Unexpected sea ice south of St Lawrence island on 6/25
Posted on by gsturdevant85

Guy Sturdevant: The Cave pt. 1, June 29, 2026

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: June 29, 2026

Weather Data from the Bridge

N 58.6° W 170.4 °, 0 AMSL

Conditions: Fog, Seas at 4’

Visibility: < 3 NM

Wind: 70°/ 9 kt

Barometric Pressure 29.9 inHg

Dry Bulb Temp: 43 ° F

Science Log

So, we’ve taken a chilly dive into the why behind the focus on the pollock. Today, I will take you into “The Cave,” where we can learn how scientists use sound to locate and count pollock. On the port side of the main deck sits a dark, windowless room lit only by the dozen or so monitors adorning its aft wall. A gentle, constant humming fills the room from racks and racks of electronics, servers, and support equipment that dominate the center of this space. While the OOD on the bridge steers this vessel, “The Cave” calls the scientific shots by determining the ship’s course as well as the timing and location of all science operations. 

a man and a woman sit in computer chairs at a desk beneath an array of 8 computer monitors; the large computer stack is visible to the right. the two scientists lean far back in their chairs to look up at the screens above.
Abigail McCarthy and Mike Levine discuss plans for the day shift. Time at sea is precious; this vessel operates 24/7 in all conditions. For the past two days, a very quiet, fishless northern extension has limited opportunities. But remember, even a null result is a result!

Acoustics 101

Since the early 20th century, scientists have used the unique ability of sound waves to transmit very efficiently through water for remote sensing. “Pings” of acoustic energy are generated by a transmitter, and then the backscatter (or reflected sound) is detected by a receiver. Early pioneers used sonar to better understand the physical geography of ocean basins in a process called bathymetry.

a graphic showing a cut-out photo of a ship (USS Stewart, DD-13) at the surface of the ocean (depicted as a blue rectangle) above the seafloor (a brown rectangle.) in the animation, upside-down orange parabolas extend from the bottom of the ship toward the seafloor; then right-side up dotted parabolas, like rainbows, extend back from the seafloor up to the ship's bottom. there is a cutout image of the antique echosounder off to the right. There is a speech bubble containing the equation for seafloor depth. The graphic is titled The North Atlantic, 1922: Acoustic Bathymetry
USS Stewart first tested an early form of echosounder in 1922 as part of preparations for the installation of the Transatlantic cable.

Not long after the first echosounders made their way aboard ships, scientists realized that as the quality of the instrument increased, they could measure the backscatter (or reflected sound) off of other objects besides the seafloor. Large backscattering layers far above the seafloor were targeted by fishing vessels using the new technology, demonstrating the effectiveness of echosounders at locating marine organisms throughout the water column.

a static graphic showing a cut-out photo of a ship at the surface of the ocean (depicted as a blue rectangle) above the seafloor (a brown rectangle.) 3 upside-down orange parabolas, representing the wave front, extend from the bottom of the ship toward the seafloor; 3 right-side up dotted parabolas, like rainbows, extend back from the seafloor up toward the ship's bottom, representing seafloor backscatter. cutout images of individual pollock fish are pasted in a "school" in the middle of the blue ocean water, and 3 blue rainbow-oriented parabolas extended up from the fish school, representing fish backscatter. this slide is titled: Acoustic Trawling.
Early innovators in Norway and England reported success in using echosounders to detect large schools of fish and began actively monitoring their behavior (Balls, 1948).

The following decades of acoustic research relied on analog, single-beam systems, which were often towed behind or below a vessel and recorded a narrow swath directly below the ship onto a paper echogram. 

composite photo of a porcelain wall showing an echogram. arrows and text have been superimposed on the photo to point out the seafloor backscatter and the school of pollock backscatter. in the lower right are the words NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION.
A 3d porcelain rendering of this now-famous echogram (the recorded chart of an echosounder) from the Shelikof Straight adorns the entry to the NOAA Alaska Fisheries Science Center in Seattle. The strong red and yellow reflections that sweep gently across the bottom represent the strong backscatter from the seafloor, and the large red cloud represents a large school of pollock.

The 1990’s welcomed a new era in echosounder technology with the release of the SIMRAD EK-500. This landmark digital echosounder combined multi-frequency operation with improved data processing and integration tools, enabling much better estimates of fish population density and biomass.

a graph of target strength (low, medium, high) v. frequency (kHz, log scale). three lines graph this relationship for fish (swim bladders) at 50-600 mm length; krill at 10-60 mm length; and copepods 0.2-20 mm length.
Larger acoustic targets, such as the swim bladder of a large fish, produce strong backscatter at relatively low frequencies, whereas smaller organisms, such as krill and copepods, reflect sound only at much higher frequencies.  Multi-frequency echo sounder measurements allow scientists to discriminate between acoustic targets of different sizes and target strengths and more accurately estimate the biomass of individual organisms as they scroll across the screen.

Next time, we will look at the echograms produced aboard Oscar Dyson and receive a crash course in interpretation from the Cave!

Personal Log

Work hard, play hard is an unofficial motto aboard Oscar Dyson. The officers, crew, and science team are keeping a fierce eye on the World Cup when off duty (Colombia’s goal call-back was a travesty!!). 

a 16-competitor bracket drawn on an old hydrographic chart. beneath the chart is the title: The Inaugural Collin McMillan Memorial Biannual Oscar Dyson Amateur Cribbage Tournament.
The “Inaugural Collin McMillan Memorial Biannual Oscar Dyson Amateur Cribbage Tournament” is underway; stay tuned for updates and potential video coverage of the championship match!
Guy, wearing overalls and long yellow gloves, holds up a flatfish pointing toward his face, and makes a kissy face at a safe distance.
The future gyotaku model, Northern rock sole (Lepidopsetta polyxystra), posing for a picture before her big debut.
fish print, in black ink, of a flatfish
Gyotaku is the traditional Japanese art of collecting fish prints. Engineer Victoria Southwick, ENS Josh Bennett, and Lt. Jesse Pierce captured the print of a Northern rock sole (Lepidopsetta polyxystra) brought up on haul 71, 06/28/26.

Wildlife sightings

highly detailed photo of an albatross floating at the ocean's surface
A Short-tailed albatross (Phoebastria albatrus) follows us during trawling operations, hoping for a fishy treat. This threatened marine bird is a tale of cautious conservation success. Their population in the 1950s dwindled to as low as 25 individuals. Today, roughly 4,200 individuals are known to exist.

Fun Fact

In the Cave, it is not uncommon for the shallow layer to be filled with a mix of non-fish backscatter. Everyone has their pet theories as to what may be the source of these shallow acoustic targets (we know they aren’t fish), but they have all agreed to call it by one name… munge. Below is my artist’s interpretation of Munge as a heavy metal album.

a comical graphic of NOAA Ship Oscar Dyson floating, algae covered, in a black ocean, above the word MUNGE (written in death-metal style lettering). at the bottom right is a play on the NOAA logo that creates an octopus-type creature beneath the word MACE
MUNGE album cover

Sources

  1. Balls, R. 1948. Herring fishing with the echometer. Journal du Conseil International pour l’Exploration de la Mer, 15: 193–206.
  2. Korneliussen, R. J. (2018). Acoustic target classification. 
  3. Benoit-Bird, K. J., & Lawson, G. L. (2016). Ecological insights from pelagic habitats acquired using active acoustic techniques. Annual review of marine science, 8, 463-490. 
  4. Mordy, C. W., Bond, N. A., Cokelet, E. D., Deary, A., Lemagie, E., Proctor, P., … & Wisegarver, E. (2023). Progress of fisheries-oceanography coordinated investigations in the Gulf of Alaska and Aleutian Passes. Oceanography, 36(2/3), 94-100. 
  5. De Robertis, A., McKelvey, D. R., & Ressler, P. H. (2010). Development and application of an empirical multifrequency method for backscatter classification. Canadian Journal of Fisheries and Aquatic Sciences, 67(9), 1459-1474. 
  6. Simmonds, J., & MacLennan, D. N. (2008). Fisheries acoustics: theory and practice. John Wiley & Sons. 
  7. Holliday, D. V., & Pieper, R. E. (1995). Bioacoustical oceanography at high frequencies. ICES Journal of marine Science, 52(3-4), 279-296. 
  8. Echoview. (2019). Acoustics Unpacked. https://acousticsunpacked.echoview.com/acoustics/AcousticsUnpacked.asp

Leave a Reply

Discover more from NOAA Teacher at Sea Blog

Subscribe now to keep reading and get access to the full archive.

Continue reading