FISHERY BULLETIN; VOL. 76, NO. 4 



Q, the species appeared to be performing an exten- 

 sive diel vertical migration (Figure 11 A, B); how- 

 ever, the average daytime catches at both stations 

 were a bit less than half the average nighttime 

 catches, though only in 1973 and 1974 were there 

 statistically significant differences (Table 3). Ex- 

 cept for the largest size class of Stenobrachius 

 leucopsarus (Figure 5C), Scrgestes similis is the 

 only species for which we found such a prominent, 

 repeated, day-night difference in catches. Either 

 S. similis is a diel vertical migrator and descends 

 below our usual range of sampling in the daytime 

 or it is capable of avoiding the nekton trawl in the 

 daytime. Our very deep daytime vertical series 

 taken at Station P (1975) bears on this question. 

 Although the species seemed considerably less 

 abundant in 1975 (Table 3), this was probably 



n 



Nl Dl N2 02 N3 D3 N4 



B 



01 02 N1.2 03 N3.4 04 



c 



N 1 



N 2 



1 



2 



Figure ll.— Vertical distribution ofSergestes similis. A. 1973, 

 Station P. B. 1974, Station Q. C. 1975, Station P. Scales repre- 

 sent 50 individuals/10' m''. Sequence of vertical series as in 

 Figure 3. 



partly due to the shallower depth to which the 

 routine vertical series extended in the daytime. In 

 the very deep daytime vertical series, S. similis 

 occurred in considerable numbers between 440 

 and 640 m (Table 4 ). Thus it probably was a migra- 

 tor and in the daytime ranged well below the 

 greatest depth of sampling on routine vertical 

 series. 



At both stations, S. similis tended to be rather 

 broadly distributed over the 0- 1 50 m layer at night 

 and often was more abundant below 50 m than 

 above ( Figure 11). In this respect its diel migration 

 differs from that of the two migratory myctophid 

 fishes and E. pocifica, which tended to aggregate 

 strongly above about 60 m at night. 



In addition to S. similis several other types of 

 malacostracans were collected in the samples: the 

 caridean decapods Hymenodora frontalis, Noto- 

 stomiis japoniciis, and Pasiphaea sp.; the penaeid 

 decapod Bentheogennema borealis; and the my- 

 sids Gnathophausia gigas, Boreomysis sp., and 

 Eucopia sp. All were rare, were collected only at 

 night, and almost always occurred below 200 m. 



Micronekton Associated With 

 Sound-Scattering Layers 



In the daytime, the position of the scattering 

 layer corresponded closely with the daytime depth 

 of occurrence of the smaller size classes of Steno- 

 brachius leucopsarus and the populations of D. 

 theta and Protomyctophum thompsoni (Figure 

 12A). For example, in the profiles shown in Figure 

 12A, the 300-400 m stratum contained an average 

 concentration of 136 fish/10,000 m^ of the three 

 species combined. Sergestes similis is distributed 

 too broadly and deeply in the daytime to contrib- 

 ute to the observed scattering layer (Figure IIB, 

 Day 3). Excluding euphausiids, in our samples no 

 other potential sound-scattering organism (e.g., 

 physonect siphonophores) consistently had its 

 center of abundance between 275 and 400 m in the 

 daytime. The large E. pacifica collected with the 

 nekton trawl had a pattern of vertical distribution 

 (Figure lOB, Day 3) very similar to that of the 

 migratory myctophid fishes. 



Comparison of the vertical distribution and diel 

 migration of Stenobrachius leucopsarus with the 

 echosounder trace indicates a correlation between 

 the fish and the migratory sound-scattering layer 

 (Figures 2, 3). The correlation is best for individu- 

 als of the small and medium size classes (Figure 5). 

 Similarly, the vertical distribution and diel mi- 



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