By Chris Taylor, Research Ecologist,
NOAA National Centers for Coastal and Ocean Science (NCCOS)
During this expedition scientists will use several forms of sonar (SOund Navigation And Ranging) to map and assess the coral reefs, seafloor habitats and fish communities within the sanctuary. Sonar is the process of transmitting sound to detect and locate objects underwater and has been used by the military since the 1950s.
With increased computing power, sonar technologies are providing researchers, managers and stakeholders with valuable information and tools to conserve, restore, better understand and enjoy our living marine resources.
Mapping Habitats. . .
Maps are very important tools for researchers, managers, and people who enjoy the great beauty of our nation’s marine sanctuaries. These maps give us a picture of what conditions and habitats exist on the seafloor and help researchers and managers better understand the distribution of coral reefs and other habitats as well as the critters that call these areas home.
This map shows the bottom relief of the
coral cap at West Flower Garden Bank. Image credit: FGBNMS
Our maps are created from data collected using a multibeam echosounder system. This system consists of a transducer and a receiver mounted on the bottom of a ship.
As the ship follows a planned straight line, the transducer sends sound pulses to the seafloor. When the pulses hit the seafloor, they reflect back to the ship and are detected by the receiver. Once the data pulses return and are analyzed by the computer, topography maps of the seafloor can be created.
A variety of tools are deployed from the NOAA Ship Nancy Foster to map seafloor habitats. Image credit: NCCOS
ROVs and drop-cameras, or divers are used to groundtruth, or double check, the composition of the seafloor.
Much of what we know about the underwater world has come from direct visual observations by scientific divers. And these direct observations are vital tools in our research in understanding of underwater habitats and inhabitants, like fish.
But, there are limitations to what diver observations can tell us about the underwater world. We need additional tools to help place diver observations in a broader context of the coral reef ecosystem. During this expedition we will also use sonar to complement fish population observations made by the dive team.
Diver observations and sonar data can create a
detailed picture of underwater habitats and sea life. Image credit: NCCOS
Sonar provides a rapid method to survey a large area, but does so in a non-destructive and minimally invasive manner.
We collect several types of information when we conduct sonar fish surveys. First, we can locate individual fish in three dimensions and determine their position relative to other fish, the bottom or other habitat features. Second, we can estimate the density of fish along a survey line or within an area.
The image produced by the fisheries sonar allows
scientists to detect schools and the underlying seafloor. Image credit: NCCOS
Fisheries sonar follows the same principle behind multibeam sonar technology-- both work by emitting pulses of sound and measuring the return strength. Anything having a different density from the surrounding water – fish, plankton, air bubbles, and the seafloor– can return a signal.
For fish, the airbladder provides the greatest contrast from the water. The strength or intensity of the returning echo is an indication of the size of the object – a larger fish will reflect more sound energy.
The real advantage of these sonar tools comes when they are used together to capture a virtual reality of how fish use the coral reef ecosystems. Because all of the collected data are georeferenced (each data point has a precise location in terms of a coordinate system), we can reconstruct the ecosystem using Geographic Information Systems and other visual software tools.
This provides a valuable tool for managers by allowing them to see the ecosystem from the perspective of the organism. Providing this virtual ecosystem can also add to the information provided to stakeholders regarding management decisions such as zoning and regulations.
Watch a "virtual reef" movie clip created by CCFHR Research Ecologist Chris Taylor from a previous expedition:
click on the > to play
This video shows a virtual 3-D representation of the sea floor created from SONAR data, which was produced by "pinging" sound from the ship and measuring the returns of the echo. The fish you see on the screen are actually representations of real fish that were detected under the ship from a single beam sonar, or basically a fish finder. The virtual fish are not actual size, they really show how concentrated fish are in that area. One large virtual fish could really be a whole school. NOAA scientists use 3-D maps like this one to figure out where fish are hanging out, what type of terrain they like, and possibly where they spawn.