Northwest Fisheries Science Center

The Main Deck

Acoustic and trawl adventures in the Northeast Pacific

This portal tracks the research and sea-going activities of the Fisheries Engineering and Acoustic Technologies (FEAT) Team from NOAA¿s Northwest Fisheries Science Center.  Follow us as we use acoustics, trawling, and oceanographic sampling to learn about the Northeast Pacific Ocean.

Back deck of Bell M. Shimada
Acoustic echogram of hake
trawl catch

E is for Echogram

By Rebecca Thomas
February 6, 2016

Image credit NWFSC-FRAM-FEAT (and Pacific hake)

During the winter survey, we have talked about “echograms” - what are they and what do they show us?  Well, you can’t see very far in even the clearest water, but sound can travel both far and quickly.  This is why sound is great for exploring the ocean, and one reason why so many marine mammals use echolocation. 

An echogram is the image of the ocean we make using an echosounder.  An echosounder is at its heart a high-tech fish-finder.  Like a radar system, it lets us see objects that are farther away than we can see with our eyes.  The echosounder transmits a “ping” of sound which travels down through the water.  The sound energy in a ping may bounce off of a fish, squid, or some other crazy creature, and then eventually hit the ocean bottom and bounce back.  We use how long it takes for the sound to bounce back to our equipment to calculate how far below the creatures and bottom are from the surface of the water.  For instance, in the picture you can see how it would take longer for the sound to get down to the bottom of the ocean (and bounce back) than it would to take to get to the fish (and back).  Each vertical line in an echogram comes from a single ping as the ship travels forward through the water.  As the ship moves, we build up enough of these pings to form an echogram image of what’s below us (sometimes the letter “E”) in the water column.  Because we have been over such deep water during the winter survey, we collect a ping of information from the echosounders on the Bell M. Shimada every 1-3 seconds.

We use echograms to help us find, identify, and estimate fish numbers, as well as to learn how deep the ocean bottom is below us.

Image credit Rebecca Thomas (NWFSC)

Tagged: Winter hake survey

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