0-8 Maintenance of the Clarke-Bumpus 

 Sampler. — After the operation is completed, 

 rinse the sampler in fresh water, dry, and lubri- 

 cate all metal parts with a light coating of oil. 

 Remove the net and dry in shade before storing. 

 (Keep oil off net.) 



0-9 The Midwater Trawl.— The Isaccs-Kidd 

 midwater trawl shown in figure 0-5 was devel- 

 oped at the University of California, Scripps 

 Institution of Oceanography. It is capable of 

 collecting some of the large and more active 

 nekton forms found in the ocean. As implied by 

 its name, the trawl was primarily designed for 

 use in midwater, that is, ocean water below the 

 surface layei-s. An ordinary net will surface be- 

 hind the towing vessel miless hauled at extreme- 

 ly slow speeds. To counteract this tendency, the 

 midwater trawl has an inclined plane surface 

 rigged in front of the net entrance. This surface 

 or vane acts as a depressor, in a manner oppo- 

 site to the elevating action of a kite surface. 



Figure 0-4. Side view of Clarke-Bumpus plankton 

 sampler. 



release the impeller. Record the time the mes- 

 senger was released. Recommended towing 

 speed is 2 to 4 knots. 



Step 7. Wlien the tow is completed (usually 

 10 to 30 minutes) drop a second messenger to 

 close the shutter. Record messenger time; hoist 

 the sampler to the surface; attach a safety line 

 to the sampler; and remove the assembly from 

 the wire. Record the reading of the digital 

 counter. 



Step 8. Wash any plankton clinging to the 

 net into the sampler bucket with sea water, and 

 empty its contents into a sample jar. If the 

 sampler bucket is full of specimens, reduce tow- 

 ing time on the next tow. If less than one- 

 quarter full, increase towing time. 



If more than one sampler is to be used on the 

 same wire, the following additional steps must 

 be performed after the second or third sampler 

 is attached to the wire. 



Step 1. Remove the safety line from the sam- 

 pler and connect it to the lowering wire below 

 the sampler. This will prevent accidental trip- 

 ping of the sampler(s) already in the water. 



Step 2. Attach two messengers from hooks 

 (I) at the base of rod (B). The wire of one 

 messenger should be 10 inches long, the loop at 

 its upper end should be passed tlirough the 

 right slot (facing sampler) in the base of the 

 frame, and then slipped over the larger of the 

 hooks. The wire from the other messenger 

 should be about 13 inches long. Pass its loop 

 through the left slot and connect it to the short- 

 er hook. 



Figure 0-5. The midwater trawl. 



The midwater trawl is essentially an asym- 

 metrical cone of 21/2-inch stretch mesh. It has 

 a 10- or 15-foot pentagonal mouth; it is 31- or 

 72-feet long; and it has a roimd opening at the 

 cod end. From a point 3 feet from the end, an 

 additional netting of i^-inch stretch mesh is 

 attached as a lining, and a perforated sample 

 container is fastened to the cod end of the trawl 

 by draw strings. 



Table 0-1 lists some of the specifications for 

 the 10- and 15-foot trawls. 



0-4 



