Surveys Shed Light on Mesophotic Coral Reefs in the Manuʻa Islands, American Samoa

In November 2012, scientists with the PIFSC Coral Reef Ecosystem Division led an 18-day expedition to survey habitats and marine life on mesophotic coral reef ecosystems around the Manuʻa Islands in American Samoa, a group of 3 lightly populated islands ~ 102 km (55 nautical miles) east of the main American Samoa island of Tutuila. The aim of the research was to document the location and characteristics of mesophotic coral ecosystems in the Manuʻa Islands, information critical to planning and decision making by marine resource managers in American Samoa.

Until a few years ago, the general perception among marine scientists was that reef-building, stony corals were limited mostly to depths of 40 m and shallower because light below that depth was insufficient to enable them to survive. Although individual live corals had been observed at much deeper depths, those coral species were viewed as oddities that were considered too rare to be ecologically important. This general perception fit with the personal experience of most scientists, as the no-decompression depth limit for a scuba dive with a single scuba tank was about 40 m and, more often than not, coral cover was observed to decline down to that depth.

Working at deeper depths of 30-150 m is technologically and logistically challenging, and as a result these deep ecosystems remain relatively unknown areas that are rarely included in ecological monitoring programs or explicitly considered in management activities.

The TOAD was lowered into the sea from the Bonavista II.  Cables running from the TOAD to the vessel transmit data.
The TOAD was lowered into the sea from the Bonavista II. Cables running from the TOAD to the vessel transmit data.

The research team included a trio of CRED scientists employed by the University of Hawaii Joint Institute for Marine and Atmospheric Research — Marie Ferguson, Jeremy Taylor, and John Rooney — and a collaborating scientist from the American Samoa Department of Marine and Wildlife Resources, Tee Jay Letalie. The group collected data at mesophotic depths using an underwater camera sled, commonly called a TOAD (for towed optical assessment device) deployed from the 12-m (40-ft) vessel Bonavista II which is owned and operated by Tutuila-based Pago Pago Marine Charters. The TOAD was launched at randomly selected points around the Manuʻa Islands to capture video imagery of corals and other organisms inhabiting the seafloor and fish species living nearby. The sled features a downward-facing still camera, a forward-facing video camera, a depth sensor, sonar altimeter, and other instruments.

The research team completed 65 camera sled operations around the islands of Ofu and Olosega, covering a length of 30.8 km of seafloor and another 27 dives around the island of Taʻu, covering 11.5 km of seafloor. With the aid of Carlo Caruso, a National Park Service ranger on Ofu for the National Park of American Samoa (NPAS), the scientific party set up a gear storage and data processing station in the NPAS laboratory. Data analysis is proceeding at PIFSC, and the results are eagerly awaited.

TOAD surveys were conducted at 28 locations around the American Samoa island of Taʻu.
TOAD surveys were conducted at 28 locations around the American Samoa island of Taʻu.