PLANKTON NETS AND APPARATUS 



i8i 



APPARATUS AND METHODS 



PLANKTON NETS AND APPARATUS 



Six different types of plankton net were used in the investigations : a small silk net 

 on a ring of 50 cm. diameter, a series of four nets of graded mesh, with diameters of 70, 

 100, 200 and 450 cm. respectively, and the Petersen young-fish trawl, which was used 

 at first with the otter boards as a trawl proper and later as a tow-net on a 200 cm. ring. 

 These nets will be referred to as the N 50, N 70, N 100, N 200, N 450 and TYF 

 respectively. 



The N 50 was almost exclusively used for vertical hauls, the N 70 vertically, hori- 

 zontally or obliquely, the N 100 horizontally, obliquely, or very occasionally vertically, 

 and the remainder horizontally only. All the nets could be closed, if necessary, by 

 messengers and closing mechanisms on the Nansen principle. 



Whilst differing in size of mesh and details of construction, according to their different 

 functions, all the nets, with the exception of the TYF, follow the same general plan 

 shown in Fig. 6. Between the ring R and the conical catching part of the net BC there 



Fig. 6. Diagram of general tow-net design; for explanation see text. The smaller figure shows 



the net closed on the Nansen principle. 



is a cylindrical portion A, separated from the latter by the canvas band D, which is 

 fitted with small rings G to take a closing rope. In the nets of graded mesh the catching 

 part BC is made of netting of two different sizes, the front part B, which is one-third 

 of the length, 1 being of a coarser mesh than the remaining two-thirds C. The 



1 Except in the N 70, where the proportions are made to conform with a similar net used by Prof. Hjort 

 in the north (see p. 183). 



