NOAA Teacher at Sea
Aboard NOAA Ship Oscar Dyson
July 24 – August 11, 2018
Mission: Pollock Acoustic-Trawl Survey
Geographic Area of Cruise: Eastern Bering Sea
Date: August 1, 2018
Weather Data from the Bridge:
Latitude: 61 55.41 N
Longitude: 172 48.34 W
Wind Speed: 2.24 knots
Wind Direction: 77.54 (NW)
Air Temperature: 9.7 C
Barometric Pressure: 998.8 mb
Visibility: 9 nautical miles
Sea Wave Height: 3 feet
“When am I ever going to use this?” This is the query of many students who are required to take mathematics courses. However, scientists aboard the NOAA Ship Oscar Dyson use mathematics every day as part of their job. As discussed in a previous blog post, underwater acoustic data are collected as the NOAA Ship Oscar Dyson navigates along the transects. These backscatter data are relied upon to decide when to take trawling net samples as well as to estimate the number and biomass of pollock in the area.
How do these underwater acoustics work? The answer can be found in mathematics and physics. As previously discussed, echosounders affixed to a centerboard below the hull of the ship send an audible ping down into the water and measure how long it takes to bounce off of an object (like a pollock) and return to the surface. The echosounders know the transmitted signal power (denoted Pt) and measure the received signal power (denoted Pr). Measuring the time between the signal transmission and reception and multiplying by the speed of sound (approximately 1450 m/s, given local water salinity and temperature conditions) will allow the calculation of distance of an object below the surface (or range denoted r). Using acoustics properties combined with known properties of pollock, we can get the equation for backscattering strength at a point as , where β is a constant and C(r) is a constant that is dependent on range.
However, since sound is measured in decibels which are arranged on a log scale, 10 times the log of both sides of the backscattering strength equation is desired. Using logarithm rules, this becomes
The value on the left-hand side of this equation is commonly referred to as target strength (TS) and is an important value to complete the survey.
The target strength is the amount of energy returned from a fish of a certain length. Since the echosounders are transmitting through the water column below the ship, the TS values are converted to backscatter strength per volume unit of water, referred to in the literature as Sv. The Sv values are graphed on the EK60 scientific echosounder, giving a picture similar to the one below. Different colors in the output are matched to various ranges of Sv values. An experienced fisheries scientist, like the ones aboard the NOAA Ship Oscar Dyson, can use the echosign data to determine a possible picture of the ocean life below the ship. While the EK60 scientific echosounder can transmit at five different frequencies (18 kHz, 38 kHz, 70 kHz, 120 kHz, and 200 kHz), the 38-kilohertz transmission frequency is the best frequency to detect pollock. Other transmission frequencies are shown to help delineate adult pollock from baby pollock and from other types of fish and smaller crustaceans called euphausiids.
The target strength is related to the length of the fish. The age of pollock is strongly correlated to their length until they are about 4 years old, so length can help the scientists determine how many of each year class are in the ocean below. Once again, logarithms come in handy, as the equation that relates the fork length in centimeters, l, of the pollock to the recorded target strength is TS = 20 log l – 66. This allows the scientists to use the echosounder data to get an approximate measure of the fish below without having to catch them.
Today we will be going on a partial tour of NOAA Ship Oscar Dyson so you can see where I spend most of my time while aboard. The first stop is my stateroom, where I sleep and relax when not on shift. The top bunk is mine and the bottom bunk belongs to my roommate, NOAA scientist Abigail McCarthy. Our stateroom has one window where we can check on the weather and sea conditions. The picture below shows our view most of the time: cloudy!
Next stop is the mess hall where three meals a day are served. The stewards do a great job of cooking creative meals for everyone aboard. Before I boarded the ship, I bought a lot of snacks because I was worried about not getting enough to eat, but boy was I wrong. There is always plenty to eat at every meal, snacks that are out if you get hungry in between, and lots of dessert!
Finally, we come to the fish lab where the trawling net samples referred to in my last blog post are processed. Before processing, we go to the ready room and put on our gear. This includes work boots as well as waterproof coveralls and jacket. Measuring the length of the pollock can get messy so we have to have the right gear. Once in the fish lab, we grab our gloves and get to measuring!
Did You Know?
Scientists aboard the NOAA Ship Oscar Dyson are part of the National Marine Fisheries Service (NMFS), which is one of the six major line offices of NOAA.