NWHI Habitat Mapping

Hawaii Archipelago

The Northwestern Hawaiian Islands (NWHI) are a chain of tiny islands, atolls and shoals that spans more than 2000 km of the Pacific. They stretch from Nihoa Island, northwest of Kaua’i, to Kure Atoll, northwest of Midway. These islands, atolls and banks are remnants of volcanic islands, originally similar to the volcanoes of the Main Hawaiian Islands, which have eroded and subsided beneath the ocean surface. The NWHI are uninhabited except for seasonal field camps on a few islands and a small year-round population on Midway. The coral reef ecosystems of the NWHI are unique in that they are some of the healthiest and most undisturbed coral reefs remaining and possibly the last large-scale, predator-dominated coral reef ecosystem on the planet. In 2000 Executive Order 13178 created the NWHI Coral Reef Ecosystem Reserve (CRER) and included conservation measures to restrict some activities throughout the CRER. NOAA’s National Marine Sanctuaries Program is currently working to designate the CRER as a National Marine Sanctuary.

Combined IKONOS, multibeam, and single-beam data around French Frigate Shoals

The CRER is approximately 2,000 km long by 180 km wide, resulting in an area of over 360,000 km2. The most recent charting in the NWHI had been done in the 1930s and many of the banks and reefs were charted miles from their actual position. To establish management boundaries for the CRER, updated and accurate bathymetric information was required, particularly for important 25-, 50-, and 100-fm contours that were needed to define Reserve Preservation Areas (RPA). In order to meet these critical needs, NOAA and the University of Hawaii conducted a 25-day mapping cruise to the NWHI in October/November 2002; data from this cruise were used to create the Bathymetric Atlas of the Northwestern Hawaiian Islands. These data provide some of the vital base maps needed for the area as well as information needed to update nautical charts as documented in “Collaborative Nautical Charting and Scientific Seabed Mapping” (Evans et al, Sea Technology, June 2004.)

Extensive spur and groove reef structures can be seen north of Midway. Also seen are scarps that may delineate previous shorelines

Since 2000 CRED and CRER scientists have been conducting annual coral reef ecosystem assessment and monitoring cruises in the NWHI. In 2001 a mapping component was added to the program, collecting single-beam bathymetry, bottom classification data, bottom photographs and video data; multibeam data collection capabilities were added in 2003. The primary focus of the mapping has been in water depths greater than 20m, because IKONOS imagery that can be used for benthic habitat mapping is available in waters shallower than 20m.

High-resolution bathymetry data collected at Midway Atoll

In 10 days of operations during August 2003 theR/V AHI surveyed 240 km2 of seafloor around Midway Island in water depths between 15 and 250 m; these data were combined with estimated depths from IKONOS imagery inside of the lagoon to better represent the overall nature of the geomorphology of Midway Atoll. Gridded bathymetry data (5 m resolution) are available for download. These bathymetry data are being combined with backscatter imagery data and photographic/video data for analysis of the benthic habitats around Midway Island.

NWHI Mapping Data Collected since 2001

Cruise # Date Areas of Study Single-beam w/QTC (km) # Still Photos # Videos Multi-beam (km2)
TC0110 Sep-01 FFS, Brooks, St. Rogatien, Gardner, Raita, Maro 2,963 297 9 -
TC0111 Oct-01 Lisianski, Midway, Pioneer, FFS, Brooks, Necker, Maro 1,717 640 5 -
TC0207 Sep-02 Necker, FFS, Laysan, Pearl & Hermes, Lisianski, Midway, Kure, Maro, Brooks, St. Rogatien, Pioneer 1,309 902 13 -
KM206 Oct-02 Nihoa, Necker, FFS, Brooks, Gardner, Raita, Maro, Lisianski, Laysan, Pioneer, Northhampton Seamounts - - - 38,3671
OES0306 Jul-Aug 03 FFS, Gardner, Maro, Laysan, Lisianski, Midway, Pearl & Hermes, Kure 650 1894 67 2402
Totals   6,639 3733 94 38.607

1 Intermediate depth surveys covering 20-5000m using 100 and 12-kHz sonars; conducted to define CRER boundaries
2 Survey of banks adjacent to Midway Atoll (10-250m) using 240 kHz sonar

Mapping in the NWHI is on-going and multibeam surveys are planned on an annual basis. The bank top areas between 20 and 100m are extensive and still mostly unmapped; for example, Necker Island has an unmapped area of almost 1500 km2 and Gardner Bank, over 2200 km2. Shallow water surveys such as these are very time consuming; typical progress is about 2 km2/hour. Individual mapping priorities are being established by CRER, CRED, and other groups working in the NWHI. Upcoming multibeam surveys are planned first in areas where RPA boundaries are still incomplete, such as Kure, Pearl and Hermes, and Nihoa Island. We will also continue to collect optical validation data whenever possible, in order to provide ground truth information to help interpret the detailed bathymetry data after it has been collected.