Trawl Gear

NOAA Teacher at Sea: Tammy Orilio

NOAA Ship Oscar Dyson

Mission: Pollock Survey
Geographical Area of Cruise: Gulf of Alaska

Date: 24 June 2011

Weather Data from the Bridge:

Latitude: 54.14 N

Longitude: -164.16

Wind Speed: 9.73 knots

Surface Water Temp: 7.0 degrees C

Water Depth: 92.75 m

Air Temp:  7.2 degrees C

Relative Humidity:  101%

Science & Technology Log:

I've been talking a lot about trawling for fish, and I realize that some of you may not know exactly what I'm talking about, so let me explain.  Trawling is a fishing method that pulls a long mesh net behind a boat in order to collect fish.  Trawling is used to collect fish for both scientific purposes (like we're doing) and also in commercial fishing operations.  We have two types of fish trawls onboard the NOAA Ship Oscar Dyson- a mid-water trawl net and a bottom trawl net.  We've used both types throughout our cruise, so let me tell you a little about each.  

The mid-water trawl net is just as it sounds- it collects fish from the middle of the water column- not those that live on the seafloor, not those that live at the surface.  The technical name for the net we have is an Aleutian Wing Trawl (AWT)- it's commonly used by the commercial fishing industry.  The end of the net where the fish first enter has very large mesh, which is used to corral the fish and push them towards the bag at the end.  The mesh gets progressively smaller and smaller the further into it you go, and at the very end (where the collecting bag is), the mesh size is 0.5 inches.  The end (where the bag is, or where the fish are actually collected) is called the codend. This is the kind of net we use when we want to collect a pollock sample, because pollock are found in the water column, as opposed to right on the seafloor (in other words, pollock aren't benthic animals).  Our particular net is also modified a little from a "normal" AWT.  Our trawl has three codends (collecting bags) on it- each of which can be opened and closed with a switch that is controlled onboard the ship.  The mechanism that opens and closes each of the 3 codends is called the Multiple Opening and Closing Codend (MOCC) device.  Using the MOCC gives us the ability to obtain 3 discrete samples of fish, which can then be processed in the fish lab.  One other modification we have on our mid-water trawl net is the attachment of a video camera to the net, so we can actually see the fish that are going into the codends.  

The MOCC apparatus, with the 3 nets extending off. 

Part of the mid-water trawl net as it's being deployed. 

The camera apparatus hooked up to the trawl. 

When we spot a school of fish on the acoustic displays, we then radio the bridge (where the captain is) and the deck (where the fishermen are) to let them know that we'd like to fish in a certain spot.  The fishermen that are in charge of deploying the net can mechanically control how deep the net goes using hydraulic gears, and the depth that we fish at varies at each sampling location.  Once the gear is deployed, it stays in the water for an amount of time determined by the amount of fish in the area, and then the fishermen begin to reel in the net.  See the videos below to get an idea of how long the trawl nets are- they're being reeled in in the videos.  Once all of the net (it's VERY long- over 500 ft) is reeled back in, the fish in the codends are unloaded onto a big table on the deck using a crane.  From there, the fish move into the lab and we begin processing them.  

The end of the trawl net.  These are the lines that basically hold on to the net!

The mid-water trawl net all reeled in. 

One of the codends before being opened up. 

Opening the codend to release the fish catch. 

The other type of trawl gear that we use is a bottom trawl, and again, it's just as it sounds.  The bottom trawl is outfitted with roller-type wheels that sort of roll and/or bounce over the seafloor.  We use this trawl to collect benthic organisms like rockfish, Pacific ocean perch, and invertebrates.  There's usually a random pollock or cod in there, too.  As I mentioned in my last post ("Today's Catch"), the net can sometimes get snagged on rocks on the bottom, resulting in a hole being ripped in the net.  Obviously, we try to avoid bottom trawling in rocky areas, but we can never be 100% sure that there aren't any rogue rocks sitting on the bottom :)  

The mesh and the wheels of the bottom trawl.

More of the bottom trawl.

The bottom trawl all reeled in.

Personal Log:

It's been a quiet couple of days.  On Wednesday, we didn't see any fish until late in my shift, then we did a mid-water trawl.  We ended up actually busting the bag- that's how many fish we ended up collecting!!  Once the codends were opened, we immediately began processing- first separating the pollock from everything else we caught.  After sorting, I got to work on sexing the fish- it's a kind of gruesome job, because you have to take a scalpel and cut them open (while they're still alive!), exposing their innards- definitely NOT like the preserved organisms we dissect in class.  I'm not a huge fan of cutting them open, so I moved on to measuring the length of the male fish- there were so many males in our catch, I was the last one working!  After I cleaned up, that was the end of my shift.  We were near some islands at the end of my shift, and the bridge called down to the lab to tell us that there some whales off the starboard side of the ship.  I grabbed my camera and ran up to the deck, scanning the water for whales.  Finally, I spotted a pod waaaay off the starboard side- they were too far off to get a good picture, and I couldn't even tell what kind they were, but I was able to see them spouting water out of their blowholes, and it looked like one of them breached.  The officers up on the bridge said they thought they were minke whales.  

Thursday we didn't see any fish (well, not enough to put our gear in the water) all day, so no fishing for me.  Right now, it's about 9:30 a.m. on Friday, and we're just cruising to begin our next set of transects.  I just read that there was an earthquake in the western Aleutian Islands last night- magnitude 7.2!  Holy moly, I was just there!  Apparently, people felt the earthquake as far east as Dutch Harbor on the island of Unalaska, and they had a tsunami warning go off.  It's crazy to think that I was in that area a couple days ago!  

Question of the Day:

• Speaking of tsunamis...What would cause the East Coast of the U.S. to be hit by a megatsunami?