METHODS OF HANDLING PLANKTON NETS 201 



afford a means of checking the accuracy of the haul, for they indicate by their sudden 

 extension the exact moment when the closure of the net is effected. 



On reaching the surface the weight is pulled up by its special rope and the net washed 

 down on each side with water from a bucket. The brass net bucket is now detached, 

 replaced by another, the net fully opened and the mechanism reset for the next descent. 



When all is working properly the routine of N 50 and the six N 70 hauls takes 2 hours 

 to complete. 



Routine Horizontal Nets. On arrival at South Georgia it was necessary in the 

 first place to find a method by which comparable hauls of Euphausta superba (whale-food) 

 and associated organisms could be obtained. This Euphausian, which is the largest in 

 the group to which it belongs, swims actively, and we soon discovered that it could not 

 be caught in nets hauled vertically. It can escape even the N 200 hauled as quickly as 

 possible, and though a vertical N 450 proved more successful, this net was too difficult 

 to manipulate. Experience showed, however, that very good catches could be obtained 

 in horizontal nets. Towed N 100 often yielded thousands of the species and it was 

 frequently taken in abundance in the N 70. Horizontal N 100 and N 70 were therefore 

 adopted as a routine method of investigation ; they were used open at the surface and 

 with a closing mechanism working on the Nansen principle at depths of about 60 and 

 120 m. At first both the N 100 and the N 70 were shot together and towed for i mile 

 at a speed of 2 knots ; but it was found that unduly large catches were then made in 

 the finer net and subsequently the two sizes were towed separately, the N 100 for 

 I mile and the N 70 for \ mile. The method proved very satisfactory: closure of the 

 nets could be effected with reasonable certainty and none of the difficulties referred to 

 below in connection with the larger nets was experienced. So far as can be ascertained 

 the catch is not appreciably diminished if the net is closed, and the material was always 

 in first-rate condition, with the majority of the organisms still alive. With nets worked 

 on this principle hundreds of hauls have been made, both in the whaling areas of South 

 Georgia and the South Shetlands and in other parts of the South Atlantic. During 

 some special investigations off the African coast repeated hauls were made with a surface 

 net and four closing nets used in series on the same wire rope, the total length of rope 

 being 400 m. 



One objection to the use of these flights of horizontal nets is that a patch of plankton 

 may occur at a level which lies between those at which the nets are fishing. Euphausia 

 superba lives in swarms or patches which are sometimes very strictly localized hori- 

 zontally, and on more than one occasion the fore part of one of the nets (in front of the 

 throttling rope) came to the surface covered with Euphausians, though few or none at 

 all were found in the bucket. In June 1927 experiments were made with oblique hauls 

 and these proved so satisfactory that in routine work they have been substituted foi 

 the horizontal hauls. It is thought that they give a better indication of the total plankton 

 which is present, and for economic purposes this is a more important consideration 

 than information, which the horizontal nets might yield, on the vertical distribution 



