METHODS 



eyesight like young tish can sec the net and still dodge it. Catches of young tish 

 are thus usually much bigger in night hauls than in day hauls, irrespective of 

 their diurnal depth distribution (see Chapter 12). One way to help overcome 

 this difficulty is to increase the towing speed, but this is not a straightforward 

 answer. The net is incapable of filtering water at more than a certain rate, so 

 to tow it faster merely means more overspill, and unless the aperture is 

 reduced the increased pressure may be more than the silk can stand and the net 

 will tear away. Faster towing also means a greater pressure cone in the water 

 immediately in front of the net which frightens more plankton away. 



A modern net, such as the Gulf III (Fig. 3 ; 4), is an attempt to get over all 

 these difficulties at once: fundamentally it is still the same basic cone of net 

 though it hardly looks like it. So that it can be towed at high speed, the net is 

 now a rigid structure of fine mesh wire adequately supported at both ends, and 

 to protect it from damage it is totally enclosed in a large metal tube. To give 

 a proper filtration ratio the mouth opening is reduced, and behmd the net the 

 outer case is also reduced in diameter to give a venturi effect. This reduction 

 affects the passage of water flowing over the instrument, causing a partial 

 vacuum which helps to suck the water through. This now means that the 

 flow meter, or depth/flow meter, can be fitted in this narrow tail part where 

 it is protected and where it will register the water-flow that has actually 

 passed through the net — a much more accurate way. It is towed by a line 

 attached only to the upper front edge of the drum and kept down in the 

 water by a depressor attached by a line to the lower edge. As in our simple 

 net and weight system, this line means that if the depressor touches bottom 

 its downward pull is no longer effective and the relatively delicate net and 

 housing will not be damaged. If the bottom is very jagged the depressor may 

 be lost, the net will then come to the surface and the loss will be compara- 

 tively slight. The water passing the drum, aided by stabilizing fins, keeps the 

 whole structure facing directly into the w^ater-flow and the opening is thus 

 clear of hindrances. The whole is somewhat streamlined to reduce towing 

 resistance and to limit the turbulence in front of it. 



Because of the different depth distribution of the various planktonic 

 forms, and because this varies according to the light (see Chapter 12) hori- 

 zontally towed nets will not give representative samples of the plankton in 

 any one place, but they are used to investigate depth distribution. If one 

 wishes to sample the whole depth range then a vertical haul is required. 

 Theoretically, assuming perfect filtration etc, if a net with an opening of i ni- 

 ls hauled vertically from the bottom to the surface it will sample the total 

 plankton living under i m^ of surface. This kind of information is essential 

 when trying to assess total plankton crop at any one time. But nets are far 

 from 100 per cent efficient, 30 per cent is about average, but it frequently 



17 



