Benthic Habitat Mapping
DATA BY LOCATION
Very little has been written on the geology and geomorphology of American Samoa. The comprehensive acoustic data sets collected by PIBHMC and its partners allow us to better understand the structures and coral ecosystems around American Samoa. The optical validation data — classified videos and photos of the seafloor — provide additional aid in the interpretation of the multibeam bathymetry and backscatter data, and in the creation of seafloor characterization and benthic habitat maps.
The Territory of American Samoa, made up of five high volcanic islands, one coral atoll, one low coral islet, and several seamounts, is the only United States territory south of the equator.
The largest islands in the Samoan archipelago belong to the Independent State of Samoa located approximately 70 km WNW of Tutuila in American Samoa. American Samoa includes Tutuila, its largest (145km2) and most populated (50, 000 including Aunu‘u Island) island, tiny Aunu‘u Island, immediately southeast of Tutuila; the Manu‘a Island group of Ofu, Olosega and Ta‘u approximately 95 km east of Tutuila, which combined have a population of approximately 3,000 and a land area of less than 51 km2; Swains Island, a tiny coral islet approximately 350 km N of Tutuila that has a varying population of less than 30; and unpopulated Rose Atoll 140 km ESE of Ta‘u, which is a wildlife refuge under U.S. Fish and Wildlife Service jurisdiction. The U.S. National Park Service leases 4,250 hectares of land (170 of which are below sea level) from Samoan families that still own the land under communal Samoan traditions. The National Park of American Samoa encompasses areas on the islands of Ofu, Ta‘u, and Tutuila. Fagatele Bay National Marine Sanctuary, on Tutuila, is a part of NOAA’s National Marine Sanctuaries program.
The Samoan Archipelago sits on the northern end of the Tongan Trench. It is at the southwestern end of the Polynesian Triangle, 550km west of the Cook Islands. The archipelago spans about 520 km east to west. The eastern islands of Samoa were renounced to the United States by the United Kingdom in 1899. On April 17, 1900, Tutuila and Aunu’u were officially ceded to the union. The Manua Islands and Rose Atoll followed on July 16, 1904. The deeds of accession were formally ratified in 1929. Swains Island was joined to the territory in 1925 by a joint resolution of Congress. American Samoa was under U.S. Navy control from 1900 to 1951.
Tutuila was initially used as a coaling station, and for a time, it was used as a whaling port. Tutuila is home to Pago Pago harbor, often considered the safest harbor in the Central Pacific. This harbor was instrumental in World War II as it was the center of the Samoan Defense Group, the largest of the Pacific defense groups. Postwar American Samoa was transferred to the Department of Interior. In 1954, the first cannery was opened resulting in significant economic and demographic changes. In the 1930s the Manu’a Islands were home to 20% of American Samoa’s population, but within 50 years that statistic had shrunk to 6% as the lure of western culture and lucrative fishing jobs on Tutuila attracted young people and families alike. The fishing industry still thrives on Tutuila as the top employer in the territory.
Formed volcanically as the seafloor moved slowly across a single hotspot, the islands of the Samoan archipelago decrease in size from West to East. In 2002, nine optical validation surveys were conducted around Tutuila, Aunu’u, Ofu, and Olosega Islands, which helped characterize the geology and ecology of the reefs beyond conventional safe diving depths.
Existing nautical charts of Tutuila showed surrounding banks (0 to 60 m) with an area of approximately 300 km2, but little was known about the coral reef and other shallow ecosystems that exist on these banks. Almost no shallow bathymetric information existed around Ofu, Olosega, Ta’u, Swains Island, and Rose Atoll; in fact, there are few soundings in less than 200 m (100 fathoms) on the nautical charts.
In early 2004 shallow-water multibeam surveying was conducted around Tutuila and the Manu‘a islands using the R/V AHI (Acoustic Habitat Investigator) during cruise OES0402. These surveys resulted in complete multibeam coverage (~95%) around Ofu, Olosega and Ta‘u (to depths of 300 m) with similar coverage of approximately 60% of the banks surrounding Tutuila.
In preparation for a 2006 cruise aboard the NOAA Ship Hiialaka’i, the 2004 Tutuila bathymetry data were combined with a limited amount of 2001-2002 multibeam data collected by Dr. Dawn Wright at Oregon State University and Dr. Dave Naar of University of South Florida. By using the combined bathymetry data, the mapping team targeted remaining offshore gaps and produced a complete multibeam coverage of Tutuila.
Three additional locations, all submerged features, were surveyed in American Samoa in 2006. Two nights of mapping at Northeast Bank (Muli seamount) — 25 miles WNW of Ofu — revealed rugged structures within 50 meters of the surface. About one half of Two Percent Bank (Tulaga Seamount), about 50 km SW of Ta’u, was mapped in just four hours. Vailulu’u is an active volcano just East of Ta’u and has been mapped three times previously. HI0602 provided a fourth map contributing to the time series of bathymetry that has revealed a significant amount of growth in the volcanic crater over the past year alone; one night completed the mapping of the Vailulu’u summit.
Multibeam mapping around all islands is complete, providing almost complete coverage of multibeam data across 1013 km2 of seafloor between depths of 20 and 3000 m. Combining the 2004 and 2006 NOAA data with academic data provides almost 100% multibeam bathymetry coverage of the entire Territory between 20 and 3000 m depths.
Of the 7 islands and atoll in the territory, photographic and video data were collected around the 5 high islands (Tutuila, Aunu‘u, Ofu, Olosega and Ta‘u) in 2004. The 2004 optical validation surveys were much more intensive than in 2002, yielding 86 tows altogether. The 2006 optical validation efforts targeted the Manu‘a Islands only, adding 15 tows to the catalog of seafloor images used to characterize marine ecosystems.
The 2006 work in American Samoa also included an important education component, sponsored by the National Marine Sanctuaries Program (NMSP). Two students from the American Samoa Community College were on board for the cruise. One student, Frances Leiato, traveled to Swains Island with the ship and the other student, Della Tuamoheloa, embarked for the Tutuila portion of the cruise.
They were able to participate in mapping on the AHI and on the ship and spent several days on the water with the rapid ecological assessment team, the towboard team and the oceanography team, learning about biological and oceanographic protocols. They participated enthusiastically, learning about the importance of ocean and coastal awareness and stewardship through hands-on experience. In addition, one day was spent with the Governor and other American Samoa officials aboard the ship outside of Tutuila; this collaborative effort provided an excellent opportunity for government officials to understand the vital resources that are being provided to promote the understanding of and research on the coral reef ecosystems of American Samoa.