NOAA Teacher at Sea
Julia Harvey
Aboard NOAA Ship Oscar Dyson (NOAA Ship Tracker)
July 22 – August 10, 2013
Mission: Walleye Pollock Survey
Geographical Area of Cruise: Gulf of Alaska
Date: 8/1/13
Weather Data from the Bridge (as of 00:00 Alaska Time):
Wind Speed: 12 knots
Temperature: 13 C
Humidity: 97 %
Barometric Pressure: 1021 mb
Science and Technology Log:
The main goal of Leg 3 of this mission is to use acoustics and trawling to survey the mid-water portion of the pollock population along the Gulf of Alaska starting near Kodiak to Yakutat Bay.
Pollock live in the an area between the middle of the water column and the seafloor. Sometimes we sample the mid-water and sometimes we will sample the bottom.
The Oscar Dyson carries three different types of trawling nets for capturing fish as part of the mid-water survey: the Aleutian Wing Trawl (AWT), a mid-water trawl net, the Poly Nor’Eastern (PNE), for bottom trawls and the Methot, which is for gathering samples of very small ocean creatures such as krill. I will focus on the AWT, although some of the video footage is from a bottom trawl.
When the net is deployed from the ship, the first part of the net to hit the water is called the codend. This is where most of the fish end up after the trawl. The mesh size of the net is smallest at the codend (about 1 cm) and gets larger as it approaches the doors (about 1 m).
A Cam Trawl goes in the water next. This is a pair of cameras that help scientists identify and measure the fish that are caught in the net. This technology can also be used to help scientists validate their biomass estimate from trawling sampling counts. This piece of equipment has to be clipped into loops on the trawl each time.
The next piece of the net to hit the water is the “kite” which is secured to the head rope. Here, a series of sensors is attached to help the scientists gather data about the condition of the net including depth, size, and shape underwater. The major acoustic sensor, called the “turtle,” can tell if the fish are actually going into the net.
Once the kite is deployed, a pair of tom weights (each weighing 250 lbs), are attached to the bridal cables to help separate the head rope from the foot rope and ensure the mouth of the net will open. Then, after a good length of cable is let out, the crew transfers the net from the net reel to the two tuna towers and attach the doors. The doors create drag to ensure the net mouth opens wide.
The scientists use acoustic data to determine at what depth they should fish, then the OOD (Officer on Deck) uses a scope table to determine how much cable to let out in order to reach our target depth. Adjustments to the depth of the head rope can be made by adjusting speed and/or adjusting the length of cable released.
The scientists use the acoustic data sent from the “turtle” to determine when enough fish are caught to have a scientifically viable sample size, then the entire net is hauled in.
Once on board, the crew uses a crane to lift the cod end over to the lift-table. The lift-table then dumps the catch into the fish lab where the fish get sorted on a conveyor belt.
Personal Log:
The Oscar Dyson needed to pick up materials for a net repair so we headed into Prince William Sound towards Valdez. The area was spectacular.
The sun was out and the skies were blue for most of the day. Although we have had very calm seas, we have been under clouds for most of the last few days.
The absolute highlight of the day was spotting Dall porpoises and filming them bow surfing.
Here are snapshots of the day. The area was so impressive that I have several hundred pictures. Here are just a few:
I am reminded of the Exxon Valdez oil spill devastation.
Did You Know?
The Exxon Valdez (oil tanker) ran aground on Bligh Reef in Prince William Sound, Alaska on March 24, 1989.
The amount of oil spilled into this pristine environment exceeded 11 million gallons of crude oil and affected over 1300 miles of shoreline. According to OCEANA, as many as 2,800 sea otters, 300 harbor seals, 900 bald eagles and 250,000 seabirds died in the days following the disaster.
Jodi, who works the night shift with me, grew up in Cordova, Alaska and as a child remembers the smell of the disaster and the fears in her town (many were fishermen).
Has the area recovered? Part of the settlement with Exxon established a fund to support research. Read more.
Great pictures!
Wait for the sharks!
Glorious Photos, Julia. We’re Thinking of you-
Folks at Camas Country Mill
The weather has cleared as we head towards Yakutat Bay. I should be able to get some gorgeous shots but may need to stay up tomorrow.