The plankton hauls ivere taken obliquely from about 70 meters deep to 

 the surface at a vessel speed of about 1-1/2 to 2 knots. In taking a 

 haul, the net was lowered on 100 meters of wire (l/U-inch cable) at the 

 rate of about $0 meters a minute, then retrieved at the rate of $ meters 

 a minute. The actual depth reached by the net varied somewhat from haul 

 to haul, depending upon the speed of the ship and the state of the sea. 

 As most of the vessels used for taking plankton hauls could not be 

 slowed down sufficiently when the sea was fairly calm, it was necessary 

 to start and stop the engine frequently d\iring a haul in order to 

 -approximate the desired towing speed. 



A film trace of the actual path of the net during hauls has been 

 obtained for the tows made on at least one vessel per cruise, by using 

 a microplankton sampler in conjunction with the regular net. The micro- 

 plankton sampler is fastened about 2 to U meters below the regular 

 plankton net. The sampler is equipped with a calibrated bellows and a 

 rotator. A continuous record of the depth of the sampler in the water 

 and the amount of water strained by it during a haul is obtained as a 

 stylus scratch on clear 35-mm. acetate film, the amount of water 

 strained, being recorded on the horizontal axis, the depth of the net 

 on the vertical, ■'^rora these traces tje have verified that the depth of 

 the net at any instant during a haul can be approximated by multiplying 

 the amotmt of wire cut by the cosine of the angle of stray of the towing 

 wire from the vertical (see fig. 2), 



WIRE OUT IN METERS 

 100 90 80 70 60 50 40 30 20 



-1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 T" 



SURFACE 



J I I L 



•— • PATH OF 1.0 METER NET BY ANGLES 

 PATH OF M.P.S. BY TRACE 



I I I I I I I I I I I I 



I I L 



Figure 2. Comparison of the path of a 1.0 -meter plankton net during 

 an oblique haul upward (as determined from the cosine of the angles of 

 stray of the towing wire) with the path of a microplankton sampler 

 (M.P.S.) attached 2.$ meters below the 1.0-meter net, as determined 

 from a film trace made by a pressure bellows. 



-I- 



