The work today started at 2:30 am when we “shot” the first OTSB trawl. OTSB stands for “Otter Trawl Semi-Balloon” and this is a large trawl, similar to the type that fisherman use, that skims over the seafloor catching demersal fish (ones that live just above the seafloor) and large invertebrates (the megafauna). The group I work in specializes in deep-sea megafauna.
Generally when trawling we use approximately three times the length of wire compared to the water depth. So, at 2700m water depth, which is the depth of the South East station, we need to pay out and then haul back in, approximately 6200m of cable. Leaving the trawl on the seafloor for around an hour, the whole operation was expected to take about 8 hours. However, there was a bit of a hitch with this (we seemed to run into something on the seafloor!), so the trawling took about 12 hours. We weren’t sure if there would be anything in the net, and when it arrived on deck it presented us with a large amount of mud which we then had to sieve. There were far fewer animals in the sample than we would have expected, although these were greatly prized. In total we caught nine fish, and a small bucket of invertebrates. The invertebrates were mainly echinoderms (sea cucumbers and starfish), together with various sorts of shrimps, worms and mollusks. All the animals were carefully labeled and preserved, to be taken back to shore to be examined further. The reason for the small number of animals in the sample could either have been because the net was only on the seafloor for a short time, or else because a lack of food at this site means that their numbers are actually very low here. We will have to carry out more trawls at this site to find out which of these is the case.
Following the trawl, we deployed the ISIT lander once more (see earlier diary entries to find out more about this), at a site on the western side of the ridge. We then finished the day by carrying out an echo-sounder (EK60) phytoplankton survey.
National Oceanography Centre, Southampton.