Week 4: 28 December - 3 January
Week 4 - Lights, camera, action!
 Map showing progress to Bruce Rise since deployment of ocean profilers in week 2 Photo: AAD |
Zig-zagging to Bruce Rise
For two days we have been slowly zig-zagging our way through unyielding sea ice towards Bruce Rise, located north-west of Casey at approximately 63 25 S, 102 27 E. After our enforced rest on Sunday while we waited for the CASA ice reconnaissance flight, we picked up the pace on Monday and made about 2.3 nautical miles in eight hours at an average speed of less than one nautical mile per hour. The flight confirmed that the ice conditions eased about five nautical miles ahead, so we pressed on and were rewarded on Monday night with open water punctuated by looser pack ice.
Trawl trialling
It's now Tuesday, 29 December, and the benthic invertebrate team begin trialling the beam trawl camera system to iron out any glitches before the serious sampling starts. The beam trawl is a one-tonne, steel, A-frame 'sled', about 1.5m high and 2 m wide, which supports a trawl net. Beam trawls are used mostly for research purposes, although they are used by some fishers in Europe to catch bottom-dwelling species.
The video and still cameras are mounted in 'crash frames' at the top of the trawl sled and the whole set-up is winched out over the stern of the ship. The still camera takes high resolution digital images of the sea floor every 10 seconds, while the video takes continuous, low resolution black and white footage.
|
|
|
The longline-mounted video camera, in comparison, is housed inside a narrow length of poly-pipe with two buoys at one end. The pipe is attached to the longline, which is essentially a thick piece of rope about the diameter of a 10 cent coin that sometimes contains lead threads to help the line sink faster and sit firmly on the sea floor. On commercial vessels, hooks are attached to this line by small lengths of rope called 'snoods' and the complete set-up is shot out of a narrow window at the stern of the ship.
The benthic team is using their camera on an experimental 'demersal' longline. On commercial fishing boats, demersal longlines drop down to the sea floor and then run along the bottom for up to 10 km, catching toothfish. Our longline has no hooks and is only 1-2 km long.
Once the longline camera reaches the required depth, a clever release system allows a long, steel arm and bracket to fall away from one end of the housing and, with the help of the two buoys, keeps the camera propped upright on the sea floor. Here it can see what happens to about five metres of line in front of it.
One of the cameras' creators, Antarctic Division research technician Robbie Kilpatrick, said both the trawl and longline cameras have been designed for easy use by the fishing industry. They are now robust and automated enough that they will shortly be deployed on commercial fishing boats by scientific observers – who monitor the quantity of catch and the types of species caught. The cameras will allow fishing vessels to see the habitat they are working in, and move on from an area if it contains species deemed 'vulnerable' to bottom fishing by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR).
|
|
The benthic team isn't aiming to catch anything today so the trawl net is removed for all but one beam trawl tests. When the net is reattached, a few worms and brittle stars are inadvertently captured and these provide the first good quality specimens for the biologists to examine. A small team gets to work identifying, cataloguing, preserving and photographing the creatures and generally trialling the wet lab set-up and procedures.
|
|
The first picture of the sea floor is also captured by the beam trawl camera, and reveals a consistently flat and muddy 'paddock' with no immediately apparent benthic communities.
Up on the bridge another part of the team is running the ship's acoustic echosounders to better define the bathymetry (depth) of the area and identify any interesting features, such as trenches or canyon heads, for us to sample. The cameras can operate to about 2000 m, but for these tests a depth of between 500 and 1500 m is desired. So far the Bruce Rise has proved to be very deep (1400 m) compared to the nearby shelf areas around Antarctica (600 m) and the search for a suitable sample site continues.
