Mission: Walleye Pollock Survey
Geographical Area of Cruise: Gulf of Alaska
Weather Data from the Bridge (as of 18:00 Alaska Time):
Wind Speed: 15.61 knots
Temperature: 13.71 C
Barometric Pressure: 1023 mb
Science and Technology Log:
How do scientists use acoustics to locate pollock and other organisms?
Scientists aboard the NOAA Research Vessel Oscar Dyson use acoustics, to locate schools of fish before trawling. The Oscar Dyson has powerful, extremely sensitive, carefully calibrated, scientific acoustic instruments or “fish finders” including the five SIMRAD EK60 transducers located on the bottom of the centerboard.
This “fish-finder” technology works when transducers emit a sound wave at a particular frequency and detect the sound wave bouncing back (the echo) at the same frequency. When the sound waves return from a school of fish, the strength of the returning echo helps determine how many fish are at that particular site.
Sound waves bounce or reflect off of fish and other creatures in the sea differently. Most fish reflect sound energy sent from the transducers because of their swim bladder<s, organs that fish use to stay buoyant in the water column.
These reflections of sound (echoes) are sent to computers which display the information in echograms. The reflections showing up on the computer screen are called backscatter. The backscatter is how we determine how dense the fish are in a particular school. Scientists take the backscatter that we measure from the transducers and divide that by the target strength for an individual and that gives the number of individuals that must be there to produce that amount of backscatter. For example, a hundred fish produce 100x more echo than a single fish. This information can be used to estimate the pollock population in the Gulf of Alaska.
The trawl data provide a sample from each school and allow the NOAA scientists to take a closer look by age, gender and species distribution. Basically, the trawl data verifies and validates the acoustics data. The acoustics data, combined with the validating biological data from the numerous individual trawls give scientists a very good estimate for the entire walleye pollock population in this location.
How I spent my shift on Saturday, July 27th?
When I arrived at work at 4 pm, a decision was made to trawl for krill. A methot trawl is used to collect krill.
Then we set to work processing the catch. First we have to suit up in slime gear because the lab will get messy. My previous blog mentioned not wanting to count all of the krill in the Gulf of Alaska. But in this case we needed to count the krill and other species that were collected by the methot trawl.
How many krill do you think we collected?
Patrick, the lead scientist, put a few specimens under the microscope so we could see the different types of krill.
The collection of krill was preserved in formaldehyde and sea water. It will be sent to Poland for further species diagnosis.
As the ship continued back on transect, I wandered in to see what Jodi and Darin were doing with the data collected last night. Jodi was processing data from the multibeam sonar and Darin was surveying the images from the drop camera. Jodi was very patient explaining what the data means. I will write more about that later. But I did feel quite accomplished as I realized my understanding was increasing.
A decision was made to do another methot trawl. This time we had a huge sample.
In an approximately 50 gram sample we counted 602 individual krill. Compare this to the 1728 individuals in a 50 gram sample from the first trawl. They were much bigger this time. The total weight of the entire sample of krill was 3.584 kilograms.
How many individuals were collected in the second trawl? (Check your answer at the end of the blog)
Around midnight, Paul decided to verify an echogram by trawling.
We collected data from the trawl net and the pocket net.
The pocket net catches the smaller organisms that escape through the trawl net.
It was after 2 am by the time we had processed catch and washed down the lab. The internet was not available for the rest of my shift due to the ship’s position so I organized my growing collection of videos and pictures.
I wasn’t sure how I would handle my night shift (4 pm to 4 am) after I dozed off during the first night. Now that I have adjusted, I really enjoy the night shift. The night science team of Paul, Darin and Jodi are awesome.
Did You Know?
People who are on the Oscar Dyson live throughout the United States. They fly to meet the boat when they are assigned a cruise. Jodi is from Juneau, Alaska. Paul is from Seattle, Washington. And Darin is from Seattle/North Carolina. There are a number who are based out of Newport, Oregon.
Something to Think About:
When we are fishing, a number of birds gather behind the boat. What different sea birds are observable this time of the year in our survey area?