FROST and McCRONE: MESOPELAGIC FISHES IN THE EASTERN SUBARCTIC PACIFIC 



reed switch was mounted on the top heam of the 

 trawl. 



The trawl was towed on a two-conductor coaxial 

 cable and its depth, angle of inclination of the 

 mouth, and revolutions of the flowmeter were 

 monitored continuously during trawling by means 

 of a shipboard display unit. The nets were opened 

 and closed at the mouth by means of a net-tripping 

 assembly which was controlled electronically from 

 the ship. The bottom net ( without cod end ) was left 

 open during deployment to eliminate kiting of the 

 trawl when a net was first opened (Clarke 1969; 

 Badcock and Merrett 1976); thus, four sequential 

 samples could be collected in one haul. The volume 

 of water filtered by each net (assuming 10O7f fil- 

 tration efficiency) was calculated from flowmeter 

 revolutions and average angle of inclination of the 

 net mouth. 



To determine vertical distributions of fish and 

 other components of the nekton, we towed the 

 trawl obliquely and collected samples on the up- 

 ward leg of a haul. We monitored the speed of the 

 ship during trawling by reference to a Doppler 

 ship's speed indicator. 



Recordings of deep sound-scattering layers were 

 obtained using an Edo-Western transducer 

 operating at 12 kHz (pulse length 3-10 ms, beam 

 width about 33°) and a Precision Depth Recorder 

 (PDR) operating on the 0-400 fm (0-732 m) depth 

 range scale. At the beginning of each cruise, echo- 

 sounder characteristics (pulse length, power out- 

 put) and recorder gain were set to give optimal 



resolution of the sound-scattering layer and were 

 not varied thereafter. 



Sampling Program 



Taylor (1968) found a close correlation between 

 distribution of abundant species of myctophid 

 fishes and distribution of deep sound-scattering 

 layers in the eastern subarctic Pacific near the 

 Queen Charlotte Islands. Relying on this correla- 

 tion, at each station we designed our sampling 

 program after observing the depth and migrations 

 of deep sound-scattering layers. The number, 

 depth, and migration of scattering layers were 

 virtually identical at Stations P and Q. We ob- 

 served no differences between years at Station P, 

 and our observations do not differ substantially 

 from those of Bary ( 1967) who also used a 12-kHz 

 echosounder in summer at Station P. In the day- 

 time, a single, diffuse, sound-scattering layer ex- 

 tended from about 275 to 375 m depth (Figure 2 A). 

 In the late afternoon and early evening, this scat- 

 tering layer became broader, chiefly by upward 

 movement of the top of the layer, and it persisted 

 with relatively little further change throughout 

 the night. At about 2130 h (local time), a single, 

 upwardly migrating layer became evident, and 

 within half an hour it merged completely with the 

 surface reverberation (Figure 2B). This migratory 

 scattering layer began descent at about 0530 h and 

 merged with the deep nonmigratory layer shortly 

 after 0600 h. Slight variations in times of ascent 



B 



-I 

 100 H 

 200- 

 300 -I 

 400 - 



1200 



2100 



2130 



2200 



Figure 2. — 12-kHz echograms typical ofthesummer period (July-August) in the sampling areas in the northeastern Pacific Ocean. A. 

 Daytime record about noon, local time. B. Evening record taken on the same day showing the upward movement of the migratory 

 sound-scattering layer and the persistence of the nonmigratory layer at depth. Local time, depth in meters. The dark areas above 100 m 

 are due to surface reverberation. 



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