Sound Science in our Sanctuaries!

Since 2004 we have used several forms of SONAR (SOund Navigation And Ranging) to map and assess the coral reefs, soft-bottom habitats and inhabitants within the Florida Keys National Marine 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, the use of SONARs are providing researchers, managers, and stakeholders with valuable information and tools to conserve, restore, and better understand and enjoy our living marine resources. 

Mapping Habitats. . .

Mapping Habitats image of ship/multibeam. (Image credit, NOAA Center for Coastal Monitoring and Assessment

Maps are very important tools for researchers, managers, and users of the Sanctuaries.  They 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 use these habitats.  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 the ship.  As the ship follows a planned straight line, the transducer sends sound pulses to the sea floor.  When the pulses hit the sea floor, they will reflect back to the ship and be detected by the receiver.  The multibeam system then calculates the angle, the time it took for the pulses to travel, the ship’s orientation to the water to determine the depth of the seafloor in a fan roughly 4X water depth.  Once the data pulses return and are analyzed by the computer, topography maps of the seafloor can be created.  ROVs and drop-cameras, or divers are used to groundtruth the composition of the seafloor.

. . . and Inhabitants
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 coral reef ecosystems.  But there are some limitations to what diver observations can tell us about the underwater world.  Divers can only be on the bottom for a limited time (about 30 minutes in our case).  Our dive protocol uses band transects that are 30m (100 feet) long.  It would take an impossible number of dives to completely survey the Sanctuary.  So we need additional tools to sample a larger area to help place these diver observations in a broader context of the coral reef ecosystem.

Using SONAR to detect and count fish has been used in scientific research for over 50 years.  The availability of “fish-finders” to commercial and recreational fishers is an example of the broad utility of this tool for finding fish and underwater features.  The SONAR we are using to detect and count fish is a scientific version of a typical fish- or bottom-finder that most fishermen have on their boat.  The greatest differences between our fish SONAR and the commercial version is the precision of the data, the links to geo-positioning systems (GPS), and the ability to record the data in digital format on a computer for analysis.  But the principle is the same as the multibeam SONAR, except the sound pulse is received in a narrower “beam”, roughly 12% of water depth (compared to 4X water depth for multibeam).  However, like the bottom, anything having a different density – fish, plankton, air bubbles, and the bottom – 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.

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 surveys.  First, we can locate individual fish in three dimensions and determine their position relative to other fishes, the bottom, or other habitat features.  Second, we can estimate the density of fishes along a survey line or within an area.

The Virtual Reef!
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 data collected 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.

Click here to watch the movie clip (no audio).

Watch a "virtual reef" movie clip created by CCFHR Research Ecologist Chris Taylor. A virtual reef is reconstructed from splitbeam and multibeam SONAR data from a survey in the Tortugas North Ecological Reserve. Individual fish are detected in 3-dimensions using a scientific fishery echosounder system. The topography of the seafloor is mapped using a multibeam echosounder system. The sizes of virtual fish are proportional to the actual fish size (but not scaled to the scale of the seafloor).