Julia Harvey: Listening to Fish/How I Spent My Shift, July 28, 2013


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:  7/28/13

Weather Data from the Bridge (as of 18:00 Alaska Time):
Wind Speed: 15.61 knots
Temperature:  13.71 C
Humidity:  91%
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.

Trnasducer

Scientists are using the EK60 to listen to the fish.

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.

The transducer sends out a signal and waits for the return echo...

The transducer sends out a signal and waits for the return echo…

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.

swim bladder

The above picture shows the location of the swim bladder. (Photo courtesy of greatneck.k12.ny.us)

Click on this picture to see how sound travels from various ocean creatures through water. (Photo from sciencelearn.org)

Click on this picture to see how sound travels from various ocean creatures through water. (Photo from sciencelearn.org)

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.

echograms

These are the echograms that are produced by the EK60.  Five frequencies are used to help identify the type of fish.

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.

echogram for krill

These echograms are similar to the ones produced when we trawled for krill. Krill have a significant backscatter with the higher frequencies (bottom right screens)

Personal Log:

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.

Methot Trawl

Survey tech, Vince and Fishermen Brian and Kelly ready the methot trawl.

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.

Counting krill

I needed my reading glasses to count these small krill.

How many krill do you think we collected?

Krill Sample

This is the total krill from the first methot trawl of the night.
How many are here?

Patrick, the lead scientist, put a few specimens under the microscope so we could see the different types of krill.

krill

Closeup look at krill.
Photo courtesy of NOAA

The collection of krill was preserved in formaldehyde and sea water.  It will be sent to Poland for further species diagnosis.

preserving krill

Scientist Darin Jones preserves the krill for shipment to Poland.

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.

multibeam data

These images are what Jodi was processing.

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.

krill

This was the haul from the second methot trawl.

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.

trawl net haul

Emptying out the trawl net right next to the fish lab.

We collected data from the trawl net and the pocket net.

squid

This trawl had a variety of specimen including Pacific Ocean perch, salmon, squid, eulachon, shrimp and pollock.

The pocket net catches the smaller organisms that escape through the trawl net.

pocket trawl

These were caught in the pocket 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?

birds

Many sea birds follow the ship hoping for some of our catch.

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