Figure l. — Sketch of the pelagic trawl in operation. 



mm poly-Dacron rope. The wings are of #60 

 thread, the body of #3 thread, the intermediate of 

 #4 thread, and the cod end of #6 thread. Nineteen 

 25.4 cm (diameter) aluminum floats (good to 1,000 

 m) are attached to the headrope. Galvanized chain 

 is attached to the footrope and one 23 kg lead ball 

 is attached to the tip of each of the bottom wings. 

 The bridles to the bottom wings are 55 m long; 

 bridles to the upper wing are 36 m long, and the 

 middle bridle is 47 m long (Figure 1). 



The trawl was designed to be towed with 1.5-1.8 

 X 2.4-2.7 m (5 x 8 ft or 6 x 9 ft) otter doors. A scale 

 model of the trawl was first constructed and tested 

 in a tank to test its design and performance. 



Observations were made on the trawl by divers 

 during trials in Puget Sound using 1.8 x 2.7 m 

 doors and towing speeds of 1.0 to 1.6 m/s at depths 

 of about 12 m. The towing characteristics of the net 

 were observed and the number of floats needed to 

 provide neutral buoyancy to the cod end opening- 

 closing device was determined by trial. At a tow- 

 ing speed of 1.6 m/s the vertical opening of the 

 mouth was measured to be about 8 m and the body 

 of the trawl was observed to be nearly circular and 

 about 8 m in diameter, providing an estimate of 

 about 50 m^ for the cross-sectional area of the fine 

 mesh netting of the trawl body. 



The depth that each net fished was determined 

 from the depth-modulated signal from an acous- 

 tical pinger mounted on the headrope of the trawl, 

 a hydrophone towed from the vessel, and a graphic 

 recorder. An EG&G pinger was used initially but 

 was replaced with an Institute of Oceanographic 

 Sciences (lOS) 0-683 m (0-100 atmospheres pres- 

 sure) acoustical net monitoring system (Baker et 

 al. 1973) with an overall accuracy of 0.1% of the full 

 depth range. 



Methods 



A timer-actuated ejection device was used as a 

 method to provide some information on the flush- 

 ing rate through the body of the trawl. This device, 

 similar to the one described by Pearcy et al. (1977), 

 has a modular timer (Evans 1975) to release the 

 contents of two 1.3 1 chambers. It was hung from 

 the headrope inside the trawl mouth and its con- 

 tents were ejected against the 19 mm mesh. Pre- 

 served juvenile salmon (10-20 cm total length) 

 were released into the net at intervals of 5, 10, 15, 

 or 25 min before closure of net 1 and opening of net 

 2. 



The pelagic trawl-MPS combination was used 

 on three chartered trawlers off Oregon. All vessels 

 had net reels and used double-warp towing. A 

 boom was used to launch and recover the MPS 

 during 1975 and 1976 cruises. The vessel chartered 

 in 1977 had a stern ramp, which greatly facilitated 

 use of the pelagic trawl. Twelve tows were made in 

 1975 to test the monitoring equipment and to 

 evaluate flushing of the net. Eight tows were made 

 in 1976 and 10 in 1977. All tows were 110-130 km 

 off the central Oregon coast; 18 of these tows pro- 

 vided the data for comparison of pelagic trawls and 

 Isaacs-Kidd midwater trawls (IKMT's) (Table 1). 



Tows were also made with a 5.4 m^ IKMT with 

 10 mm stretch mesh and a 1 m^ MPS opening- 

 closing device (Pearcy et al. 1977) at 1.5-2.0 m/s at 

 a similar location and within 10 days after each of 

 three cruises that used the pelagic trawl (Table 1). 

 Volume of water entering the IKMT was moni- 

 tored with a modified TSK flowmeter on all tows. 

 One of the purposes of these IKMT tows was to 

 enable comparisons of the catches by the two dif- 

 ferent types of nets. 



530 



