8-3 Assembly of the Half-Meter Net. — 



The opening at the large end of the net is 

 one-half meter in diameter and fitted with re- 

 inforced eyes so that it can be lashed to the 

 metal ring. The metal ring is bridled to the 

 towline. At the small, or cod, end of the net 

 is a metal headpiece; the bucket is attached to 

 this. It is necessary for the operator to make 

 certain that the mesh nimaber stamped on the 

 metal bucket is the same as the mesh number 

 on the canvas edge at the large end of the net. 



8-4 How to Operate the Net — Lowering 

 and Streaming. — The net should be lowered 

 over the weather side of a drifting ship, keeping 

 the bucket clear of the side of the ship. From 

 an anchored vessel, however, the net should be 

 streamed to leeward. The operator should 

 ascertain that the net is open and streaming 

 properly before lowering it out of sight. Care 

 should be taken to prevent the net from fouling 

 in the ship's screws. In some instances it may 

 be necessary to add weights to the net to 

 stream it more than 2 or more meters below 

 the surface. Ordinarily, the net is streamed for 

 30 minutes; however, if the plankton are sparse, 

 the time should be increased to 60 minutes. 



8-5 Retrieving the Net. — The net is hauled 

 in by a slow but steady puU on the line. Caution 

 must be taken to keep the net from turning 

 inside out. By catching the net just above the 

 bucket with one hand and by swinging the 

 mouth ring aboard with the other, the net 

 can be brought aboard in safety. 



When the net has been retrieved, any plank- 

 ton clinging to the side of the net should be 

 rinsed down into the bucket with sea water. 

 The bucket is then detached, and its contents 

 emptied into the sample jar. The inside of 

 the bucket should be rinsed with sea water 

 and the rinsings also emptied into the sam- 

 ple jar. The plankton sample is then ready for 

 preservation. 



8-6 Preservation and Storing of Speci- 

 mens. — The method for preserving and storing 

 plankton samples are given in sections 8-32 

 and 8-33. 



8-7 Labeling the Plankton Sample. — A 

 label is the only means of identifying a plankton 

 sample. Labels are made of a special material 

 and must be filled out in pencil rather than ink. 

 The following information is included on the 

 label: date, time (GMT), latitude and longitude, 

 duration of the tow in minutes, depth of tow, 

 mesh number of the net, diameter of the mouth 

 ring, the name of the collector, and the sample 

 number. Include the water temperature, 

 salinity, and oxygen content at the depth of 

 tow, if available. Any other pertinent remarks 



such as "Raining," "Sargassum Patches," etc., 

 should be added. This label must be placed 

 inside the sample jar, facing outward. The 

 bottle must be recapped tightly. This informa- 

 tion should be transcribed to a log sheet, using 

 consecutive lines for each tow. 



8-8 How to Secure the Net After a Tow.— 

 After using the sampler, rinse the net and 

 bucket carefully in fresh water. Care must be 

 taken to rinse the cod end carefully since some 

 plankton may adhere to it. The net should be 

 hung to dry in a place where it will be protected 

 from fraying or snagging. 



8-9 Care and Repair of Nets. — Owing to 

 the fragility of the nets, great care must be 

 used in handling and storage. They are ex- 

 pensive and difficult to replace because of their 

 special mesh sizes. Oil and grease affect the 

 silk used in making the nets and great care 

 must be taken to prevent contamination. 

 Should oil or grease get on a net, it must be 

 washed with soap and warm water immediately. 



Small holes in a net may be repaired tem- 

 porarily with a drop of rubber cement, shellac, 

 or other suitable material. Large holes must 

 be patched with linen, closely sewn with thin 

 thread. When badly damaged, rotten, or 

 excessively patched, the net must be replaced. 



8-10 Spare Parts Needed.— The following 

 spare parts are recommended for the half- 

 meter net: 



a. Silk plankton net — one of each size used. 



b. Detachable metal bucket — one of each 

 size used. 



c. Brass mouth ring — one. 



d. Sash cx)rd or manila line — about 100 feet. 



e. Formalin (37-40 percent solution of for- 

 maldehyde). 



f. Sodium Bicarbonate — one box. 



g. Linen for patching — 1 square yard. 



h. Silk thread for patching — one spool. 



8-11 CLARKE-BUMPUS QUANTITA- 

 TIVE PLANKTON SAMPLER.— This sampler 

 is equipped with a flow meter so that quantita- 

 tive plankton investigations can be made. 

 An impeller is geared to the meter so that the 

 number of revolutions made by the impeller is 

 recorded by the counter. From the reading, 

 the volume of water which has passed through 

 the sampler can be determined. This quantity 

 of water filtered by the sampler per revolution 

 of the impeller is determined by calibrating the 

 meter, either in a laboratory equipped with 

 flume tanks or in the field. As a result of many 

 calibrations, an approximate rating of 4 liters 

 per revolution has been found to be satisfactory 

 for all instruments whose impellers spin freely 

 when blown on by the operator. 



H. O. 607 



79 



