LARGE PLANKTON NETS 109 



constructed kite, the bucket lashing from side to side and causing serious damage to the 

 contents". Such erratic movement is doubtless aggravated by the behaviour of the 

 heavy iron ring, which, no longer towed squarely by the bridles or buoyed up and held 

 in steady control by the open net, is free to surge about at random. But that is not all 

 that may happen : if, for instance, the moment of closing should escape notice, so that 

 there is some delay in hauling after the net has actually been closed, or if some hitch 

 should occur while hauling necessitating a stoppage of the winch, there is danger of the 

 collapsed net with its cumbrous unsupported frame sinking and fouling the warp. Such 

 a possibility cannot be overlooked, for it has been seen time after time when working 

 flights of small silk and stramin nets that, for all their lightness, they must be hauled 

 at fair speed as soon as they are closed, otherwise in their collapsed and unbuoyed state 

 the mere force of towing alone is not enough to keep them from sinking and getting 

 round the warp. In view of its far greater bulk and weight a mishap of this nature is 

 more liable to happen with the N 450 than with smaller nets, especially in a very deep 

 haul when the exact moment of closing may readily pass undetected by those on deck. 



Owing to the failure of the Nansen principle the original idea of employing the N 450 

 as a routine deep-water net was temporarily abandoned, and for a long time during the 

 earlier voyages of the 'Discovery IT it was put only to rare and occasional service, 

 mainly as an open net. During the fourth voyage of the 'Discovery IT, however, in 

 response to increasing demands for more accurate information regarding the depths 

 frequented by the larger pelagic creatures, the various problems involved in working 

 this large apparatus began again to receive attention. 



The chief problem, that of closing, had already been brought much nearer solution 

 as a result of experiments with the young-fish trawl conducted towards the end of the 

 third voyage of the 'Discovery 11'. The Nansen principle had been abandoned and a 

 system of internal throttling introduced in which the heavy net ring was so effectively 

 controlled that all possibility of surging after closing was eliminated. Under the new 

 system, which was recently described by Mackintosh,^ the throttling rope operates on 

 the inside of the net and when the bridles are slipped the net falls back bodily until the 

 fore part, tightly strangled, is drawn in a cone up through the mouth of the ring. The 

 latter does not capsize and turn end on as it does in the Nansen method, but falls back 

 fairly until arrested, still maintaining its original fishing position, by a stray line or 

 preventer, one end of which is attached to the release gear and the other to the towing 

 shackle at the apex of the bridles. Thus, after closing as before, the ring is held in 

 steady control by the bridles from start to finish of the haul. 



The length of stray line required, which is the distance the net must be allowed to 

 fall back, depends on the circumference of the net at the point where it is to be 

 throttled. If the throttling rope works as a running noose the net must be dropped a 

 distance at least equal to the circumference at the point of closing. On the other hand 

 if the throttling rope is arranged to work " on the bight " the distance need only be half 



1 Mackintosh, N. A., and Ardley, R. A. B., 1936, The Royal Research Ship 'Discovery //', Discovery 

 Reports, xiii, pp. loo-ioi. 



