Lindsay Knippenberg: Acoustics Day! September 13, 2011

NOAA Teacher at Sea
Lindsay Knippenberg
Aboard NOAA Ship Oscar Dyson
September 4 – 16, 2011

Mission: Bering-Aleutian Salmon International Survey (BASIS)
Geographical Area: Bering Sea
Date: September 13, 2011

Weather Data from the Bridge
Latitude: 56.91 N
Longitude: -169.08 W
Wind Speed: 10.07 kts
Wave Height: 4 – 6 ft
Surface Water Temperature: 6.5 C
Air Temperature: 7.5 C

Science and Technology Log

The Oscar Dyson uses several different types of sonar to get the best image of what is beneath the ship.

The Oscar Dyson uses several different types of sonar to get the best image of what is beneath the ship.

Today I learned about acoustics with Paul. The Oscar Dyson is one of NOAA’s newer ships and has a hull-mounted sonar system which uses sound waves to “see” what is underneath the ship. The Oscar Dyson was also built to have a low acoustic signature and be “quiet” in the water. This is helpful to the scientists using acoustics to study fish onboard the Dyson because the fish don’t hear the ship and swim away. On our cruise the acoustics data is used to get a picture of where there is life in the entire water column. For the most part we have just been trawling on the surface, but the ocean is much deeper and there could be a lot more life underneath our nets that we will never catch. If we get very few fish in our nets, it could mean that the fish are just at a deeper depth and not that there are not any fish in that area. Since the scientists are getting a better picture of what is really going in that ecosystem, they can make more accurate stock assessments. All throughout the cruise I have been curious about the images displayed on the screens in the acoustics room and on the bridge. Today I would finally learn what they were all about.

Since the sonar is attached to the bottom of the boat, the top 14 meters aren't seen in the images. To solve that problem, a sonar transducer is lowered over the side to get the top 14 meters when we at station.

Since the sonar is attached to the bottom of the boat, the top 14 meters aren't seen in the images. To solve that problem, a sonar transducer is lowered over the side to get the top 14 meters when we at station.

Basically how acoustics work is that a sound or ping is sent from the ship and it travels through the water. When it hits something in the water column or the bottom of the ocean it bounces back and the ship’s echosounder records the length of time that it took for the sound wave to travel there and back. Depending on the temperature and depth of the water, the pings are sent at different time intervals and pulses. The pings can also be sent at different frequencies to “see” different types of organisms. For instance zooplankton can be viewed best at one frequency and jellyfish can be viewed best at another frequency. As the sound waves are returning to the vessel, the computer translates the returning sound waves into images for the scientists to analyze.

A sonar image at dawn. The dark red line at the bottom of the screen is the ocean floor. Notice all the greens and blues at the top of the water column. Those are pollock.

A sonar image at dawn. The dark red line at the bottom of the screen is the ocean floor. Notice all the greens and blues at the top of the water column. Those are pollock.

On our cruise Paul is comparing the sonar signatures produced by the different organisms under the boat to what we are actually catching in the nets. The use of acoustics technologies for stock assessments is fairly new and individual species can’t be recognized by the sonar images, but Paul can use the images to detect if an area will have a greater density of organisms. We are also selecting several locations between stations to do mid-water trawls. Paul selects areas that have a high density of organisms underneath the depth that our surface trawl nets reach and we do a mid-water trawl. He then compares what we find in the trawl to the sonar signatures that he saw in the images to see if he can find any patterns between specific species and sonar signatures. It will be amazing if some day fisheries biologists will be able to assess the stock of fisheries by using sonar instead of net trawls which are a lot more work and often result in the death of the fish.

Personal Log

Today's weather after the two low pressure systems had entered the area. The weather was pretty crappy the last two days, but today it is beautiful.

Today's weather after the two low pressure systems had entered the area. The weather was pretty crappy the last two days, but today it is beautiful.

We have had several lo- pressure systems blow through during our cruise and so far we have had two gale warnings. The first one occurred when we had only been out to sea for a day so it was easy to head back in to Dutch Harbor. The last one occurred a couple of days ago and we were too far out into the Bering Sea to turn back. We had no choice but to ride it out. Two low-pressure systems were colliding and the Bering Sea turned into a washing machine. There were consistent 10 – 13 ft waves coming from one direction, large 20ft swells coming from another direction, and the occasional 8 – 10 ft wave coming from a different direction. The ship just kind of bobbed from side to side and up and down and we were all along for the ride. Thank goodness I didn’t get sick, but I definitely didn’t sleep well.

Face to face with some angry seas.

Face to face with some angry seas.

I was also amused by how life went on for everyone onboard the ship. Dinner was hilarious as everyone held onto their dishes and your chair moved from side to side with the waves. Walking around was pretty funny too. There was no way that you could walk in a straight line. I would choose something to grab onto, walk another couple of steps, and then grab onto something else. As I tried to sleep at night I could hear the things that we had thought we had secured roll around the room. Who knew that a roll of paper towels could make so much noise? The curtain on my bed was making me crack up because it would roll open with one wave and close shut with another. It just kept opening and closing all night and there was nothing that I could do about it but laugh. Thankfully by today the seas had calmed down significantly and the sun is actually out.

Francesco was a lost shorebird who found his way to our ship in the middle of the Bering Sea.

Francesco was a lost shorebird who found his way to our ship in the middle of the Bering Sea.

There was one casualty though, and that was Francesco. Francesco was a shorebird, an American Pipit, that was blown way off course during the storm. He ended up cold and hungry on our back deck last night. We were able to catch him and we put him in a warm box with some dead flies, water, and crackers. He managed to eat and drink, but he was a juvenile and had very little body fat. He was pretty much skin and bones. He lasted until this afternoon and when we went to check on him, he was dead. We gave him a burial at sea and were reminded that the Bering Sea is a harsh, harsh environment.