280 



FISHERY BULLETIN OF THE FISH AKD WILDLIFE SERVICE 



^.^"CABLE 



FiGTJKE 11. — Towing bridle and spreader bar used with the 10-foot Isaacs-Kidd trawl. 



sections of %-incli pipe welded alternately to the 

 towing arm and to the end plate of the vane. 

 This plate is fabricated from i/4-inch plate 1% 

 inches wide. All leading edges of the towing 

 arms are ground to a thin edge and the hinge pin 

 head is turned to a point to minimize resistance 

 and turbulence during towing. 



After final assembly the vane is thoroughly 

 sandblasted and galvanized. 



Towing Bridle and Spreader Bar 



Details of the rigging of the towing bridle are 

 shown in figure 11. All sections are made of y^^- 

 inch-diameter 7X19 preformed stainless steel 

 wire rope. The spreader bar is a 78-inch length 

 of heavy pipe with 4-inch bai-s welded at each end 

 and drilled to take %-inch shackles. 



Hauling Methods 



The 10-foot Isaacs-Kidd trawl was hauled by 

 the Smith at speeds of 4 to 6 knots, on a towing 

 line of %-inch wire rope. At 5 knots, with about 

 800 meters of wire out and a wire angle of 71°, the 

 towing tension ranged from 1,750 to 2,600 pounds ; 

 at 6 knots, with the same amount of wire out and a 

 wire angle of 73°, the towing tension was 2,000 

 to 3,000 pounds. 



As previously mentioned, data on the shape of 

 the towing wire during hauling of 10- and 15-foot 

 Isaacs-Kidd trawls have been given by Devereaux 



and Winsett (1953). We have independently cal- 

 culated the trawl depth by the method described 

 by Hida and King (1955), based on measurements 

 of wire angle and wire out. Depth conversion 

 factors derived from these measurements are listed 

 in table 5. 



Table 5. — Depth conversion factors used to estimate depth 

 of haul for 10-foot Isaacs-Kidd trawl 



[Wire out (in meters) X factor = trawl depth (in meters)] 



LABORATORY PROCEDURES 



The trawl collection, drained of the preserving 

 liquid, was spread out in a shallow, white-enameled 

 pan and sorted into two size categories : organisms 

 less than 2 cm. and organisms greater than 2 cm., 

 greatest dimension. This separation was made 

 since the first category was not judged to have 

 been sampled in a quantitative manner because of 

 the coarse mesh of the nets used. Each size group 

 was further sorted into kinds of organisms, with 



