shown that MCEs are found around many islands and have demonstrated the uniqueness of the mesophotic reefs in the Auau Channel. The reef complex there is much larger than any other found to date within these island groups.

The July 2010 Sette cruise, and other cruises associated with this study, are designed to gather information about the communities of coral, other invertebrates, reef fish, and algae in the Auau Channel. Scientists hope to learn what ecological roles these reefs play in island and archipelago-wide coral reef ecosystems, and what it is about this site that makes the reefs there so luxuriant. Oceanographic instruments placed earlier at the study site to record information about currents, ocean temperatures and the passage of tagged fish will be recovered during the cruise. A number of scuba dives using mixed gas are also planned, to collect high-definition video of segments of the seafloor to measure the size and species of corals. Other divers will identify, count and measure sizes of fish associated with the reefs. Dives using an underwater camera sled will be conducted during the night to better characterize the distribution of mesophotic reefs, and acoustic surveying (using sonar) will be used to look for communities of small fish, squid, and other organisms that live offshore but may be contributing nutrients to the Auau's mesophotic reefs.

Results of this cruise will improve our scientific understanding of these little known mesophotic reefs and our ability to manage coral reef ecosystems in Hawaii and elsewhere across the Pacific.