The two-boat mid-water trawls were supplied and operated by the Institut für Fangtechnik, Hamburg, under the direction of Dr E. Dahm. The two boats, each powered with 40 hp engine, were skippered by two Finnish masterfishermen.
The details of the trawls are given in Appendix Figures 1 and 2. Further details and an account of the fishing procedures are given by Nédélec (1975) and a description of the way the trawls were used is given by Steinberg and Dahm (1975) and Dahm (1980). The depth of the mouth opening from head rope to foot rope, and speed-depth relationship was estimated during preliminary day-time trials using an echo-sounder. The speed was measured by a float and line long and calibrated with the throttle positions of both boats. These preliminary trials and calibrations took three days from 2 to 5 September. The calibrations obtained during these day-time trials were then used to maintain speed and depth when the trawls were used by night.
The catches were labelled and transported in polyethylene bags to the shore where they were processed immediately.
The lengths were measured in 5 mm size groups. Within each size group, the first five fish were weighed to the nearest 0.5 g. After the first five fish in a length group had been weighed, only the number of subsequent fish in that group was recorded.
If a sample of one species from a trawl catch was too large for treating in the standard way described above, and this was usually when the bulk weight was above 2 kg, three subsamples of 50 fish were weighed and one of these was treated in the standard way. The total number was then estimated from the mean weight of the subsamples by proportion. The length distribution of the total catch was also estimated by proportion from the measured subsample.
