Kathleen Harrison: First Trawl, July 7, 2011


NOAA Teacher at Sea
Kathleen Harrison
Aboard NOAA Ship  Oscar Dyson
  July 6– 17, 2011

Location:  Gulf of Alaska
Mission:  Walleye Pollock Survey
Date: July 7, 2011

Weather Data from the Bridge
True Wind Speed:  18.7 knots
True Wind direction:  145.55°
Sea Temperature:  8.12° C
Air Temperature:  9.65° C
Air Pressure:  1013.2 mb
Ship’s Heading:  299°, Ship’s Speed:  11.8 knots
Latitude:  54.59°N, Longitude:  145.55°W

Science and Technology Log
The primary mission of the Oscar Dyson Walleye Pollock Survey is to estimate the biomass (mass of the living fish) of the Pollock in the Gulf of Alaska.  Read about why Pollock are important here:  Pollock    Now, you can’t exactly go swimming through the Gulf of Alaska (brrrr) and weigh all of the fish, so the NOAA scientists on board use indirect methods of measuring the fish to come up with an estimate (a very accurate estimate).  Two of these methods include using nautical charts, and trawling.

Nautical charts are used for navigation, and location.  The Oscar Dyson has several systems of charts, including electronic and paper.  Each chart contains latitude, longitude, and ocean depth, as well as lands masses and islands.  A chart that shows ocean depth is called a bathymetric chart.

bathymetric map

Here is a bathymetric map for part of the Gulf of Alaska. The change in color from green to blue shows the edge of the continental shelf.

These need updating continually, because the sea floor may change due to volcanic eruption or earthquakes.  The Officer of the Deck (OOD, responsible for conning and navigating the ship) needs to know how deep the ship sits in the water, and study the bathymetric charts, so that the ship does not go into shallow water and run aground.  The lines on the bathymetric chart are called contour lines, depth is shown by the numbers on the lines.  Sometimes every line will have a number, sometimes every 5th line will have a number.   A steep slope is indicated by lines that are close together, a flat area would have lines that are very far apart.  The OOD also need to know where seamounts (underwater volcanoes) and trenches (very deep cracks in the ocean floor) are because these may affect local currents.  GPS receivers are great technology for location, but just in case the units fail, and the ship’s technology specialist is sick, the OOD needs to know how to use a paper chart.  He or she would calculate the ship’s position based on ship’s speed, wind speed, known surface currents, visible land masses, and maybe even use star positions.  Here in Alaska, star position is helpful in the winter, but not in summer.  (Do any of my readers know why?)

The Oscar Dyson’s charted course follows a series of parallel straight lines around the coast of Kodiak Island, and other Aleutian Islands.  These are called transects, and allows the scientists to collect data over a representative piece of the area, because no one has the money to pay for mapping and fishing every square inch.

The Chief Scientist on the Oscar Dyson is always checking our location on the electronic chart at his desk.  It looks something like this:

map of transects, Gulf of Alaska

This chart shows some of the transects for the Oscar Dyson in the Gulf of Alaska.

Several things are indicated on this chart with different symbols:  the transect lines that the ship is traveling (the straight, parallel lines), where the ship has fished (green fish), where an instrument was dropped into the water to measure temperature and salinity (yellow stars), and various other ship activities.  It also shows the ocean depth.  This electronic version is great because the scientists can use the computer to examine a small area in more detail, or look at the whole journey on one screen.

They can also put predicted activities on the map, and then record actual activities.  The scientists also use several systems for the same thing;  recording the ship’s path and activities in the computer, as well as making notes by hand in a notebook.

When the scientists want to catch fish, they ask the crew to put a trawling net into the water.  The basic design of the trawl is a huge net attached to 2 massive doors.

otter trawl

This is the basic design for a trawl net, showing the doors that hold the net open, and the pointed end, where the fish are guided, called the cod end.

The doors hold the net open, as it is dragged behind the boat.  There are 2 different trawling nets aboard the Oscar Dyson:  one that trawls on the bottom called the PNE (Poly Nor’Easter), and one that trawls midway in the water column called the AWT (Aleutian Wing Trawl).  Another net called the METHOT can be used to collect plankton and small fish that are less than 1 year old.  The scientists determine the preferred depth of the net based on the location of fish in the water column; the OOD gets the net to this requested depth and keeps it there by adjusting the ship’s speed and the amount of trawl warp (wire attached to the net).
A trawl typically lasts 15 – 20 minutes, depending on how many fish the scientists estimate are in the water at that point (more about this later).  Today, a bottom trawl was performed, and 2 tons of fish were caught!  The net itself weighs 600 pounds, and is handled by a large crane on the deck at the stern (back) of the ship.  Operating the trawl requires about 6 people, 3 on the deck, and 3 on the bridge at the controls.  When the scientists judge that there are the right amount of fish in the net, it is hauled back onto the deck, weighed, and is emptied into a large table.

poly nor'easter

Here is the PNE being weighed with the cod end full of fish.

Then the scientists (and me) go to work:  sorting the fish by species into baskets, counting the fish, and measuring the length of some of them.  NOAA technology specialists have designed a unique data collection system, complete with touch screens.  A fish is placed on a measuring board, and the length is marked by a  magnetic stylus that is worn on the finger.  The length is automatically recorded by the computer, and displayed on a screen beside the board.  I measured the length of about 50 Atka Mackerel after the first trawl.

using the measuring board

In the fish lab, this mackerel is having his length measured. The data goes directly into the computer, and shows up on the screen in front of me.

By sampling the fish that come up in the trawl net, the scientists can estimate the size of the population.  Using the length, and gender distribution, they can calculate the biomass.

Personal Log
Some great things about living on the Oscar Dyson:  the friendly and helpful people, the awesome food, the view from the bridge.

Some challenging things about living on the Oscar Dyson:  taking a shower, putting on mascara, staying in bed while the ship rolls.

I started my 12-hour shifts, working from 4 am to 4 pm.  Well, maybe working is not the right word, I actually worked about 3 hours, and asked a lot of questions during my first shift.  The scientists are very patient, and explain everything very well.  We did one trawl today, and it was a good one.  I enjoyed sorting and counting the fish, and then measuring the length of them.  I will probably take a shower, eat dinner, and read for a short time before climbing into bed.  I have the top bunk, and it is plenty of room, except I can’t sit up straight.  Here is a picture of the stateroom.  After my shift, I will probably take a shower, eat dinner, watch a movie and fall asleep around 8:30.

view of my room

Standing at the door, this is the view into my stateroom. The bunks are on the right, the desk and closets are on the left. There is a tiny bathroom, as well as a small refrigerator.

The weather today has been windy, so there are 6 – 8 foot swells, and the ship is rolling a bit.  I have not been seasick yet – yippee!  The wind is supposed to calm down tomorrow, so hopefully we will have a smoother ride tomorrow night.

I learned the difference between pitch, roll, and heave:  pitch is the rocking motion of the ship from bow to stern (front to back), roll is the motion from side to side, and heave is the motion up and down.  The Oscar Dyson is never still, demonstrating all 3 motions, in no particular pattern.  Imagine standing in a giant rocking chair, and someone else (that you can’t see) is pushing it.

Here is a view from the bridge:

from the aft deck

View from the deck in front of the bridge, showing a gyrorepeater (the white column on the right), and a windbird (anemometer and wind vane) on top of the forward mast. You can also see a horizontal black bar in the center of the picture - that is the provisions crane.

Species seen today:
Northern Rockfish
Dusky Rockfish
Walleye Pollock
Pacific Ocean Perch
Kelp Greenling
Atka Mackerel
Pacific Cod
Fanellia compresson (octocoral)
Sea Urchin
Kelp

One response to “Kathleen Harrison: First Trawl, July 7, 2011

  1. Hi Mrs.Harrison,

    Reading your experiences, it seems quite interesting! I would NOT like the pitch, roll or heaving. Being sea sick is no fun, you must be a trooper! I found the trawls interesting, for I had only seen them on TV and never knew their proper name nor how heavy they were! I hope you enjoyed your trip,

    Shayla Marrow.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s