METHODS 



Edinburgh now cover annually many thousands of miles, as may be seen 

 from the route chart, Fig. 6. 



To sample the bigger and usually more infrequent species much larger 

 nets are used with a correspondingly coarser mesh, the idea being to filter the 

 maximum amount of water possible under the circumstances. Here the 

 development of techniques has been largely towards decreasing the difficul- 

 ties of manhandling and storing large nets. A fixed ring to keep the mouth 

 of the net open is therefore dispensed with and its place taken by a flexible 

 rope framework kept apart by the water pressure. The principle is the same 

 as used for the kite, the depressor and the otterboards of a trawl net, i.e. an 

 impervious surface is kept at an angle to the water-flow which thus pushes 

 it to the side, hi the 'Corbin' net this is a continuous canvas sheet kept rigid 

 at the correct angles bv struts attached to the lower half of the mouth of the 

 net; in the 'Isaacs-Kidd' net there is a 'kite' attached to the footrope. As we 

 saw earlier the difficulty is to get the net to go down and it is therefore 

 usually unnecessary to have any special 'kite' arrangement on the upper 

 edge. The construction of the nets unfortunately necessitates the use of 

 bridles attached to the struts or kite that disturb the water in front of the net, 

 a serious disadvantage when we are trying to capture the faster moving 

 creatures. Alternatively, the net may simply be of a pattern similar to a 

 commercial fishing net but with a 'cod end' of particularK' small mesh — 

 about I inch mesh. 



If a pump is used, all the water can be passed through a net or filter on 

 board the ship and accurately measured. As the length of suction tube is 

 known, the precise depth of origin of the sample is also known. Small 

 pumps are adequate for the finest plankton, a 2 or 3 inches diameter pipe 

 with a power driven centrifugal pump of about 6 h.p. can give sufficient 

 water for estimating the small to medium sized organisms but only a very 

 powerful pump could deliver the large volume necessary for the rarer and 

 fast swimming species. A i -metre net, towed at 2 knots ideally would cope 

 with nearly 3,000 cubic metres of water in an hour, or 640,000 gallons an 

 hour which would indeed require a large pump ! Although pumps are suitable 

 for some surface or sub-surface sampling, they become quite unwieldy when 

 long lengths of tubing of large diameter have to be handled in order to 

 reach to the deeper layers. 



At the other end of the range we have to sample the smallest organisms 

 that pass through the finest of our silk meshes. There are three possible 

 methods. The first is to use fine filters, the chemical filter papers are not fine 

 enough for this purpose and we use 'membrane filters'. These are composed 

 of incompletely cross-linked polymer molecules, as the chemist says, and 

 can be had in a variety of pore sizes from i jjl upwards (i /x = i micron = 



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