Although the design and shape of the 2- and 3-m 

 IKMTs in general use are similar, it is obvious 

 from the literature that net construction may vary 

 considerably. The size of the mesh and the thread 

 of the outer net may differ widely, as may the 

 pattern of a graded mesh. Liner mesh size, type, 

 and placement within the outer net, as well as 

 size, shape, and mesh of the cod end of the net, 

 may also differ. As shown in Table 1, the cross- 

 sectional area of the net mouth may also vary. 



Since the influence of such factors on the filter- 

 ing efficiency of a given net can be considerable, 

 the net used in the present experiment is 

 described in detail. Dimensions and material 

 specifications are shown in Table 2. The net was 

 made of No. 21 thread nylon and has outer walls 

 of 6.36-cm stretch mesh. The entire inner surface 

 of the net was lined with No. 42 thread knotless 

 nylon having a 0.95-cm stretch mesh, which was 

 sewn to the outer walls of the net at every foot. 



The aft tube of the IKMT was fitted with four 

 rings made of 0.95-cm-diameter stainless steel 

 rod spaced as follows: one 0.81-m-diameter ring at 

 the aft end of the funnel, one 1-m-diameter ring 

 at the aft end of the tube, and two 0.66-m-diameter 

 rings in the aft tube centered between the other 

 rings. The mouth of the net was hung on 1.59-cm- 

 diameter Polydac^ net rope with four legs extend- 

 ing 0.61 m and the center bosom leg extending 

 0.41 m. Riblines, composed of 0.95-cm-diameter 

 nylon rope, were rigged down each of the five 

 seams from the mouth opening to the cod end. 

 A standard 1-m conical nylon plankton net (1-m 

 mouth diameter tapering to 19 cm over its length 

 of 3 m) of No. 00 mesh (0.752-mm aperture) was 

 attached to the aft end of the main body of the 

 IKMT. Dimensions of the IKMT mouth are shown 

 in Figure 1; cross-sectional area was 7.08 m^, 

 and principal dimensions of bridle and paravane 

 were as specified for the 3-m IKMT in Aron ( 1962). 



Table 1. — Mouth area of Isaacs-Kidd midwater trawl. 



Table 2. — Dimensions and material specifications of 10-foot 

 IKMT net used for filtering-efficiency studies. 



Item 



Dimension 



Mesh size 



Forward section 



Intermediate section 



Cod end 

 Cross-section area 



Mouth 



Intermediate section 

 Forward end 

 Mid section 

 Aft end 



Cod end 

 Filtering area 



Forward section 



Intermediate section 



Cod end 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



'Knotless nylon liner — stretched mesh size 



When the net is in regular use, a Mark III ( 15-cm- 

 diameter) GM discrete-depth plankton sampler 

 (DDPS) (Aron et al., 1964) is attached to the aft 

 end of the 1-m plankton net. 



Estimates of the filtering efficiency of the 3-m 

 IKMT described here were obtained by two dif- 

 ferent methods. In the first method, a calibrated 

 TSK (Tsurumi-Seiki Kosakusho) flowmeter was 

 mounted in the net mouth as shown in Figure 1. 

 The meter (A) was tautly suspended by 0.32-cm 

 steel cable (B) inside a 1-m-diameter ring of 1.9- 

 cm-diameter brass rod (C). The 1-m ring, in turn, 

 was suspended in the net mouth by three legs of 

 0.95-cm-diameter shock cord (D). Each leg of the 

 shock cord was tensioned so that the ring and 

 suspended flowmeter were positioned in the ap- 

 proximate center of the net mouth and maintained 

 at right angles to the water flow during net 

 towing. During the tows, the shock cord stretched, 

 positioning the ring and flowmeter about 1 m 

 inside the net mouth. It is assumed that the water 

 flow at the center of the net mouth was represen- 

 tative of the average flow through the entrance. 

 A second TSK meter of identical design and 

 calibration characteristics was mounted on the 

 spreader bar outside the net. The difference in the 

 number of revolutions registered by the two 

 meters was used in arriving at the estimate of 

 filtering efficiency. The four-bladed impeller of 

 each meter was restrained from turning until 

 after the net was launched and in its towing 

 position. 



In the second method used to determine filtering 

 efficiency, a Clarke-Bumpus (C-B) plankton 

 sampler (Clarke and Bumpus, 1950), with shutter 



619 



