NOAA Teacher at Sea
Nick Lee
Aboard NOAA Ship Oscar Dyson
June 29 – July 20, 2024
Mission: Pollock Acoustic-Trawl Survey
Geographic Area of Cruise: Eastern Bering Sea
Date: July 6, 2024
Weather Data from the Bridge:
Latitude: 61° 15.0 N
Longitude: 174° 56.8 W
Wind Speed: 13 knots
Air Temperature: 5.3° Celsius (41.5° F)
Science and Technology Log:
On NOAA Ship Oscar Dyson, the science party’s mission is to understand the population of walleye pollock in the Eastern Bering Sea. To collect data, scientists rely on two main tools: acoustics and targeted trawling. Before any trawling can happen, scientists must first locate fish using acoustics, so I’ll be focusing on acoustics in this blog post – stay tuned for a post on trawling next time!
Scientists use two kinds of acoustics: active and passive. Many of my students are familiar with how bats use echolocation to navigate in the dark – active acoustics relies on the same principle. First, the echosounder on the ship emits a pulse of sound, or ping. This sound travels through the water and bounces off of objects that have different densities than water (such as fish, krill, or the ocean floor). The echosounder then “listens” for and records these echoes, also known as backscatter. Passive acoustics work similarly, except the echo sounder only listens for sound and doesn’t emit any itself.
The greater the distance between the echo sounder and the object reflecting the pulse, the greater the amount of time between when the signal was emitted and backscatter. Based on this time, echosounder can determine the depth of the object producing the backscatter. This information is represented visually in an echogram:
The echogram shows depth on the y-axis and time on the x-axis. The intensity of backscatter is color-coded, where more intense backscatter is represented with red and brown, and less intense backscatter is represented with blue and green. The vertical grid lines represent all the backscatter from one ping, and the space between lines represent 100 pings.
On the cruise, pings are typically emitted at a rate of 1 Hz, or once every second. With every new ping, the echo sounder adds data to the right end of the echogram. This means that the horizontal grid lines represent the backscatter at one depth over time (or distance, if the ship is traveling at a constant speed).
At least one scientist monitors the backscatter throughout the duration of the transect. During the first day, the echogram was blank except for some lower-intensity backscatter near the surface and high-intensity reflection from the ocean floor. Because the mission of this cruise is to survey pollock, which tend to live at greater depths, we don’t pay much attention to the backscatter near the surface which is comprised of smaller organisms like krill. However, when scientists notice backscatter consistent with scattering from pollock, they may trawl to collect a sample for more detailed biological information.
As we traveled along the first transect line, there was very little backscatter that the science team thought represented pollock. Our CTD (conductivity, temperature, depth) measurements also showed that the water temperature was cold, right around freezing. This may suggest that we were traveling through the Bering Sea cold pool, a mass of cold water that forms from melting ice. This water tends to be too cold for pollock and other fishes, however, other animals, such as snow crabs, can still survive the lower temperatures. Fish like cod prey on snow crab, so the cold pool offers these crab an important refuge from predators. Read more about the importance of the cold pool for crab here!
Personal Log:
The start of the cruise has been busy learning new faces, maritime practices, and scientific terms. However, in the past few days, with the help of meclizine (seasickness medication), I’ve begun to feel more settled and like I have some sense of routine.
When I’m on shift, I bounce around between a few different places. The science team tends to be in the acoustics lab, where we monitor backscatter and make decisions on when to fish.
Once the scientists decide to fish, we first go up to the bridge, where NOAA officers control the direction and speed of the ship. The bridge has windows on all sides, so we’re able to make sure there are no marine mammals before putting the net in the water.
From the bridge, you can also see the trawl deck, where the deck crew works in collaboration with NOAA officers to put the net in the water. Once the fish are caught and hauled back to the ship, the science team processes the catch in the fish lab.
When we’re not working, we’ll grab food from the galley / mess deck. The stewards on the ship serve three meals a day, but since I’m on the night shift, I often heat up leftovers or take advantage of the wide selection of snacks they leave out. There’s also a lounge, two gyms, and places to do laundry while at sea!
Did you know?
NOAA Ship Oscar Dyson has six onboard laboratories including a wet lab, dry lab, electronics lab, bio lab, acoustics lab, and hydrographics lab. Read more about the ship here!