11 February 2001 - Looking for Krill
RRS James Clark Ross Diary
Noon Position : 60° 40' South, 57° 11' West.
Distance travelled since Grimsby: 20,059 nautical miles
Air temperature @ noon: 1.2 degrees Celsius
Sea temperature @ noon: 2.7 degrees Celsius
ANOTHER SUCCESSFUL CRUISE
The Autosub came back every time, plenty of krill was fished and TUBA worked well. All in all lots of new data was gathered and we have a feeling that we will be seeing Autosub again in the future.
Once Autosub had completed all the under-ice missions it was time to head north to more krill-infested waters for some avoidance work with the sub and to calibrate the echosounders, both on the vessel and on Autosub itself. The decision was made to relocate to Bransfield Strait and King George Island instead of Signy Island due to the sheer amount of ice still present reported by RRS Ernest Shackleton last week. This was a shame as there were two MYRTLE pods floating around down there which we were going to retrieve, on an opportunistic basis, for Proudman Oceanographic Laboratory. We will have to leave these for Shackleton on her return from Halley. MYRTLE ? Multi Year Return Tide Level Equipment. What's one of those then ? See diary for ">31st October 1999.
As we are writing this we are steaming north for the Falkland Islands once more. Despite the Captain's best efforts overnight to take us into Admiralty Bay in the South Shetland Islands, the Antarctic weather has conspired against us. The 50 knot winds for the previous 24 hours, coupled with horizontal snow, had driven all the ice north from the Antarctic Peninsula to collect along the southern coast of the islands, making access to any of the bays extremely hazardous. The conditions were bad enough for us to issue a navigational warning, due to the unusually large amounts of nasty pieces of ice lurking everywhere you looked. Technical description of the conditions from the Bridge " 'orrible ". So we head for Cape Pembroke, off Stanley, and hope to complete the instrument calibrations around the Falkland Islands, once we have identified the best location. This is not an ideal solution due to the differing water temperatures, which will mean extra work for the scientists when post-processing their data, but is one of the problems that has to be faced when working in this part of the world.
Some Images of yesterday's rough weather, Click on each to enlarge.
DOUG'S LOVELY KRILL LAYERS
For those regular readers of the page, you will no doubt have had drummed into you the importance of krill in the food chain down here. EVERYTHING revolves around these little creatures. But in case you were not paying attention earlier on, here we have a brief history :
The following is plagiarized from a piece written by our Principal Scientist, Andy Brierley, last season. See week 23rd January 2000 for which we hope he'll forgive us.
Krill is an important food for whales, penguins, seals and many other Antarctic animals, and is also caught by fishermen. In order to work out how much krill the fishermen can be allowed to catch without depriving natural predators of food, we need to know both how much krill is eaten and how much krill there is. By integrating three instruments - the ship's echo sounders, the AMT net and TUBA, we are attempting to develop easier ways to measure the amount of krill in the ocean, without the need to keep throwing nets over the side.
Instrument Number 1 (Sorry gone fishing!)
When scientists go fishing, they have to have a much more sophisticated net than anybody else in the world.
The AMPS net, above, consists of a frame rather like a scaffold tower lying on its side. The net proper is mounted inside this frame and leads back to a mechanism that carries five 'cod-end' nets. The frame has a tail plane and fins to keep it level and stable in the water as it is towed at speeds between 3 and 4 knots. Instruments giving the depth of the net, water temperature, salinity, light level and height off the bottom are mounted on the net. The data produced by these instruments are available on the surface via the towing cable which has an electrical conductor up the centre, and help us to fish the net effectively. The mesh of the net is moderately fine and designed to capture planktonic animals up to 60 mm in length. When the net is launched any plankton that enters it washes straight through and out the back, but, when the net has reached the required depth, a command is sent from the surface via the towing cable that releases the first cod-end net which stretches across the back of the main net collecting the catch. Further commands are sent to open the rest of the nets in turn. Finally the back of the net is left clear again.
Instrument Number 2 (No sound no matter how hard you blow)
So Who's "TUBA" Then?
The Towed Undulating BioAcoustic Sensor, TUBA, developed at the Southampton Oceanography Centre, is an acoustic instrument for detecting small biological targets, up to a few centimetre in size, such as krill and small fish. TUBA fires short bursts or 'pings' of underwater sound from its transducers at ten times a second. These penetrate to around four metres away from the transducer head, and are reflected back by the targets in the water. The amplitude of the return 'echos' provides us with information about the size and abundance of the targets that it sees.
Our experiments on this cruise involve TUBA being mounted on the AMPS net (see below). The transducer head is mounted right in the centre of the mouth of the net, so that we can compare the acoustic information from TUBA with what is caught in the net. Different kinds of targets, have different acoustic signatures - i.e. the amount of sound energy that is reflected back depends on the frequency of the sound used. TUBA uses 7 different frequencies, and so, as well as studying the structure and distribution of krill swarms in the Southern Ocean, our experiments will enable us to evaluate the acoustic 'signatures' for krill and other animals that we have caught during this trip.
One of the reasons for studying krill and other zooplankton acoustically, is that nets can only be towed at slow speeds. In some cases, the animals that we are attempting to catch can swim faster than this, and can avoid being caught, which means that the nets do not always provide us with a good representation of the abundance, or species present. By using multi-frequency in-situ acoustic instruments such as TUBA, which can be mounted on undulating vehicles which survey the top 500 m of the ocean and can be towed at eight knots, a much better estimate of the abundance can be achieved. Increased knowledge of the acoustic signatures of important species means that,ultimately, we may also be able to estimate the species present as well as their abundance and size (e.g adult versus juvenile). Currently, however, net hauls are still required to validate the acoustic techniques, and to provide actual samples for physiological study. As an extra feature Nick has included Tuba's own sound effect. Click here - PING
Further details of TUBA
Instrument Number 3
The ship's echo sounders. These are exactly the same instruments that we have put into Autosub, so over the last few weeks the scientists have been able to see what is in the open ocean using the ship's echo sounders. Thus they provide a direct comparison with what the echo sounders on Autosub have been seeing under the ice.
This is why calibration is so important - ensuring that both sets agree with each other.
So it's good bye from the JR58 Cruise Team
The Ship's routine continues......
We might have had quite a busy week all in all with the varied science taking place onboard, but we have still found time to fine tune our emergency techniques. We see below Charlie Smith of Bravo fire party demonstrating his skill with a foam making hose branch during a fire drill.
As an aside Charlie wished us to pass on his best wishes to his daughter who was married yesterday in the Falkland Islands. We all wish them all the very best for the future.
On a lighter note.....
We like to think we maintain our standards regarding customer satisfaction and equal opportunities. To this end we despatched Gwyn Griffiths to interview Tuba to ensure every need was being met. (Photograph Copyright. Andy Harris)
The Autosub web site