ON THE OPERATION OF LARGE 

 PLANKTON NETS 



By James W. S. Marr, m.a., b.Sc. 

 (Plate II; Text-figs. 1-4) 



FOR the exploration of the deeper layers of the sea and the capture of the bigger 

 and more active pelagic animals such as fishes, squids and the larger decapod 

 Crustacea, which through their strength and speed can readily elude the ordinary tow- 

 net, a net of exceptional size is necessary. The larger the apparatus the better, especially 

 if investigations are to be made in deep water, for there are many indications that the 

 ocean depths, while predominantly the haunt of relatively large and swiftly-moving 

 creatures, are on the whole more thinly populated than the upper layers, so that the 

 chances of their successful exploitation by ordinary nets are incUned to be small. 

 However large the net may be it must be possible to close it, otherwise it can only be 

 assumed, and, as will be shown later, assumed on inadequate grounds, that the species 

 captured were taken from the particular level at which the net was fishing and not 

 during its ascent to the surface. 



Although large plankton nets have for long been in general use, until within com- 

 paratively recent years they were employed for the most part as simple open nets. The 

 problem of closing them satisfactorily first appears to have received serious attention 

 during the early days of the Discovery Committee's work when, between 1925 and 1927, 

 closing experiments with several types of large mid-water net were conducted from the 

 auxiliary barque ' Discovery ' and formed an important part of her scientific programme. 

 These experiments were concerned chiefly with two types of net — one a pelagic trawl of 

 the well-known Petersen young-fish type, the other a tow-net of graded mesh, 60 ft. long 

 and mounted on a ring 450 cm. (nearly 15 ft.) in diameter. The latter, usually referred 

 to as the 4I m. net, or N 450, is believed to be the largest plankton net that has yet 

 been fished. A third net, also of graded mesh and very similar in design to the N 450 

 but with an opening of 200 cm. diameter, was used for some time in the ' Discovery ' ; 

 but after a short trial it was discarded in favour of the all-stramin pelagic trawl, which, 

 mounted on a 200 cm. frame and used as an ordinary tow-net, proved in many ways a 

 more efficient apparatus, and, being of stouter construction, better suited to the 

 boisterous conditions of the Southern Ocean. 



The dimensions and structural details of the N 450 are shown in Fig. i . Since the 

 first description of this net was published one important structural alteration has been 

 made. As recommended by Kemp,i the longitudinal support roping (Fig. i,^), originally 



1 Kemp, S., and Hardy, A. C, 1929, Discovery Investigations: Objects, Equipment and Methods, Part II, 

 Discovery Reports, I, p. 187. 



