Seafloor Studies Will Fill Data Gaps to Improve Scientific Knowledge of Northwestern Hawaiian Islands Marine Ecosystems

May 2, 2008
A composite map of known bathymetry at French Frigate Shoals combining data derived from multibeam sonar and the Ikonos 
                 satellite. The Hi'ialakai expedition will try to fill in data gaps (white patches within the map) to get a complete 
                 picture of substrate depth and type at FFS.
A composite map of known bathymetry at French Frigate Shoals combining data derived from multibeam sonar and the Ikonos satellite. The Hi'ialakai expedition will try to fill in data gaps (white patches within the map) to get a complete picture of substrate depth and type at FFS. Click image for higher resolution

The NOAA Ship Hi'ialakai departed Honolulu on May 2, 2008, with a team of thirteen scientists for a 27-day cruise to the Northwestern Hawaiian Islands. The expedition is a cooperative study with participating scientists from the NOAA Pacific Islands Fisheries Science Center's (PIFSC) Coral Reef Ecosystem Division (CRED), the Papahānaumokuākea Marine National Monument (PMNM), and NOAA's Biogeography Branch from Silver Spring, Maryland. Chief Scientist for the investigation is Joyce Miller, a CRED scientist employed by the University of Hawaii Joint Institute for Marine and Atmospheric Research.

The primary goals of the expedition are to fill gaps in several types of data needed to accurately describe the coral reef ecosystem including the benthic habitat and associated biological communities. Operations will include multibeam sonar mapping of the seafloor, collection of optical data for validation of sonar results and for detection of alien invasive species, and SCUBA diving to better delineate different bottom types and understand the nature of the seafloor. The data collected by divers and from camera deployments help "groundtruth" the information derived from the multibeam sonar and satellite instruments.

One of the primary targets of interest during the research cruise is French Frigate Shoals (FFS). This area has been partially mapped using data from satellite imagery and data on bathymetry (seafloor depth) and other seafloor characteristics (e.g., hardness and roughness of the bottom) collected by multibeam sonar. Nevertheless, there are still gaps in the satellite and multibeam data, as shown in the accompanying map. These gaps occur in areas where a wealth of biological data has already been collected. To better interpret these valuable biological data and understand FFS ecosystems, complete base maps of bathymetry and substrate type are needed in water depths ranging from 0 to 500 m and deeper. The major data gaps are in waters in the 10-20 m depth range around FFS. Thus the highest priority activity for the cruise is to collect multibeam data using NOAA's 25-ft survey launch R/V AHI, which is able to operate safely in the shallower areas not accessible by the Hi'ialakai. If FFS mapping is completed early, the AHI will be used to fill data gaps in waters of similar depth at Pearl and Hermes Atoll, about 500 miles northwest of FFS. If time does not allow the Hi'ialakai to transit to Pearl and Hermes Atoll, researchers on the Hi'ialakai plan to collect multibeam data in deeper habitats and camera data in other areas nearby, including St. Rogatien Bank, Necker Island, Necker Ridge, and Nihoa Island.

The NOAA R/V AHI will collect multibeam sonar data in shoal areas where the Hi'ialakai cannot safely operate.
The NOAA R/V AHI will collect multibeam sonar data in shoal areas where the Hi'ialakai cannot safely operate.

In the optical survey work, scientists from CRED and the Biogeography Branch are deploying two independent camera systems and tracking systems to test and compare results from different equipment. The primary goal in this work is to better understand multibeam bathymetry and backscatter data in order to create continuous benthic habitat products, including interpretive maps of hard and soft substrates. Another goal is to collect additional optical data documenting substrate type, extent of biological cover, and mobile fauna. The Biogeography Branch has been contracted by the PMNM to collect data to study and document alien invasive species in the NWHI. A team of 2-4 divers from CRED, PMNM, and Hi'ialakai will conduct studies to document the depth of sediment in areas that have been identified as "hard" based on returns from high frequency multibeam sonar, and other areas identified as "soft" based on backscatter data collected using low frequency multibeam. These data will enable us to test the hypothesis that the "harder" or higher f requency return data are typical of a thin (1 cm thick) sediment cover over a hard flat substrate, while the "softer" or lower frequency backscatter data represent thicker sediment cover perhaps meters in depth. The Hi'ialakai dive team will also try to identify transition zones between distinct types of substrate, furthering the goal of developing more accurate benthic habitat maps.