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 10, 2024
Weather Data from the Bridge:
Latitude: 50° 40.9 N
Longitude: 178° 29.9 W
Wind Speed: 20 knots
Air Temperature: 6.2° Celsius (43.1° Fahrenheit)
Science and Technology Log:
Last blog post, I talked about acoustic backscatter, which scientists on board use to locate fish. When scientists notice high-intensity backscatter – or backscatter that they’re interested in collecting more biological data about – they’ll call the bridge and ask to go fishing. The bridge then makes the announcement over the radio:
“All stations. This is the bridge. We will be fishing, fishing, fishing.”
This announcement sparks a flurry of action from scientists, NOAA officers, and the deck crew. A few scientists go up to the bridge for a marine mammal watch, where they make sure that there are no marine mammals in the area of the operation. NOAA officers navigate to the science team’s target fishing area, and the deck crew prepares the net to go in the water.
Before my cruise, I thought fishing nets were relatively simple and uniform. However, I’ve since learned that the net has many different components and sensors, which help scientists collect additional information about the fish seen with acoustics.
Codend
During the trawl, the net is dragged behind the boat. Near the opening at the mouth of the net, the net’s mesh is over a meter wide. This helps reduce drag from the water, while still funneling fish toward the back of the net. The net gradually gets smaller until the very end of the net – called the codend – where the fish are collected. At the end of each trawl, the net is hauled out of the water, and the contents of the codend are emptied into a sorting table for further processing in the fish lab, where length, weight, sex, and maturity are recorded for a representative sample.
Pocket Nets
In portions of the net with larger mesh, small fish and other organisms can escape through the holes in the mesh. This creates a problem for scientists – a trawl could show that only adult pollock are present in a certain area when in reality the population is mixed, but all of the juveniles escaped! Since scientists will be using trawl samples to understand the overall population of pollock, they want to avoid bias as much as possible in their data.
To get around this problem, scientists are studying the rates at which different sized pollock (and other organisms) escape from the net. They use pocket nets, or small nets made of the same fine mesh as the codend, to get an idea of what escaped from each trawl. Nine pocket nets are attached to the side, top, and bottom of three different sections of the net with varying mesh sizes. As the trawl net is being hauled back on the boat, one of my jobs is to help empty these pocket nets and collect what’s inside.
We’ve mostly found krill and jellyfish, but occasionally we’ll find a larval fish or squid!
CamTrawl
Near the codend, there is also a camera, referred to as CamTrawl. This camera provides scientists with a visual of what is going into the net, and can be used to help identify species and length of fish that are caught.
On this cruise, scientists are also testing a camera that they lower over the side of the ship (without a net), known as PelagiCam. They are hoping that PelagiCam may be able to collect species and length data, supplementing the data captured when processing fish from the trawl. If PelagiCam can record this data accurately, it could provide an efficient complement to trawling, which requires a lot of time and collaboration between different teams of people.
FS70 Net Sounder
The FS70, nicknamed the Turtle, collects acoustic data and produces a live image of the net’s opening when it is in the water. This data allows scientists and the deck crew to monitor the shape of the net while fishing, ensuring that the net opened correctly. It also monitors when fish enter the net.
Personal Log:
Going fishing can sometimes be a lot of “hurry up and wait.” After the marine mammal watch, at least one scientist stays on the bridge to monitor the net using the FS70, and the others get ready to process the trawl. Letting the net out and hauling it back in is far from simple, however. It requires constant communication between the bridge and the deck crew, and it can be made more complicated by the weather or equipment malfunctions. Once the net is in the water, trawling can take anywhere from 15 minutes to over an hour.
Opening the codend is always exciting, because we’re never quite sure what we caught. While our target is always pollock, we’ll often find other interesting organisms mixed in as well. Some highlights include rockfish, squid, and a smooth lumpsucker.
Did you know?
The net used on NOAA Ship Oscar Dyson was specifically designed for this survey!