CHAPTER 8 



OBTAINING BIOLOGICAL SPECIMENS 



8-1 GENERAL REMARKS.— There are 



different ways by which biological organisms 

 can be taken out of the oceans. Some of the 

 most commonly used samplers are plankton 

 nets and mid-water trawls. The former has a 

 net mesh size which is capable of sieving micro- 

 organisms such as zooplankton and phyto- 

 plankton from water; the latter has a net-mesh 

 size which permits plankton to pass through, 

 but filters out larger forms. 



Plankton nets vary in that there are some 

 types which can be used only when a ship is 

 stopped or at anchor, while other types are 

 designed to take samples while a ship is under- 

 way. Plankton nets are further diversified in 

 construction to allow sampling at one or 

 several depths. For example, some nets, such 

 as the qualitative plankton nets, can be used 

 only at the surface level. On the other hand, 

 there are plankton tows that can be used at 

 different depths, as well as at the surface. Such 

 a sampler is the Clarke-Bumpus sampler. 



Plankton nets can be subdivided into quahta- 

 tive and quantitative samplers. A qualitative 

 sampler sieves organisms from the water, but 

 does not measure the volume of water that has 

 passed through the net. A quantitative sampler 

 measures the volume of water filtered by the 

 net; thus, the concentration of the organisms 

 in a unit volume of water can be determined. 



The midwater trawl is a specially designed 

 net for rapid trawling at great ocean depths. 

 This trawl can move at such speed through the 

 water that active, fast-swimming fish are unable 

 to swim out of the net once caught. Ships 

 towing a certain type of trawl can travel as 

 fast as 5 knots. 



The plankton nets and trawl used by the 

 Hydrographic Office survey ships and discussed 

 in this chapter are the half-meter plankton 

 net, the Clarke-Bumpus sampler, and the 

 Issacs-Kidd midwater trawl. 



8-2 THE HALF-METER PLANKTON 

 NET. — A typical net is a half-meter (diameter) 

 conical net with a No. mesh. A special bottle 



or bucket fits the small end of the cone. The 

 bottle is secured to the net by several turns of 

 strong thread or twine. The bucket is secured 

 by fastening to a metal ring on the net. Three 

 lines, each about 18 inches long, are attached 

 at equidistant points to the brass ring at the 

 large end of the net and are joined at 1 point 

 to the towline, forming a bridle. The towline 

 should be approximately 150 feet long (fig. 8-1). 



The half-meter plankton net. 



The mesh size of a plankton net varies. 

 The selection of mesh size depends on the 

 size of the plankton organisms sought. An 

 identification number is on every net; the 

 larger the number, the finer the mesh. 



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