Anne Mortimer: Shelikof Strait, July 16, 2011


NOAA Teacher at Sea
Anne Mortimer
Onboard NOAA Ship Oscar Dyson
July 4 — 22, 2011 

Mission: Pollock Survey
Geographical area of cruise: Gulf of Alaska
Date: July 16, 2011

Weather Data from the Bridge
Sat. July 16, 2011—sunny and windy
Air Temperature:  10.8⁰C
Sea Temperature: 9.3 ⁰C
Wind direction: 208.9⁰C
Wind speed: 23 knots

Science and Technology Log

Everyday on the ship there are many other research projects that are occurring, in addition to the pollock survey. Other scientists (currently not on this leg) are collecting data from a multiple beam system to look at the characteristics of the ocean floor, such as roughness or sound reflectivity, using 30 sound beams (of various frequencies between 100 and 115kHz) in a fan-shaped configuration. For this project, the researchers use several devices. First, they need updated temperature and depth data, which allows them to calculate the speed of sound and the attenuation coefficient (how easily a fish is penetrated by a beam – a large attenuation coefficient means that the beam is quickly weakened as it goes through the fish), which vary as a function of temperature and salinity. To do this, they have chosen select locations to release an expendable bathythermograph, or “XBT.”

This chart shows locations for XBT drops. The yellow and blue stars show where on the transect the XBT should be released.

This torpedo-shaped device is launched overboard with a gun-like dispenser. It has a long coil of fine, copper wire that begins spinning out when it’s released and the wire transmits temperature data back to the ship through the cable in the launch dispenser, and then to the database in the lab. The depth is calculated based on the assumed descent rate of the torpedo.

Getting Ready to Launch

Getting Ready to Launch

Link to Graph of XBT data

In order to confirm the suspected bottom composition from the multi-beam measurements, a drop camera is deployed at specific locations. The drop camera is usually performed off the side of the ship at night, so it doesn’t interfere with operations that can only happen during the day.  The deck crew will deploy the drop-camera using a hydraulic winch, where it is lowered to the bottom. The camera then records for 5 minutes of time at the bottom. Several camera drops are usually completed in an area.


Another operation that happens mostly at night, is using the “Drop TS” or Drop Target Strength echo-sounder. The DTS is used to get a stronger signal at closer range to fish. This helps the scientists differentiate the signals, or echo, that individual fish may give. Many fish have swim bladders (or air bladders) that allow them to regulate their buoyancy in the water. There is a large difference in the sound velocity in air and in water, so this swim bladder causes fish to give strong echo returns.  The DTS can give them a better idea of fish counts when looking at the echograms, but they aren’t perfect. No fish will remain still or perfectly straight. Just like the echograms from the single source mounted on the hull of the ship, the colors red and brown show strong signals, yellow is medium, and blue and green are weak.

This echogram was created with data from the DTS. Each wavy line is probably a separate fish.


Shelikof Strait

We are now traveling south through Shelikof Strait. This body of water runs northeast to southwest along the Alaska Peninsula on the east side of the Kodiak Archipelago. It extends about 150 miles and is dominated by many glaciers, cliffs, and both active and dormant volcanoes. The Alaska Peninsula and Aleutian Islands are part of the Pacific “Ring of Fire.” This is a seismically active area because the Pacific plate is subducting below the North American plate. This has been occurring for millions of years, also giving glaciers time to scour away at the mountains, creating U-shaped valleys and sharp peaks. We’ve had particularly good weather the past few days and caught a great sunset behind the island-volcano Augustine.

Sunset on Augustine

Raspberry Island

Personal Log

So far we are on day 2 (3?) of fair weather and partly sunny skies and I love it. Shelikof Strait is just amazing–there are volcanoes every direction you look and we’ve had beautiful sunset after beautiful sunset. The transect lines we are running in these waters run east-west so we are very close to shore every few hours which means lots of time for pictures. Tonight I went to the flying bridge with Kathleen, the other teacher, so we could whale watch. She had been up earlier (she works the day shift!) and saw a fin whale not too far from the shore and boat. We saw lots of whale blows far off in many directions, but none again that close.

Later after the sun went down and I had started my laundry and next blog entry. The net was in the water for another trawl. Luckily it wasn’t a big catch (I was tired and not ready to slice open tons of fish), but a very little one — literally! We caught mostly juvenile pollock and some smelt fish called eulachon and capelin. We also got our token salmon — we seem to catch one with every trawl — and some squid and jellies. We had some technical difficulties with the catch-processing program, so we were a little delayed in getting started and we had a team of two rather than three. Needless to say, we didn’t finish until after 2 am. Just in time to have some Cheerios for dinner.

The highlight of the night was Dall’s porpoises, which were following the boat to four different drop-camera sights! They were darting everywhere — it was fantastic!

Species Observed

Humpback whales, seagulls, storm petrel, northern fulmar, Dall’s porpoises, juvenile pollock, eulachon, capelin, squid, adult pollock, chum salmon

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