As the mean size of the zooplankter is larger than the phyto- 

 plankter, coarser filter s may be used. The THREE-QUARTER 

 METER (diameter) QUANTITATIVE PLANKTON NET is essen- 

 tially a cone of nylon netting with a mesh size of 0. 16 nrim. 

 ( fig. 18. 5). It is possible to purchase material with mesh dia- 

 meter ranging from 1.3 mm. down to 0.064 mm. The net shown 

 is equipped with a flow meter to record the amount of water pass- 

 ing through as it is towed. From the data obtained and the catch, 

 the density of the plankton in the area may be computed in terms 

 of numbers per unit volume of sea water. Such nets are towed at 

 approximately 2 to 4 knots and some ZOO to 300 cubic meters of 

 water are filtered. In an open ocean area we get approximately 

 10 to 20 ml. of packed volume of plankton from such a haul. 

 Nets may be used for towing at higher velocities, but the intake 

 opening must be much smaller than the filtering area (fig. 18. 6). 

 The intake of this HIGH SPEED PLANKTON SAMPLER is ap- 

 proximately 1 inch, and the appropriate filtering area is much 

 larger. This net is towed at 7 to 10 knots with a small depressor. 



18.5 



FIGURE 



THREE-QUARTER 



METER 



QUANTITATIVE 



PLANKTON 



NET 



203 



