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Watching The Ups and Downs of the Sea

Category: General Article

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Monday, August 24, 2015

Summary: Dr. Joseph Warren describes how scientists use echolocation — a sense found in dolphins for example — to map the migration of animals in the deep-sea.

Hi Virtual Science Team,

My name is Joe Warren. I count animals using sound. Hearing that, your first thought might be, “Cool!” Your second and third thoughts might be, “Why and how?”

Let’s start with the why. If you wanted to count how many fish were in an aquarium tank, which of your senses (sight, taste, touch, hearing, or smell) would you use? Most of us would use sight to see how many fish were in the tank. That would work pretty well. But what if I asked you to count the fish in the tank at night with no lights on? Then it would much more difficult.

The only light in the ocean is from the sun and moon. That light can only travels down so far. Once you get a couple of soccer fields deep, the ocean is dark. Though it is dark 24/7, we know from towing nets and using submersibles there are many animals living there. As a marine biologist, we need to know how many animals live in the dark sea and where they live. But how do you count how many fish, squid, or crustaceans live in the dark parts of the ocean.

The technology I use to “see” in to the dark sea is called an “echosounder.” An echosounder works like the echolocation of a dolphin or bat, using sound to see. Like a dolphin’s echolocation, the echosounder sends out a beam of sound. When the sound hits something, it echoes or bounces back. A dolphin uses the returning sounds to find food or its way in the dark sea. With echolocation the dolphin can determine how far away something is, how fast it is moving, which way it’s moving, and so much more.

How does the echosounder work? We transmit a short pulse of sound. It’s so short it sounds like a “snap.” The sound travels down toward the ocean bottom. Like the dolphin, we listen for echoes as the sound bounces off objects in the water column. We measure how long it takes for the echo to return to us. From that echo we can determine how deep the fish are located. Usually, the strongest echo comes from the seafloor, but if there are marine animals, they’ll produce echoes as well.

Many ocean animals live deeper when the sun rises. Then when the sun sets, they swim up toward the surface. This is daily up and down movement is called “vertical migration.” Vertical migration happens every day all over the world, making it the largest migration on Earth! During the day, these animals are very deep in the ocean (about a half mile below the surface). Using our echosounder, we can watch these animals move up and down in the ocean.

I’ve attached one of the pictures of the deep created by our echosounder which can be downloaded as a PDF from the righthand sidebar. It’s an example of what we see when the sound bounces back. As you’ll see, the time is noted along the bottom. The depth on the left. Find 6:30 p.m. then draw your finger up to find the fish (indicated in red colors). What depth is that? Now, follow the fish through time, to 8 p.m. As the sunsets and it gets darker, do they move up or down?

Thanks for joining us. Remember if you have any questions be sure to send them our way.

Dr. Joseph Warren, Team Acoustics

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Download Echosound Image

Download this document discussed by Dr. Warren in his blog.

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