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Nov 28 - The Western Core Box

 

Noon Position : lat 53 16.4 S, long 50 01.6 W

Bearing  108 T, 302 Nm from Stanley, Falkland Islands

Air temperature @ noon today :  4.1 degrees C

Sea temperature @ noon today :  5.5 degrees C

Wind: Direction WSW, Force 5


This week in brief

After a very brief visit we left the sunny shores of South Georgia in the early evening.  Such was the social whirl (a yacht and a cruise ship were in) there were only a few hardy souls to wave us off.  The scientific objectives of the cruise were started very soon afterwards.  We spent a pleasant five days towing a yellow sledge around the area!  This is the carrier for the acoustic equipment for the western core box, which is detailed below.  Luckily we had calm seas and very clear weather so, for much of the transects we were treated to views of South Georgias' snowy, mountainous skyline.  As soon as this part of the operation was finished we headed towards Shag Rocks and the weather deteriorated.  The wind picked up (gusting to 63 knots) and the swell was about 10 metres.  The personnel were split between those who retired to their bunks because they felt unwell and those who visited the bridge to watch the heavy seas breaking, and occasionally landing on the  fo'c'sle.  For those of us who are not used to being at sea there is something fascinating about riding the roller coaster of waves and watching the green wall of water coming towards the front of the ship.  Breaking seas land on the deck and then stream down the sides and out through the scuppers.  The cargo tender even became a temporary swimming pool, although it looked a bit choppy for practising front crawl!  Supper was a very quiet meal that evening.  Gradually, over the next day, the weather settled and by the time we came to the moorings for retrievel all had settled again.  We picked up the mooring buoys, as described below, and are now on our way back to Stanley.  Apparently the Dash has 'dashed' and there are fewer people in Stanley the last time we were there.

Waves crashing over the bow; photo C Waller


Science bit in the middle - fishing for buoys

The last week has been spent on science operations.  We started the week by doing the western core box, which I mentioned last week and this is explained by Doug Bone below;

The western core box is a rectangular area, approximately 80 x 80 kilometres, that we survey three times in each field season. The surveys are carried out as early in the spring as possible, in high summer and as late in the autumn as possible. As the name western core box implies there is also another, the eastern core box, but that is surveyed much less frequently. The significance of the western core box is that it covers the principal feeding area of the huge number of krill-eating birds and seals that breed each year on Bird Island. Krill is a name of Norwegian origin that covers a number of shrimp-like organisms that are generally found swimming in mid-water in the worlds Oceans. In this case it refers specifically to euphausia Superba  which occurs in vast ‘swarms’ in parts of the oceans surrounding Antarctica. The waters around South Georgia are particularly rich in krill, which is why huge numbers of whales migrated to the area each summer before their numbers were drastically reduced by the intense whaling industry in the first half of the 20th century. There is now a commercial fishery for krill, albeit on a small scale, so it is important that krill stocks are monitored to prevent the wildlife that depends on it being starved. Each survey consists of steaming along eight, 80 km long ‘transect’ lines spread across the box. The amount of krill present is measured by using a fish-finder type echo sounder. This system uses three different sound frequencies, 38kHz, 120kHz, and 200kHz, the echoes returning from organisms in the water are converted into an electrical voltage and measured by the sounder; these data are then sorted and stored by complex computer software for later analysis. Organisms of different sizes and types respond differently to the frequencies used, thus some give a stronger response to 38kHz, others to 120 or 200kHz. Over time it has been possible to work out the characteristic response of certain organisms, particularly krill, to the three frequencies. This characteristic response can now be used to identify the krill without having to catch them. At least once in each season, usually during the mid-summer survey, we do catch samples of the krill in order to study the age structure and breeding status of the population.

Once the western core box had been completed, we moved on to the area near Shag Rocks where we having been fishing for buoys.  We were due to pick up the mooring buoys, which we had been unable to retrieve on the way to South Georgia due to the fog.  Rob McLachlan has written a short explanation of the scientific equipment on the buoys;

One of the many scientific objectives of cruise JR110 is the recovery of a series of moored scientific instrumentation. The moorings were deployed during the month of May 2003 on board the JCR during cruise JR80. The geographical location of these moorings is the area known as Shag Rocks.  The six moorings in question are of a similar construction although not identical, the array of scientific instruments include;

ADCP (Acoustic Doppler Current Profiler) - The purpose of this instrument is to measure and record current speed and direction in a vertical water column.

Current Meter (type 8) - The purpose of this instrument is to measure and record the current speed and direction as well as temperature, pressure and conductivity. Unlike the ADCP, the type 8 measures and records in the immediate locality of the unit as apposed to the full water column.

XR420-TD - The purpose of this instrument it to measure and record the temperature and pressure in the immediate locality of the unit.

ARGOS Beacon - The purpose of this instrument is to detail the position of the mooring once it has been released from its anchor using satellites to give an accurate position.

Acoustic release - The purpose of this instrument is to release the mooring from its anchor using an acoustic signal received from the ship.

Diagram of the mooring buoys retrieved; by R McLachlan

So what actually happens when you 'pick up' a mooring buoy?  When the ship arrives at the known location of the buoy she stops and remains in that location.  A hydrophone is then lowered over the side of the ship into the water and sends a coded release signal to the mooring (commonly known as a ping!).  This causes the mooring to release and float to the surface.  The speed of ascent is related to the buoyancy, of course, and there was a betting system for the first buoy.  How long will it take to the surface?  Proud winner of a mars bar was Steve Mack with 25 minutes!  Once the buoy is on the surface it has to be spotted, easy if the visibilty is good and the sea is calm.  The ship then manoeuvres alongside and the rope attached to the buoy is picked up using a grappling hook.  A series of shackles and wires are attached and the buoy is pulled in using the stern gantry.  Once landed the buoy is secured to the deck and the equipment described above can be retrieved and the data downloaded.  With calm seas and sunshine we were able to retrieve all six buoys and combine them with three more CTD's to calibrate the data.

Andy Liddell and Peter Enderlein securing a mooring on the stern gantry; photo L Handcock

Buoys lashed to the deck after collection; photo L Handcock


So as I write this on Sunday afternoon, the scientists are all heaving a sigh of relief after a very successful cruise and taking a well earned break.  We are well on our way back to Stanley and tomorrow we will begin again, saying goodbye to some of the personnel, loading cargo for Port Lockroy and Rothera, then picking up new personnel ready for their trip south down the peninsula.  I am looking forward to a couple of days meeting friends and walking in the Falklands before we set off.  So, 'til next week, goodbye.