Our Mission Part 2a: The What And The How

Now that you are familiar with stock we are studying and the vague reason why we are here (See “Our Mission Part 1”), I will now give a brief rundown of what we are actually doing out here.  The primary objectives of this particular survey, from the official cruise plan, are:

“…to map the distribution and migrations of herring, blue whiting and other pelagic fish and to assess their biomass. Other objectives are to contribute to the monitoring of the hydrographic and plankton conditions of the Norwegian Sea and adjacent waters and the description of how feeding and migration of herring and other pelagic fishes are influenced by these conditions.”

Basically: count fish, check the water.

We accomplish this task (more or less) with four different activities.  I will go through the other three later. But first, this cruise is, primarily, a standard acoustic survey.

“Acoustic data will be collected with EK60 using the 38 kHz paravane-mounted transducer as standard. During fisheries operations acoustic data will be collected by the EK60 using the hull-mounted 38 kHz transducer. Echo integration will be conducted 24 hours a day. The data will be scrutinized regularly using BI500-software during the survey.”

Simple as that.  So what is ‘acoustics’? To put it simply, acoustics is underwater sonar.  The transducer sends out sound waves that bounce back off anything that may be in the water, or the bottom of the ocean if it doesn’t hit anything before that.  The strength of the echo (bounced back sound) is then measured. It’s like a fishfinder, but highly calibrated and deeply scrutinised.  Basically, we drop this thing in the water:

The Paravane A.K.A. "The Fish"

And it gives us these pretty pictures on two of the screens:

Acoustic Readings

The yellow thing you can vaguely see being dragged alongside the ship is the paravane.  That contains the transducer sending out the sound waves.  Different things in the water column bounce back the sound at different strengths, called target strengths.  Because of this it is possible to distinguish zooplankton from fish and even sometimes possible to tell different fish species apart.  Larger fish tend to have greater target strengths and come out as red on the acoustics screen, plankton tends to come out more blue, or greenish yellow if it is particularly dense.  Some signals come back as single spots, but, because herring is a schooling fish, we are more interested in dense aggregations (‘red tomatoes’) that would signal a school of herring. Although sometimes you just get nothing, like the screen in the bottom left.  Disappointingly common this cruise.

Five Miles Of Acoustic Backscatter Ready For Scrutinisation

We also have a few other tricks up our sleeves like sending sound waves at different frequencies.  Besides the standard frequency of 38kHz, we also have a 18kHz transducer with which it is easier to distinguish what are fish from dense areas of plankton and is more effective at deeper ranges.  The 120kHz transducer is effective near the surface and is helpful in rough weather when air bubbles can cause a lot of ‘static’ in the readings.

We collect all this acoustics data constantly as we go along and scrutinise each nautical mile of the track.  This means we (well, Bram) sits in front of his monitor and assigns species and values to the various aggregations and layers of backscatter that we have recorded. Because we roughly know the target strength of herring we can link acoustic backscatter to fish biomass.  And thereby we get a rough estimate of the overall herring biomass in the water below us, without having to kill a single fish.  That comes later.

Me At Mission Control

This is where I spend most of my twelve hour shift, the acoustics room.  There are six monitors on the table in front of me, with my laptop making it seven.  The two on the left show the live acoustic backscatter feeds from the three transducers and the two on the right are used for scrutinising the data.  The one in the middle is basically an electronic navigation chart:

The Navigation Screen

The black arrows show our heading and course (which may differ because of currents, wind etc.). The red line shows our cruise track, which is a fixed plan that we follow except when fishing.  If we do go fishing we return to the spot on the track where we left off and start from there again.  This is where I come to the good during my shifts, cunningly pausing the recording of acoustic backscatter while we fish, and smartly starting it again once we are back on track.  The swiggly bit behind us on that picture is where we did a surface trawl. The screen also shows other boats in green.  This photo was taken when we were near Bear Island and there were lots of fishing boats around, usually there are no other boats.

The other side of the acoustics room has more desks with more computers.  These are also used for scrutinising acoustics data and one has a live feed of various navigational information, weather conditions, water conditions etc. in various graphs and maps.  There are also paper navigational charts of the areas we are covering and binoculars for bird and whale watching out the windows.  The acoustics room is near the top of the boat so it has proper windows instead of small portholes like the cabins in the lower floors.  It’s a nice view and has lots of light because the sun never sets.

The Acoustics Room

On the walls we have a pictures of a woman sliding down a net full of fish, the first mate looking all captain like (although every time I’ve seen him walking around the ship he’s been wearing jeans with no belt), and a cat (nobody is quite sure exactly who’s cat it is).  Note the brightness outside and the time on the wall, that’s AM.  The other side of the room was probably full of over-sized computers during the 1980s.  Now it is just like excessively large storage space for four computers and a few external harddrives.  We use it as a bookshelf too, and a kind of museum for old acoustics machines. 

More Of The Acoustics Room

I’m glad I don’t have to operate this guy:

HP’s Finest Oscilloscope

My job is essentially to look at the screens and see if anything happens.  If I think I see an interesting school of fish I can tell the captain and we can drop a net in and try catch some of them.  While we can use target strengths of fish to try tell them apart in the acoustic backscatter, this is not a perfect science.  Sometimes you need to have a proper look at what is causing all the echoes you are reading. So doing a trawl and checking everything in it helps us to interpret what the acoustics are actually picking up.  Sometimes we do a trawl even when the acoustics suggest that there is unlikely to be very much, partly because the crew like to do it and partly because it’s important to make sure you aren’t missing anything.  Like the first trawl I described in this blog.  That wasn’t actually a failure.  In fact, if we had caught a whole lot of fish it would have be a cause of serious concern because it would have meant that our machines were jerking us around.

Phew, it’s taken me a lot longer than I thought it would to explain just the acoustics part of what we do. I will leave the rest for Part 2b.  That stuff is more interesting than computers screens and echoes.