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)
Chinook (King) Salmon
Egg yolk jelly fish
North Pacific sea nettle
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.
11 Replies to “Kathleen Harrison: Finding Fish, July 12, 2011”
Hi Mrs. Harrison!
I’m enjoying reading your blog and learning about all the interesting technologies that you’re seeing being used on the ship. It’s great hearing about all the information that we learned last year being used in the real-life scientific field. I hope you’re having a great time and can’t wait to hear even more about your trip once school starts again!
Thanks for reading my blog, I am enjoying writing it, and I have learned so much. Yesterday I saw fin whales, minke whales, humpback whales, and Dall’s porpoises. I can’t wait to share my trip with your class. Hope you are having a good summer.
Hey Mrs. Harison ! I love the blog ! Save me one of those Baby Octopuses please:)
I really like the octopuses too. Actually, one of the Survey Technicians on the Oscar Dyson put the baby octopus in a vial with some preservative. She will keep it to show to students who come aboard in the winter time when the ship is docked in Seattle.
Hello Mrs. Harrison,
I find those juvenile squid and octopus really cute. How did the crew collect them? They seem incredibly small for a net. I also enjoyed reading your blog.
These juvinile jellyfish and squid look adorable Mrs. Harrison. How did you catch them? They seem to small for a net.
Good question – the smaller items were caught in a special net that was attached to the big trawl net. It had very small mesh, and its purpose was to provide a sample for one of the scientists to study the smaller organisms that were found in the same area as the pollock.
Hey Mrs. Harrison!
The baby octopus is adorable.. It’s weird to think an animal like an octopus can be cute!
I’ve never heard of a DTS, it seems like echolocation on steroids. It look a lot more effective than just SONAR or something similar.
Love the blog!
those squids are so cute I Love them I hope you guys didn’t eat them but how do you catch those little things they look like they could get out of anything and what would you use to mark them?
btw waves are one of my favorite sounds 🙂
These are taking forever to post I just got a notification from my first one
Question did you eat anything out of the sea?
yes we did have some sea food on board the ship – rockfish, pollock, and halibut – it was all delicious!
Those juvinile squid are awesome looking. I was going to ask you how they were caught but I looked at the response you gave to Henrath.
Looks like you had a very interesting trip that was informational (:
– Lillie Parker