Discovery 

 net 



fma fwsh \ 

 net Inylon'l 



Tow Nets 



Because no single device can sample 

 the diverse forms of plankton, the 

 oceanographer must use a variety of 

 nets and instruments, each designed 

 for a particular type of sampling. 

 Collectively these provide a cross 

 section of the pelagic plant and 

 animal life. One of the most effective 

 qualitative samplers is the tow 

 net, which filters large volumes of 

 water and strains off small organisms. 

 The area of the mouth opening, size 

 of mesh, and over-all shape of the 

 net are varied to suit requirements. 

 The nets may be towed horizontally, 

 obliquely, or vertically. Because the 

 plankton is distributed unevenly 

 horizontally and vertically, it is 

 important that these variations are 

 expressed quantitatively. The nets 

 usually are fitted with meters that 

 register the volume of water filtered ; 

 they also may have closing 

 devices which permit any desired 

 depth range to be sampled. 



Isaacs-Kidd Midwater Trawl 



To capture the larger marine organisms 

 a net larger than the tow net 

 is needed. The Isaacs-Kidd midwater 

 trawl (right), which can be towed at 

 speeds up to six knots, has a mouth 

 opening so arranged that animals in 

 its path are not disturbed. 



^ 



Pi 



jm^ 



\ to depresser 



Hardy Plankton Recorder 



This continuous recorder (below) is 

 designed to be towed by merchant ships 

 without interfering with their normal 

 routine. The plankton is collected on a 

 continuous roll of silk gauze that 

 moves across the sampler's mouth at a 

 rate proportional to the ship's 

 speed. As the sample is collected, it is 

 preserved automatically in formalin. 



towing cable 



ibration dampei 



propeller turned by 

 passing water 



water 4 plJ(lkI6n 



main driving plane 



High-Speed Plankton Sampler 



High-speed samplers (above) are rigid 

 nets of metal mesh mounted in stream- 

 lined metal or plastic containers 

 having small mouth openings. Unlike 

 conventional tow nets, these high- 

 speed samplers can be towed at or 

 near normal cruising speed. 



Petersen Grab 



The Petersen type "grab" is used for 

 quantitative studies of bottom-living 

 (benthos) organisms. The heavy metal 

 jaws of the sampler remain open until 

 it strikes bottom; when the grab is 

 hauled the jaws come together and 

 trap a sample from a standard area. 

 The animals are then sieved, washed, 

 identified, counted, and weighed. 



formalin tank 



ZJ3 



