Ocean Discoveries: 2012 Nancy Foster Mapping Mission/Day 7 Part 2 – Fish Acoustics

By Chris Taylor

Dr. Chris Taylor

NCCOS Blogger Biography: Dr. Chris Taylor is a research ecologist with NOAA’s Center for Coastal Fisheries and Habitat Research in Beaufort, North Carolina. In his research he uses underwater acoustic technologies (like fisheries sonar, fish-finding echo sounders, and multibeam sonar) to remotely detect fish and observe their behaviors in estuaries and the ocean. Dr. Taylor earned a Ph.D. and an M.S. and Zoology from North Carolina State University, and a B.S. in  biology (marine biology) from University of Wisconsin-Stevens Point.

Understanding Fish Populations Through Acoustics
To get the best indications of the condition and abundance of the fish communities in coral reef ecosystems, we try to get eyes in the water whenever we can.  Often we use scuba divers or remotely operated vehicles (ROV) with high-definition video cameras.  These direct visual observations provide the highest level of detail on the diversity of species of fish and biological organisms that make up the coral reef habitats in these complex ecosystems.

But to cover an area as large as our current study area (over 88 square nautical miles!) using divers or ROVs alone would take more time and personnel than we can afford on our 20 day mission. Similar to how we map the seafloor using sonar, we can use the same principles to detect and map the fishes in the water column.  On the NOAA Ship Nancy Foster we are operating 3 frequencies of fishery sonars that are mounted under the hull of the ship: 38kHz, 120kHz and 200kHz.  The lower the frequency, the deeper we are able to sample.  Using these three frequencies, we can detect fish from right under the boat to almost 1000m (3280 feet) deep.  We transmit pulses of sound, or pings, rapidly through the water column.  The sound bounces off anything that has a different density than the water.  This is easy to imagine in the case of sound reflecting off the hard seafloor.  For fish, we rely on the swim bladder, the organ that fish use to stay buoyant in the water column.  Since it is filled with air, it reflects sound very well.   The bigger the fish, the bigger the swim bladder; the bigger the swim bladder, the more sound is reflected and received by the transducer.  We call this backscatter, or target strength, and use it to estimate the size of the fish we are detecting.

The SONAR equipment uses the fish’s swim bladder (the dark area in the middle of the body) to reflect the sound and provide us with data. Credit: University of Wisconsin/J. Horne

Unfortunately, while we can estimate the size of fish, we can’t tell what species of fish we are “seeing” with the sonar.  So we have to once again, get eyes in the water.  On this mission, we are using the ROV to dive to areas where we have seen large numbers of fish on the sonar.  We’ll write more later about our observations using sonar and the ROV.

An image of the fish sonar echogram providing us with information on the location and size of fish in an area. The red line is the sea floor, green and orange blips are fish, and the faint blue is likely plankton and very small fish. Credit: NOAA/CCFHR

For this mission, we are operating the fishery sonar alongside the multibeam sonar as they map  the large areas of the Grand Reserve.   The multibeam team is surveying the areas in the Reserve using very closely spaced lines, which gives very high resolution information on the seafloor.  We add a “layer” of fish detections over this new map.  Our maps of will show areas of high fish density, and the habitats they are associated with.   We know that not all habitats are created equal.  Knowing how the reef fish use the variety of habitats in the coral ecosystem will help managers make decisions on how we use and conserve the available resources.

To see the the Nancy Foster throughout the 2012 mapping mission, visit the NOAA ship tracker site and click on “Enter NOAA’s Ship Tracker link, then scroll down to “NF – Nancy Foster” in the box on the upper right of the screen to see where she is at any given time!

Be sure to visit this blog often for field updates, pictures and videos posted by members of the science team.

This entry was posted in Benthic Mapping, Biogeography Branch, Caribbean, Caribbean Research, Center for Coastal Fisheries and Habitat Research, Center for Coastal Monitoring and Assessment, Coral, General, Marine Regional Planning, Nancy Foster Exploration, National Centers for Coastal Ocean Science, NOAA Marine Protected Areas, NOAA's National Ocean Service, Ocean Exploration, Ocean Field Work, Ocean Research and tagged , , , , , , , , , , , , , . Bookmark the permalink.

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