Fisheries Acoustics

NOAA Teacher at Sea: Tammy Orilio

NOAA Ship Oscar Dyson

Mission: Pollock Survey
Geographical Area of Cruise: Gulf of Alaska

Date: 28 June 2011

Weather Data from the Bridge:

Latitude: 57.11 N

Longitude:  -155.58 W

Wind Speed:  3.61 knots

Surface Water Temp:  9.0 degrees C

Water Depth:  271.10 m

Air Temp:  8.3 degrees C

Relative Humidity: 84%

Science & Technology Log:

Today we will look at the acoustic system of the NOAA Ship Oscar Dyson! Acoustics is the science that studies how waves (including vibrations & sound waves)  move through solids, liquids, and gases.  The Oscar Dyson uses an acoustic system to find the pollock that we process.

The process begins when a piece of equipment called a transducer converts an electrical pulse into a sound wave.  The transducers are located on the underside of the ship (in the water).   The sound travels away from the vessel at roughly1500 feet per minute, and continues to do so until the sound wave hits another object such as a bubble, plankton, a fish, or the bottom. When the sound wave hits an object, it reflects the sound wave, sending the sound wave back to the Oscar Dyson as an echo. Equipment onboard listens to the echo.

The computers look at two critical pieces of information from the returning sound wave. First,  it measures the time that it took the echo to travel back to the ship. This piece of information gives the scientists onboard the distance the sound wave traveled. Remember that sound travels at roughly 1500 feet per minute. If the sound came back in one minute, then the object that the sound wave hit is 750 feet away (the sound traveled 750 feet to the object, hit the object, and then traveled 750 feet back to the boat).

The second critical piece of information is the intensity of the echo. The intensity of the echo tells the scientists how small or how large an object is, and this gives us an idea of what the sound wave hit. Tiny echos near the surface are almost certainly plankton, but larger objects in the midwater might be a school of fish.

An image of the computer screen that shows a great number of fish. This was taken underneath the boat as we were line fishing in Sand Point.

The same spot as above, but with practically no fish.

An image of the screen during a trawl. You can actually see the net, it is the two brown lines that are running from left to right towards the top of the screen.

One of the things that surprised me the most was that fish and bubbles often look similar enough under water that it can fool the acoustics team into thinking that the bubbles are actually fish. This is because many species of fish have gas pockets inside of them, and so the readout looks very similar.  The gas pockets are technically called “swim bladders” and they are used to help the fish control buoyancy in the water. 

Personal Log:

Well, it's now Tuesday morning, and we are making excellent time on our way back to Kodiak.  The water has not been as rough as expected, thank goodness!  Yesterday's forecast said we'd encounter winds up to 35 knots and seas up to 18 feet, but I have definitely not felt anything like that.  It's not quite over yet, though, so I'm not getting my hopes up too much.   

We're scheduled to arrive in Kodiak sometime tomorrow (I don't know the approximate time yet), or maybe even later tonight, which means I'll have a day to kill there.  I'm looking forward to it because I didn't get a chance to explore when I first arrived.  When I made it to Kodiak, I only had the clothes on my back, and it was raining for nearly the entire two days I was there, so I didn't want to go outside and explore because if my clothes got wet, I had nothing else to change into!  One animal I haven't seen on this trip is an eagle, and I hear they're very easy to spot in Kodiak, so hopefully I'll get a chance to look around tomorrow!  

Question of the Day:

• What is one way that bony fishes can control the amount of gas in their swim bladder?