May 4 2008
May 4 2008
You win some, you lose some:
What we didn’t want to see...
This is where the trawl should be.
Greg Buikema, Josef Kuehnast, Rhian Waller and Zim Zimmermann discuss the lost trawl.
After the last week pretty much exclusively surveying and sampling for geophysics, the coral group today decided we would put down a trawl on top of an unexplored promising-looking seamount in the Scotia Sea. The maps showed a large flat top at about 750m water depth, perfectly suited to trawling. An air of excitement went around our group, samples to sort and process and new discoveries from an unknown seamount to be had.
The Blake trawl went down at around 11pm last night. After a great looking tow on the bottom we started to haul in. Before the winch operator could even start, the tension on the wire began to rise. Putting any equipment over the side of a ship into the ocean is a risky business, once we watch it descend into the depths we never really can predict what exactly will happen – we can’t watch it go all the way down or watch it on the bottom, we can only watch the multitude of sensors we have and plan each deployment responsibly. Each wire on the ship has a tensiometer, this lets you know how much weight is on the wire and gives you a good idea of what is happening many hundreds and thousands of meters below the ship. As the tension rises this can tell us that the trawl is on the bottom, it can also tell us that things are going into the net (the more things in the net the heavier it gets, so the higher the tensiometer reads). However it can also tell us if there is a problem, if the equipment on the end of the wire has hit something and become stuck.
Last night at around 2am that is what happened, the wire tension rose and continued to rise fast. Within a few seconds the tension reached over 22000lbs and suddenly dropped back to just over 1000lbs. The bridle between the wire and the trawl had broken, leaving the trawl behind on the seafloor, 750m deep. It all happened just too fast for anything to be done to help. Though all our maps and planning suggested a smooth top to the seamount and perfect trawling conditions, it doesn’t take more than a small outcrop or overhang facing just the wrong way to snag our gear forever.
It is always a point of reflection when this happens, it is always sad to lose equipment, lose samples or to lose a trawl to the seafloor, and though it doesn’t happen too often, it is always a risk of sampling at sea, especially in unknown and extreme areas. The important thing is that when the tension was released no-one was hurt, the breakage was at the seafloor at a designed weak point, rather than on the ship where it could have damaged both people and equipment, and all safety procedures set up by Raytheon and the ECO crew were in place. Though we are disappointed to have lost our trawl and samples, we have a spare that will be rigged ready for the next deployment so our science will continue, all with a renewed sense of respect for Neptune’s realm.
Rhian G Waller
The Blake trawl in its heyday, bringing up a load of specimens from the seafloor earlier in the cruise.
One end of the broken cable, which finally snapped with a tension of 22000 pounds.
Greg Buikema radios in to the bridge when the wire is recovered safely back on deck.
The seas have become rougher today, with frequent waves breaking over the ship.