NOAA Teacher at Sea
Johanna Mendillo
Aboard NOAA ship Oscar Dyson
July 23 – August 10
Mission: Pollock research cruise
Geographical area of the cruise: Bering Sea
Date: Tuesday, August 7, 2012
Location Data from the Bridge:
Latitude: 59○ 52 ’ N
Longitude: 177○ 17’ W
Ship speed: 8.0 knots ( 9.2 mph)
Weather Data from the Bridge:
Air temperature: 7.3○C (45.1ºF)
Surface water temperature: 8.4○C (47.1ºF)
Wind speed: 4 knots ( 4.6 mph)
Wind direction: 75○T
Barometric pressure: 1018 millibar (1 atm)
Science and Technology Log:
We are wrapping up our final few sampling transects. Now that you are practically fisheries biologists yourselves from reading this blog, students, we must return to the fundamental question— how do we FIND the pollock out here in the vast Bering Sea? The answer, in one word, is through ACOUSTICS!
Hydroacoustics is the study of and application of sound in water. Scientists on the Oscar Dyson use hydroacoustics to detect, assess, and monitor pollock populations in the Bering Sea.
Now, you may have heard of SONAR before and wonder how it connects to the field of hydroacoustics. Well, SONAR (SOund Navigation and Ranging) is an acoustic technique in which scientists send out sound waves and measure the “echo characteristics” of targets in the water when the sound waves bounce back— in this case, the targets are, of course, the pollock! It was originally developed in WWI to help locate enemy submarines! It has been used for scientific research for over 60 years.
(PLEASE NOTE: The words sonar, fishfinders, and echosounders can all be used interchangeably.)
On the Dyson, there is, not one, but a collection of five transducers on our echosounder, and they are set at five different frequencies. It is lowered beneath the ship’s hull on a retractable centerboard. The transducers are the actual part of the echosounder that act like antennae, both transmitting and receiving return signals.
The transducers transmit (send out) a “pulse” down through the water, at five different speeds ranging from 18-200kHz, which equals 18,000-200,000 sound waves a second!
When the pulse strikes the swim bladders inside the pollock, it gets reflected (bounced back) to the transducer and translated into an image.
First of all, what is a swim bladder? It is simply an organ in fish that helps them stay buoyant, and, in some cases, is important for their hearing.
Now, why do the pulses bounce off the swim bladders, you ask? Well, they are filled mostly with air and thus act as a great medium for the sound waves to register and bounce back.
Think of it this way: water and air are two very different types of materials, and they have very different densities. The speed of sound always depends on the material through which the sound waves are traveling through. Because water and air have very different densities, there is a significant difference in the speed of sound through each material, and that difference in speed is what is easy for the sonar to pick up as a signal!
It is the same idea when sound waves are used to hit the bottom of the ocean to measure its depth- it is easy to read that signal because the change in material, from water to solid ground, produces a large change in the speed of the sound waves!
Interestingly, different types of fish have different shaped and sized swim bladders, and scientists have learned that they give off different return echos from sonar signals! These show up as slightly different shapes on the computer screen, and are called a fish’s “echo signature”. We know, however, that we will not encounter many fish other than pollock in this area of the Bering Sea, so we do not spend significant time studying the echo signatures on this cruise.
So, what happens when these signals return to the Dyson? They are then processed and transmitted onto the computer screens in the hydroacoutsics lab on board. This place is affectionately known as “the cave” because it has no windows, and it is, in fact, the place where I spend the majority of my time when I am not processing fish! Here it is:
We spend a lot of time monitoring those computer screens, and when we see lots of “specks” on the screen, we know we have encountered large numbers of pollock!
When the scientists have discussed and confirmed the presence of pollock, they then call up to the Bridge and announce we are “ready to go fishing” at a certain location and a certain depth range! Then, the scientists will head upstairs to the Bridge to work with the officers and deck crew to supervise the release, trawling, and retrieval of the net.
Now, in addition to the SONAR under the ship, there are sensors attached to the top of the net itself, transmitting back data. All of the return echos get transmitted to different screens on the bridge, so not only can you watch the fish in the water before they are caught, you can also “see” them on a different screen when they are in the net! As I told you in the last post, we will trawl for anywhere from 5-60 minutes, depending on how many fish are in the area!
Personal Log:
In these last few days, we have crossed back and forth from the Russian Exclusive Economic Zone (EEZ) and the U.S. several times. There were some nice views of Eastern Russia before the clouds and fog rolled in!
In addition, we crossed over the International Date Line! It turns out that everyone on board gets a special certificate called the “Domain of the Golden Dragon” to mark this event. This is just one of a set of unofficial certificates that began with the U.S. Navy! If you spend enough time at sea, you can amass quite a collection- there are also certificates for crossing the Equator, Antarctic Circle, Arctic Circle, transiting the Panama Canal, going around the world, and more…
I will award a prize to the first person who writes back to tell me what does it mean when one goes from a “pollywog” to a “shellback”, in Navy-speak!
Here is a picture of me with the largest pollock I have seen so far- 70cm!
Lastly, on to some, perhaps, cuter and more cuddly creatures than pollock- pets! Here in the hydroacoustics lab, there is a wall dedicated to pictures of pets owned by the officers, crew, and scientists:
Clearly, this is a dog crowd! I did learn, however, that our Chief Scientist, Taina, has her cat (Luna) up there! Students, do you remember the name of my cat and, what do you think, should I leave a picture of her up here at sea?
sloan at sea!
It would take about 2.3 70cm pollocks to fit you.