NOAA Teacher at Sea
Guy Sturdevant
Aboard Oscar Dyson
June 21 – July 15, 2026
Mission: Summer Pollock Acoustic Survey, Leg 2
Geographic Area of Cruise: Bering Sea, Alaska
Date: July 6, 2026
Weather Data from the Bridge
N 59.52° W 172.60 °, 0 AMSL
Conditions: Overcast, Seas at < 1’
Visibility: >5 NM
Wind: 90°/ 5 kt
Barometric Pressure 1016.1 mBar
Dry Bulb Temp: 45.3 ° F
Science Log
In my last post, we left off our acoustics 101 with the emergence of the first modern echosounders in the 1990s. Today, we will look at the current system aboard Oscar Dyson and learn how the science team can use their knowledge of acoustics to estimate fish populations. First, let’s look at the physical components that make up the EK80 echosounder system.
Transceiver – a combination of a transmitter and a receiver; in other words, it both produces an electrical pulse to be sent to the transducer and converts the backscattered signal into usable data a computer can understand. You can think of the transceiver as the electronic brain that manages all of the signal inputs and outputs.
Transducer – Just like you might plug a microphone into your laptop to record audio, each transceiver needs a transducer to first convert the electrical pulse into an acoustic pulse that is transmitted into the water, and to measure the acoustic backscatter that returns. You can actually see the transducers in the photo of the centerboard below. The transceivers measure frequencies ranging from 18 kHz (those really annoying mosquito ringtones that only young people can hear are around 18 kHz) to 330 kHz.
The red circles on the bottom of the centerboard are the faces of the transducers. These sensitive instruments are mounted at the lowest point of the ship to isolate them from the vessel’s noisy hull. (Photo credit: NOAA)
The Echogram
Once the transceivers process the acoustic backscatter, the data is displayed on a screen for interpretation.
Our old friend munge is making an appearance in this echogram! It is the heavy backscatter layer just beneath the surface that is strongest at 18 kHz. Lower in the water column, we see that most backscatter occurs at higher frequencies, with only sparse backscatter in the lower-frequency plots. Backscatter that is observed only at higher frequencies indicates smaller organisms, such as krill or copepods. Backscatter that appears across all frequencies is likely generated by fish.
As you spend more time looking at this scrolling echogram, you can begin to recognize patterns and draw reasonable inferences. Below are some examples of the variety you can see in just a few hours in the cave.
Now that we can read echograms, we are ready to call for our first trawl! Come back next time to see what we data we can scoop up in “The Anatomy of a Midwater Trawl”.
Personal Log
Things aboard Oscar Dyson have settled into a routine. We travel along acoustic transects during daylight hours, stopping 2-3 times a day to do a midwater trawl. Routine doesn’t mean boring, though! Maintaining a ship of this size and complexity is more than enough to keep everyone busy. The checklist for this leg included checking on the smaller craft that service and support Oscar Dyson on her mission. Conditions cleared on 06/29, and the Peggy D, the workboat that lives on the starboard hero deck, was given a thorough check and taken for a 30-minute voyage.
Safety drills and practice are a part of the routine as well. ENGR Connor Rauch practices recovery during a man-overboard drill on Peggy D. In the case of an actual man overboard, the smaller vessels are used for recovery, as they can respond much more nimbly and are far safer in close quarters with a swimmer.
Wildlife
