Tag: bottom trawl

NOAA Teacher at Sea
Johanna Mendillo
Aboard NOAA ship Oscar Dyson
July 23 – August 10

Mission: Pollock research cruise
Geographical area of the cruise: Bering Sea
Date: Sunday, August 5, 2012

Location Data
Latitude: 61º 10′ N
Longitude: 179º 28’W
Ship speed: 4.3 knots ( 4.9 mph)

Weather Data from the Bridge
Air temperature:  11.1ºC (52ºF)
Surface water temperature: 8.1ºC (46.6ºF)
Wind speed: 5.4 knots ( 6.2 mph)
Wind direction: 270ºT
Barometric pressure: 1013 millibar ( 1.0 atm)

Science and Technology Log:

So far, you have learned a lot about the pollock research we conduct on board.  You have learned:

• How to age fish (with otoliths)
• How to measure fish (with the Ichthystick)

and

• How to identify fish gender (with your eyes!)

Now, we are going to backtrack a bit to the two big-picture topics that remain:

• How do we CATCH the pollock (hint hint, that is today’s topics… NETS!)

and

• How do we even find pollock in the Bering Sea (that is the next blog’s focus: acoustics!)

So, to begin, there are several types of nets we are carrying on board.  Remember, when a net is dragged behind a ship in the water it is called trawling, and the net can be considered a trawl.  The most-used is the Aleutian Wing Trawl, or AWT, which we use to sample the mid-water column (called a midwater trawl).  We are also using a net called the 83-112, which is designed to be dragged along the ocean floor as a bottom trawl, but we are testing it for midwater fishing instead.  In fact, sometimes during my shift we do one AWT trawl, and immediately turn around and go over the same area again with the 83-112 to see differences in the fish sizes we catch!

If the 83-112, which is a smaller net, proves to be adequate for midwater sampling, NOAA hopes it can be used off of smaller vessels for more frequent sampling, especially in the years the NOAA does not conduct the AWT (NOAA currently does AWT surveys biennially).

Now, for each type of net, there is some new vocabulary you should know:

The codend is the bottom of the net.  A closed codend keeps the fish inside the net and an open cod end allows them to swim through.  It may seem odd, but yes, sometimes scientists do keep the codend open on purpose!  They do this with a camera attached to the net, and they simply record the numbers of fish traveling through a certain area in a certain time period, without actually collecting them!  Here on the Dyson, the NOAA team is testing that exact type of technology with a new underwater camera called the Cam-Trawl, and you will learn about it in a later post.

The headrope is the top of the opening of the net.

The footrope is the bottom of the opening of the net.

(The 83-112 is called such because it has an 83 ft headrope and an 112 ft footrope.)

The trawl doors are in front of the headrope and help keep the net open.  Water pressure against the trawl doors pushes them apart in the water column during both setting of the net and while trawling, and this helps spread out the net so it maintains a wide mouth opening to catch fish.

There are floats on the top of the net and there can be weights on the bottom of the net to also help keep it open.

Lastly, the mesh size of the net changes: the size at the mouth of the net is 3 meters (128in.), and it decreases to 64in., 32in., 16in.., 8in., etc. until it is only ½ inch by the time you are holding the codend!

Here is a diagram to put it all together:

If you think about the opening of the net in terms of school buses, it will help!  It turns out that the AWT’s opening height, from footrope to headrope, is 25m, which is 2 school buses high!  The AWT’s opening width, is 40m across, about 3.5 school buses across!  Now, you can see why positioning and maneuvering the net takes so much care– and how we can catch a  lot of pollock!

Now, when the scientists decide it is “time to go fishing” (from acoustic data, which will be the topic of the next blog) they call the officers up on the Bridge, who orient the ship into its optimal position and slow it down for the upcoming trawl.  Meanwhile, the deck crew is preparing the net.  The scientists then move from their lab up to the Bridge to join the officers– and they work together to monitor the location and size of the nearby pollock population and oversee the release and retrieval of the net.

Along the headrope, there are sensors to relay information to the Bridge, such as:

• The depth of the net
• The shape of the net
• If the net is tangled or not
• How far the net is off the bottom and
• If fish are actually swimming into the net!

The fish and the net are tracked on this array of computer screens.  As the officers and scientists view them, adjustments to the net and its depth can be made:

The start of the trawl is called “EQ” – Equilibrium and the end of the trawl is called “HB” – haul back.  The net can be in the water anywhere from 5-60 minutes, depending on how many fish are in the area.

Now, sometimes an AWT catches so many fish that there are simply too many for us to measure and process in a timely fashion, so it is deemed a “splitter”!  In a splitter, there’s an extra step between hauling in the net from the ocean and emptying it to be sorted and processed.  The codend of the AWT is opened over a splitting crate, and half of the pollock go into a new net (that we will keep and sort through) and the rest of the pollock are returned to the water.

Personal Log:

Let’s continue our tour aboard the Oscar Dyson!  Follow me, back to the bridge, where the OOD (Officer on Duty) is at the helm.  As you already know, the first thing you notice on the bridge is the vast collection of computer screens at their disposal, ready to track information of all kinds.  You will learn more about these in an upcoming blog.

In addition to these high-tech instruments, I was very happy to see good old-fashioned plotting on a nautical chart.  In class, students, you will have a special project where you get to track the changing position of the Oscar Dyson!

Here is a sample of the hour-by-hour plotting, done by divider, triangle, and pencil:

I will end here with a sea specimen VERY different from pollock, but always a fan favorite— jellyfish!  Interestingly, there are a large number of jellyfish in the Bering Sea- something I never would have assumed.  The one that we catch in almost every net is the Northern Sea Nettle (Chrysaora melanaster).  In one net, we collected 22 individuals!

When we collect non-pollock species such as these, we count, weigh, and record them in the computerized database and then release them back into the ocean.  Here they are coming down the conveyor belt after the net has been emptied:

The so-called bell, or the medusa, can be quite large- some are the diameter of large dinner plates (45cm)!  Their tentacles can extend to over 3m in length.  They consume mostly zooplankton, small fish (including juvenile pollock), and other jellies.  How so, exactly?  Well, when the tentacles touch prey, the nematocysts (stinging cells) paralyze it.  From there, the prey is moved to the mouth-arms and finally to the mouth, where it’s digested.

This same mechanism is used by sea nettle when it encounters danger like a large predator.  It stings the predator with its nematocysts and injects its toxins into its flesh.  In the case of smaller predators, this venom is strong enough to cause death.  In larger animals, however, it usually produces a paralyzing effect, which gives the sea nettle enough time to escape.

Now in the case of me handling them… and other humans…their sting is considered moderate to severe.  In most cases, it produces a rash, and in some cases, an allergic reaction.  However, we wear gloves on board and none of the scientists have ever had an issue holding them.  In fact, they offered to put one on my head and take a picture… but I declined!  If a few students email me, begging for such a picture, maybe I will oblige…