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: 8/7/13
Weather Data from the Bridge (as of 21:00 Alaska Time):
Wind Speed: 10.42 knots
Temperature: 13.6 C
Humidity: 83%
Barometric Pressure: 1012.4 mb
Current Weather: A high pressure system is building in the east and the swells will increase to 8 ft tonight.
Science and Technology Log:
Before I begin, I must thank Paul for educating me on the calibration process. Because calibration occurred during the day shift, I was not awake for some of it.
The EK60 is a critical instrument for the pollock survey. The calculations from the acoustic backscatter are what determines when and where the scientists will fish. Also these measurements of backscatter are what are used, along with the estimates of size and species composition from the trawling, to estimate fish biomass in this survey. If the instruments are not calibrated then the data collected would possibly be unreliable.
Calibration of the transducers is done twice during the summer survey. It was done before leg one in June, which began out of Dutch Harbor, and again now near Yakutat as we end leg three and wrap up the 2013 survey.
As we entered Monti Bay last night, Paul observed lots of fish in the echosounder. This could pose a problem during calibrations. The backscatter from the fish would interfere with the returns from the spheres. Fortunately fish tend to migrate lower in the water column during the day when calibrations were scheduled.
This morning the Oscar Dyson moved from Monti Bay, where we stopped last night, into Sea Otter Bay and anchored up. The boat needs to be as still as possible for the calibrations to be successful.
Map by GoogleEarth
Calibration involves using small metal spheres made either of copper or tungsten carbide.
photo courtesy Richard Chewning (TAS)
The spheres are placed in the water under transducers. The sphere is attached to the boat in three places so that the sphere can be adjusted for depth and location. The sphere is moved throughout the beam area and pings are reflected. This backscatter (return) is recorded. The scientists know what the strength of the echo should be for this known metal. If there is a significant difference, then data will need to be processed for this difference.
The 38 khz transducer is the important one for identifying pollock. A tungsten carbide sphere was used for its calibration. Below shows the backscatter during calibration, an excellent backscatter plot.
The return for this sphere was expected to be -42.2 decibels at the temperature, salinity and depth of the calibration The actual return was -42.6 decibels. This was good news for the scientists. This difference was deemed to be insignificant.
Personal Log:
Calibration took all of the day and we finally departed at 4:30 pm. The views were breathtaking. My camera doesn’t do it justice. Paul and Darin got some truly magnificent shots.
As we left Yakutat Bay, I finally saw a handful of sea otters. They were never close enough for a good shot. They would also dive when we would get close. As we were leaving, we were able to approach Hubbard Glacier, another breathtaking sight. Despite the chill in the air, we stayed on top getting picture after picture. I think hundreds of photos were snapped this evening.
Did You Know?
According to the National Park Service, Hubbard Glacier is the largest tidewater glacier in North America. At the terminal face it is 600 feet tall. This terminal face that we saw was about 450 years old. Amazing!
Read More about Hubbard Glacier
