Integrated Mapping Expedition Improves Understanding of Seafloor Habitat in Northwestern Hawaiian Islands

Multibeam sonar backscatter data collected in shallow habitats at French Frigate Shoals. Blue areas are emergent land. Gray areas indicate where sea bottom was surveyed. In general the lighter gray areas correspond to softer bottom, while the darker gray areas indicate harder substrates. Inset shows entire area of sonar surveys at FFS.

Scientists in the PIFSC Coral Reef Ecosystem Division (CRED) recently led a 28-day benthic habitat mapping cruise aboard the NOAA Ship Hi'ialakai in the Northwestern Hawaiian Islands. They were joined by staff of the National Centers for Coastal Ocean Science's Biogeography Branch and the Papahānaumokuākea Marine National Monument. The group completed an ambitious list of operations including mapping of the seafloor using multibeam sonars, surveying the bottom with towed cameras, and collecting various information by SCUBA diving.

The scientific objectives of the cruise were to complete multibeam mapping of shallow water habitats at French Frigate Shoals (FFS), validate information on the types of bottom substrate in the survey area, and identify alien invasive species of plant and animal life. Each goal was achieved. Multibeam mapping was accomplished by launching the NOAA survey boat R/V AHI from the Hi'ialakai. Sonar information collected by the AHI filled gaps in the multibeam data previously collected and information provided by IKONOS® satellite imagery. Scientists collected 145 km² of multibeam data in shallow waters at FFS, and 3197 km² of multibeam data from deeper habitats at Necker Island, FFS, and Brooks Banks. Multibeam sonar signals provide information on depth of the seafloor, and backscatter data provide information about type of bottom. Camera surveys of the seafloor habitat to validate bottom types involved 35 tows and collected over 100 hours of video data along a survey path nearly 160 km long. In 28 dives, scientists collected data on thickness of the bottom sediments, grain size of seafloor materials, and observations of sea bottom types at FFS. When integrated with the multibeam sonar information, the dive observations enable improved understanding of the acoustic signatures of various bottom types and improved interpretation of the backscatter data. The expedition was the first to integrate sonar mapping, camera tows, and diving to more precisely define benthic habitats and clarify multibeam backscatter imagery results. It should serve as an excellent model for future integrated research cruises to develop more detailed habitat models.

For more information contact: John Rooney