Survey of mesophotic coral reefs completed in the Manu`a Islands, American Samoa

Scientists Marie Ferguson, Jeremy Taylor, and John Rooney from the PIFSC Coral Reef Ecosystem Division (CRED) and Tee Jay Letalie of the American Samoa Department of Marine and Wildlife Resources (DMWR) recently completed an 18-day project to survey habitats and communities of mesophotic coral reef ecosystems around the Manu`a Islands in American Samoa. The Manu`a Islands are a group of 3 lightly populated islands ~102 km (55 nautical miles) east of the main island of Tutuila (Fig. 1).

Figure 1. Map of American Samoa.

Figure 1. Map of American Samoa.

A similar survey was conducted around Tutuila in 2008, leading to the publication of a paper, “Mesophotic communities of the insular shelf at Tutuila, American Samoa,” in the journal Coral Reefs and contributing to other products designed to support management of coral reef ecosystems. The success of that project inspired CRED and DMWR scientists, with the support of NOAA’s Coral Reef Conservation Program, to conduct surveys around the Manu`a Islands.

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 too rare to be ecologically important. This general perception fit with the personal observations of most scientists: this maximum depth of 40 m coincided with the approximate maximum depth of a no-decompression limit for a scuba dive with a single scuba tank, and, more often than not, coral cover did decline with depth.

In 2003, however, reports surfaced in both scientific literature and news services about a healthy reef at depths of 65 m off the coast of Florida. Over time, more and more stories have appeared about light-dependant coral reefs found at depths well below 40 m. Today, these communities of corals, algae, fishes, invertebrates, and other organisms are referred to as mesophotic coral ecosystems (MCEs). They often contain well-developed coral reefs and diverse and abundant fish communities, and they are ecologically important components of overall coral reef ecosystems.

Still, working at depths of 30–150 m is technologically and logistically challenging, and these deep ecosystems remain relatively unknown areas that are rarely included in ecological monitoring programs or explicitly considered in management activities. The aim of CRED’s recent study was to characterize MCEs in the Manu`a Islands and gain an understanding of where they are found to provide information critical to resource managers in their decision making about these ecosystems.

The team towed an underwater camera sled (Fig. 2) at randomly selected points around the Manu`a Islands on Nov. 1–13 to capture video imagery of organisms growing on the seafloor and fish species living nearby. The sled, more commonly referred to as TOAD (for towed optical assessment device), features a downward-facing still camera, a forward-facing video camera, a depth sensor, sonar altimeter, and other instruments. While the sled was in the water, the operator sat in front of a control console (Fig. 3) on the boat and watched a live feed from the sled’s video camera. With a switch mounted on the right side of the control console, the operator could automatically raise or lower the TOAD in the water. Video from the onboard video camera was recorded on a VCR in the console, and data on the sled’s track were recorded on a laptop computer. After each day of surveying, still photos were downloaded from the camera once the TOAD was back aboard the vessel.

Figure 2. The towed optical assessment device is lowered into the ocean with a 12-V pot hauler and an umbilical cable.

Figure 2. The towed optical assessment device is lowered into the ocean with a 12-V pot hauler and an umbilical cable.

Figure 3. Scientist Marie Ferguson stands next to the control console for the camera sled. NOAA photo

Figure 3. Scientist Marie Ferguson stands next to the control console for the TOAD. NOAA photo

Figure 4. Bonavista II, docked alongside the pier in Ofu Harbor. NOAA photo

Figure 4. The chartered vessel, Bonavista II, docked alongside the pier in Ofu Harbor. NOAA photo

The TOAD was deployed off the 12-m (40-ft) vessel Bonavista II (Fig. 4), which is owned and expertly operated by Tutuila-based Pago Pago Marine Charters. The seaworthy Bonavista II is regularly used by resource management personnel from several agencies to transport their scientists and managers to work around the Manu`a Islands and Rose Atoll. While the captain and first mate stayed aboard the vessel each night, the scientists stayed at the Vaoto Lodge on the southeastern coast of Ofu. Carlo Caruso, a park ranger on Ofu for the National Park of American Samoa, was very helpful and generously allowed the scientists to use space at the National Park Service’s laboratory to set up a data processing station and store spare equipment.

Around the islands of Ofu and Olosega, 65 camera sled dives were completed, covering a length of 30.8 km of seafloor (Fig. 5). Another 27 dives were made around the island of Ta`u and covered a length of 11.5 km of seafloor (Fig. 6).

Figure 5. Map showing tracks of the camera sled during surveys around the islands of Ofu and Olosega.

Figure 5. Map showing tracks of the camera sled during surveys around the islands of Ofu and Olosega.

Figure 6. Map showing tracks of the camera sled during surveys around the island of Ta`u.

Figure 6. Map showing tracks of the camera sled during surveys around the island of Ta`u.

The scenery around the Manu`a Islands, both above and below the surface of the ocean, is often dramatic and beautiful. The scientists found a number of areas with diverse, well-developed coral reefs (Figs. 7–10). Many of these reefs had abundant fish populations with communities of jacks and snappers, in particular, at several spots around these islands at the edge of steep ledges, generally at depths of 90–100 m. Abundant sea fans also were observed along these steep ledges.

Rooney gave a presentation on preliminary results of this survey on Nov. 14 in Pago Pago to an audience of about 30 natural resource managers and stakeholders. Over the next year, seafloor substrates and the organisms growing on them, as recorded in video imagery from this survey, will be classified through the use of a standardized method that CRED has used for imagery collected across the Pacific Islands Region. Local partners from the DMWR are particularly interested in fish communities at mesophotic depths, so fish species observed in the imagery will be identified to the lowest taxonomic resolution possible, their lengths will be estimated, and this information will be entered in a database.

Results of these classification efforts will be made available for download along with other data that CRED has collected in American Samoa. In time, other products, such as benthic habitat maps and scientific publications, may be created as well.

By John Rooney
Figure 7. Foliose coral species off northeastern Olosega at a depth of ~60 m. NOAA photo

Figure 7. Foliose coral species off northeastern Olosega at a depth of ~60 m. NOAA photo

Figure 8. Various coral species and crustose coralline algae off northwestern Ofu at a depth of ~50 m. NOAA photo

Figure 8. Various coral species and crustose coralline algae off northwestern Ofu at a depth of ~50 m. NOAA photo

Figure 9. Soft coral species with crustose coralline algae off northwestern Ofu at a depth of ~50 m. NOAA photo

Figure 9. Soft coral species with crustose coralline algae off northwestern Ofu at a depth of ~50 m. NOAA photo

Figure 10. Various coral species and macroalgae off northern Ta`u at a depth of ~45 m. NOAA photo

Figure 10. Various coral species and macroalgae off northern Ta`u at a depth of ~45 m. NOAA photo

 

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