Author: Ingvar Huse, IMR, Norway and Řyvind Knutsen, Geophysical Institute, University of Bergen, Norway
The bottom trawling for fish and invertebrate samples has so far gone very well. We have got good samples from all but one station. The one we missed was shallow (1300 m) and rocky, and we had to abandon it after tearing one trawl. We still have 5 bottom trawls left though, so still more strange shapes from the deep should continue to come aboard on a daily basis. The deepest haul so far has been down to 3400 m. Most hauls have been on soft bottom, but two tows down a semi-hard slope at around 900 m were also successful, bringing up valuable genetic samples of orange roughy, black scabbardfish and other hard bottom species. With success comes boldness, and we will probably try the odd impossible haul that may very well deflate our present state of mild hybris through a sound helping of humple pai.
We place cameras on the trawl also. Still and video. Lights are required, meaning batteries. Batteries have limited capacities, and trawl deployment time is unpredictable. The result is that when the trawl finally is at the bottom at 3000 m, the batteries are flat, and we have one hour of pelagic out-of-focus recordings. But some times we are lucky, and have one hour of video from the headline of the trawl down towards the bottom (distance 4 m). Fish encountered are generally docile and do not react much to the approaching trawl. The bottom is generally soft detritus, a pale yellow homogenate of crustacean exoskelletons and algal remains and the ground gear travels half burried in this substrate, providing us with most of the present epifauna as well as fish. An occasional volcanic rock protrudes, crating a mild shock when the trawl film is reviewed (although the trawl is already safely on deck). But as the catch is already on deck the reviewer is left in a mild state of tedium save for the albeit less than realistic hope of a glimpse of the deep sea monster that got away.
The figure (Fig 1) shows a CTD-rosette, which is lowered to ca 20 m above the bottom on each station. On this there are mounted two current measurement devices (yellow and red) in addition to the sensors for temperature, salinity, pressure, oxygen, and Chlorophyll A and 24 bottles for water samples. The current measurement devices are called LADCP, lowered acoustic doppler current profiler. They transmit 4 acoustic beams out in the water, and use the doppler shifted return signal from particles floating with the water to estimate the current. The other figure (Fig 2) shows the resulting current profile throughout the water column. The red lines represents eastwards velocity components, the green lines represents the northwards velocity components, and the dotted lines (red or green) are shear in velocity. There are four velocity components in each direction because the two LADCPs measure on both down cast and up cast. This profile shows that the current is towards southeast from the surface and down to about 1800 m, south-southwest from 1800 m to 2700 m and northeast from 2700 -2900 m. The speed of the current near the bottom is ca 3 cm/s on this station.
The resulting current profile throughout the water column
There are also two hull mounted "regular" ADCPs, one that sees down to about 400 m and one that sees about 300 m further down through the water column. The latter is continuously logging data, giving us an overview of the currents between the stations.