Kathleen Harrison
Aboard NOAA Ship Oscar Dyson
July 4 — 22, 2011
Location: Gulf of Alaska
Mission: Walleye Pollock Survey
Date: July 12, 2011
Weather Data from the Bridge
Air Temperature: 10.15° C, Sea Water Temperature: 7.6° C
True Wind Speed: 12.26 knots, True Wind Direction: 191.38°
Very foggy, visibility < 1/4 mile
Door open on bridge to hear other fog horns
Latitude: 56.07° N, Longitude: 158.08° W
Ship Heading: 24°, Ship Speed: 11.7 knots
Science and Technology Log: Finding Fish
In a previous log, I talked about using nautical charts and trawling as 2 methods used in calculating the biomass of Walleye Pollock in the Gulf of Alaska. Finding the fish to catch is tricky business in the ocean, they don’t usually come up to the surface and say hi. The NOAA scientists working on the Walleye Pollock Survey spend a lot of time looking for fish, so that their trawling efforts won’t be wasted (that is the general idea, anyway). How do you look for fish in the ocean? With acoustics, of course, another method used in calculating biomass.
Acoustics is the use of sound, which will travel through the water, and bounce off of objects that it hits. There is Simrad ER60 echosounder that operates 5 transducers mounted on the center board under the ship, and it continuously sends out sound waves.
In the Acoustics Lab of the Oscar Dyson, the data from the multi-beam echosounder is being studied all of the time. The sound waves leave the device, travel down, hit the swim bladder in a fish (the fish doesn’t even know), and reflect back to the ship. The time it takes for the sound to return is used to calculate the distance down, and a computer generated picture called an echogram is produced.
The echogram tells the scientists several things. The surface of the water is shown, with surface dwelling organisms such as krill, phytoplankton, zooplankton, and juvenile fish. The fish that are mid-water are shown as well, showing up as red or blue dashes or blobs. This is where the Pollock usually are. Some fish are bottom feeders, and the red and blue dashes on the bottom represent those. The ocean floor is also shown, which is very important when choosing which type of trawl to use. If the bottom is flat, the Poly Nor’Easter could be used to capture to fish on the bottom. The Aleutian Wing Trawl might be used in mid water if the bottom is rocky and irregular.
Now, looking at the fish from the surface is nice, but wouldn’t it be better to see them close up? Of course! The scientists have another tool at their disposal, and no, it isn’t me diving down to the fish (brrr). This tool is called a Drop Target Strength, or DTS.
About once a day, or every other day, the DTS is lowered over the side, and it starts sending out sound waves (3 pings/second), just like the echosounder mounted on the ship. The advantage with the DTS, though, is that it is closer to the fish, giving a more detailed and accurate picture. Individual fish can be sighted. Taking a picture of a fish is kind of like taking a picture of a toddler, they don’t hold still very well. So, a count of the fish on the echogram might not be exact. Also, they might change the angle of their body, making the sound wave reflect off their swim bladder at a different angle. The colors on the echogram are significant: brown and red mean a strong signal, yellow is medium, and green and blue indicate a weak signal.
The scientists will study the echograms to determine where the fish are, and make a decision to fish or not. Once fishing begins, they will move from the acoustics lab to the bridge, and study the echograms there. An estimate of how many fish are in the net is made, and then the scientists will ask the crew to “haul back” the net. (I am learning a whole new language!) Then, things get very busy as we head to the fish lab to process the fish.
New species seen:
Giant Pacific Octopus (juvenile, 1 cm)
Opalescent Squid
Chinook (King) Salmon
Egg yolk jelly fish
Sculpin (juvenile)
North Pacific sea nettle
Spud sponge
Personal Log
My days have developed a routine now: wake at 3:30 am (ugh), start my shift in the acoustics lab about 4:00, breakfast at 7:30, lunch at 11:30, end my shift at 4:00 pm, dinner at 5:30, shower, in bed by 8:00.
In between these times, I work on my Teacher at Sea log, post pictures on Facebook, read and answer e-mail, visit the bridge and ask lots of questions, and of course, process fish whenever there is a trawl (very fun). Today marks the halfway point of our cruise! The ship is quieter than I thought, even though there are 35 people on board, the most that I ever see might be 10 during mealtimes. There is constant background noise of the ship’s engines, waves hitting the bow of the ship, creaks and groans of the furniture as the ship rolls, but I am used to it now, and hardly notice it. I am thankful for the calm weather that we have had so far.