LARGE PLANKTON NETS 



117 



In oblique work of this kind, where the vertical ascent of the net is inclined to vary 

 considerably from one haul to another, it is most important that the exact depth of 

 closing should be known. In recording this depth the depth gauge has proved 

 indispensable, the shock of closing causing the pen to swing violently and so produce a 

 vertical stroke on the paper across the line representing the path of the net. 



One of the more recently obtained depth gauge charts illustrating the features 

 described above is reproduced in Fig. 4. The chart represents the path of the N 450 

 during the operation of a deep-water haul in which 4000 m. of wire was paid away 



Fig. 4. Reproduction of a depth gauge chart showing the path of the N 450 during a 

 2000 m. obhque haul (i actual size), abc, the path of the net while fishing; c, closing 

 point; s, zero line. For further explanation see text. 



in all and 2000 m. hauled before closing. The path of the net while fishing is indicated 

 by the letters abc, a being the point where fishing actually began, and c the closing 

 point clearly shown by the vertical stroke of the pen. It will be seen that in this 

 particular haul the net for more than half the time (from a to b) was travelling almost 

 horizontally through the water. Another feature especially well illustrated in Fig. 4 

 is the remarkably narrow horizon that can sometimes be examined by these large 

 oblique closing nets in spite of the great length of wire that is brought in. In this 

 particular instance the depth gauge reading was 1950-1550 m., showing a vertical 

 ascent for the net of only 400 m. 



In the ' Discovery II ' the N 450 is worked over the stern through the fair-lead in the 

 rail on the port side. When in daily service it is carried, ring uppermost, flat on the 

 spacious poop, with the closing mechanism attached and the gear all clear for running 

 —the stray line coiled and stopped off, the throttling wire carefully slacked away to 

 ensure that the net will open to its full capacity on entering the water. When shooting, 

 the weight^ is attached to the end of the warp and 20 or 30 m. paid away. The warp, 

 1 No weight other than the depth gauge, which weighs nearly a hundredweight, is used. 



