7 o DISCOVERY REPORTS 



METHODS 



The construction and method of working the i m. oblique net have been fully de- 

 scribed by Kemp and Hardy (1929). The essential points are as follows. It is a conical 

 net with an opening 1 m. in diameter and the principal fishing part of the net is of 

 stramin. The filtration of the stramin is roughly equivalent to that of silk bolting cloth 

 having 15 meshes to the inch. " For oblique hauls open N 100 and N 70 were attached 

 to the warp close together and 3 or 4 m. above the lead. With the ship steaming at 

 2 knots 200 m. was paid away, and as soon as this had been done hauling commenced. 

 The rate of hauling was 10 m. per minute; each haul thus took 20 min. and the 

 distance covered was two-thirds of a mile" (Kemp and Hardy, he. cit.). By this method 

 the net is fished obliquely from a depth of about 100 m. to the surface. The samples 

 are preserved in formalin and stored in screw-top bottles. 



The method of analysis of a plankton sample must be adapted to the type of net used. 

 The vertical N 70, for example, is fished at a very uniform speed through an accurately 

 measured column of water, the organisms caught are small and a comparatively refined 

 technique can be used for quantitative estimations. The depth and speed at which the 

 oblique N 100 is fished, however, are greatly affected by the difficulty of adjusting the 

 ship's speed to varying weather conditions, so that precise quantitative comparisons 

 between different samples are of questionable value, and a sufficiently accurate estimate 

 may be obtained by less refined methods. 



There is much variation in the richness of the Antarctic plankton, and a single haul 

 may yield anything from a dozen to 200,000 organisms. Different methods must there- 

 fore be used for different catches. In small samples, containing up to 200 or 300 

 organisms, the total number of each species is counted without difficulty. Samples of 

 average size, however, are much bigger than this, and a typical one might contain about 

 5000 organisms of which perhaps 4000 would be copepods. These have generally been 

 treated as follows. The bulk of the formalin is strained off and the sample is washed 

 into a glass dish with plenty of water. The large animals such as Amphipoda, large 

 Siphonophora, Polychaeta and Salps can generally be quickly picked out. It is then 

 necessary to go very carefully through the whole sample to find any of the species 

 which are both small and present only in small numbers. It is very easy to miss such 

 organisms as Limacina or the small siphonophore, Dimophyes arctica, whose presence 

 or absence may be of some importance, and it is often advisable to turn over the whole 

 sample several times before dealing with the more numerous organisms. The medium- 

 and large-sized organisms such as Euphausians can all be picked out and counted if 

 there are not more than (say) 60 or 70 of them. For the more numerous organisms sub- 

 samples are taken, and it may be necessary to take first a quarter or an eighth and count 

 perhaps the Euphausians and Chaetognatha in that, and then to take anything from 

 1/16 to 1/256 to count the various species of Copepoda. For the Copepoda it has 

 generally been customary to take a fraction which will contain 100 to 200 specimens. 

 Even then there is a possibility of missing one or two of the rarer species. For taking 



