NOAA Scientists Test Laser Line Scanning Technology to Map Marine Life in Coral Reef Ecosystems around Maui

November 8, 2006
Multibeam sonars can produce detailed maps of the depth and hardness of the seafloor, like this bathymetric map of waters around Baker Island, a US island in the equatorial Pacific.

One of the critical early steps in managing the nation's coral reef ecosystems is to determine what kinds of marine organisms inhabit the seafloor associated with coral reefs. Describing and mapping the coral reef biological communities and their habitats is a high priority research topic that will require a diverse set of scientific tools.

In the main Hawaiian Islands, scientists on the NOAA Ship Hi'ialakai are testing a digital imaging tool with great potential for mapping coral reef biota -- laser scanning. In waters around the islands of Maui County, digital images are being collected on the Hi'ialakai using a Laser Line Scan (LLS) system. The research cruise is under the direction of John Rooney, a scientist with the University of Hawaii Joint Institute for Marine and Atmospheric Research affiliated with NOAA's Pacific Islands Fisheries Science Center. The project is funded by a grant from the NOAA Office of Ocean Exploration. Results of the tests will help determine whether LLS technology can meet the needs of researchers and marine resource managers for describing and understanding coral reef ecosystems.

The Hi'ialakai survey is part of a systematic effort by NOAA scientists to map coral reefs in U.S. waters, including those around Pacific islands and atolls. The mapping is one of 13 integrated conservation strategies identified in the National Action Plan for Coral Reef Conservation released in 2000 by the U.S. Coral Reef Task Force.

Many different technologies have been used to survey and map coral reef habitats. Rough maps of coral reefs have been derived from satellite imagery and aerial photographs, but these methods are able to cover only very shallow physical features and do not provide spatial detail or information about bottom depth. Such shortcomings have been overcome using high-resolution multibeam sonars which yield detailed maps of bathymetry and substrate type.

Optical methods are needed to distinguish a coral reef, like the one shown here, from a seafloor of uncolonized rock pavement.

In addition to detailed maps of the seafloor, scientists and managers need information about the marine organisms living in and on the reef. Data on the coral reef biological communities have been collected using conventional optical methods, including towed or dropped cameras and visual sightings by scientists equipped with scuba gear or submersibles. However, these methods can cover only a limited area of the seafloor and because they require relatively strong lighting for effective photography and visual observation, they tend to alter the behavior of some species of marine life, biasing the results.

Laser Line Scanning technology can help overcome these problems. Scientists on the Hi'ialakai are testing the Northrop Grumman SM-2000 (Scientific Applications International Corporation), currently the only commercially-available LLS System. As the LLS System is towed through the water, a rotating a mirror in the instrument reflects a blue-green laser light in a 70 degree beam directed toward the seafloor. The width of the swath of seafloor covered by the beam depends on the altitude of the instrument. In the clear waters around Hawaii, towing the instrument 30 m above the bottom produces a swath width of up to 43 m. Detector optics synchronized with the mirror track the illuminated point, minimizing the effect of backscatter in the water column. When each discrete signal from the optical sensors is amplified and processed, the result is a scan of a thin swath of seafloor. When these scans are combined, the adjoining swaths produce a continuous grayscale image of the seafloor similar to a black and white photograph.

Because of the laser light source and the fact that the blue-green light penetrates the water better than other colors, the LLS System has a depth range far superior to conventional still or video cameras, allowing high-quality observation of the bottom from a much higher position in the water column than is possible using camera sleds or submersibles. Because the laser flashes so rapidly and the laser light is much more directed than light from conventional underwater lamps, the LLS System has less effect than other optical methods on the behavior of fish, crabs and other reef inhabitants being studied. The Northrop Grumman SM-2000 LLS has been used successfully in other marine ecosystems to map schools of fish near the seafloor. In the clear waters of coral reefs in Hawaii and other Pacific islands, data collected by the LLS System will enable scientists to better describe the coral colonies on the reefs and associated components of the biological community.

The Hi'ialakai surveys are a partnership between researchers of the NOAA Pacific Islands Fisheries Science Center, the University of Hawaii's Hawaii Undersea Research Laboratory and Botany Department, the State of Hawaii Division of Aquatic Resources, NOAA's National Marine Sanctuary Program, and the Pacific Island Benthic Habitat Mapping Program. The scientific team on the Hi'ialakai will deploy the LLS System to survey various targets including aggregations of bottomfish and potential bottomfish nursery areas, deep coral reefs made up of stony or hard (scleractinian) corals, communities of black coral, and deep beds of algae. A historically significant WWII-era aircraft wreckage will also be surveyed. If the LLS imagery collected meets expectations, it will add impetus to further development of laser line scan technology and use of the tool in other coral reef mapping applications.