The trawling was almost exclusively conducted by the consultant himself (except the last tow No. 58), applying established aimed midwater trawling techniques. The opening height and the fishing depth of the net were monitored by netsonde. The opening width (distance between wingtips) was determined from the divergence of the warps and the gear dimensions. The towing speed was taken from the ship’s log (Bergen Nautic) which also steers the distance covered on the scientific echosounders. Counterchecks with a sail-ship - or Dutch log, made up on board did not show systematic differences so that a correction was considered inadvisable. With the exception of the netsonde the absolute reliability of these measured data is obviously limited but they are the best that can be had without a considerable increase in instrumentation and effort. They are generally accepted as suitable for educated estimates of trawl gear performance and, in particular, for comparing the performance of similar trawls. It has also to be kept in mind that the need for frequent changes in towing power (and speed) for depth regulations of the net and of course changes for aiming at promising targets detected by sonar, make the establishment of average values for opening shape and size a rather difficult task and that more than educated estimates cannot be expected unless the trawl is fully instrumented, which was not possible for these trials.
A major target of the trials was to study the effectiveness of large meshed netting in guiding and shepherding the small lanternfish species. One good measure for this feature is the catching efficiency, i.e., the percentage of the fish available in the path of the trawl that is being caught. The means for determining this most important characteristic of any trawl were provided by the capability of the scientific echosounder equipment to integrate echotargets in layers of any desired width and depth. By monitoring the setting of one integrator to the changing depth and opening height of the trawl, the amount of fish available in this layer under the vessel could conveniently be determined within the limits of accuracy of the up-to-date acoustic stock assessment technology. For the purpose of these trials, the accuracy of integration and of the formula for conversion into fish weight per unit volume was taken for granted. The results are reported in Appendix 4. Detailed information on the acoustic stock assessment technology applied have been published in earlier cruise reports and will also be included in the separate report by the cruise leader.
The time towed given in Appendix 4 is counted from the moment the trawl had reached the desired fishing depth and integration was started until the begin of hauling in the warps.
The amount of catch was determined by the Norwegian scientists according to the standard used on board, i.e., by measuring the dimensions and calculating the volume of the loaded codend on deck and multiplying the volume by 0.9 for conversion into fish weight. Only in two instances did the amount of catch have to be estimated, i.e., tow 10 when the codend burst on the chute and tow 58 when the catch was far too big to be hauled on board; most had to be released by cutting holes into the netting and only 13 t were salvaged (Fig. 9). It should be mentioned that the catches were surprisingly clean throughout. There were only a few buckets of large jellyfish which got into the net near the surface during shooting and hauling and very small amounts of squid and ribbonfish. A detailed account of the composition of the catches will probably be given in the report of the cruise leader.
The lighting experiments were conducted by the Norwegian scientists, using mainly the ship’s illumination supplemented by some improvised outboard lamps. The attempt to improvise underwater lamps failed. Fish reaction was observed by echosounder and underwater photography. The results, which indicate repulsion rather than attraction, will be reported by the cruise leader. The repulsion of the fish in the N1 layer, leading to some concentration at greater distance from the surface, was utilized for the new technique of trawling with light, which is described below in Section 5.4.
