FROST and McCRONE: MESOPELAGIC FISHES IN THE EASTERN SUBARCTIC PACIFIC 



300 



+ #T 



,■,.;, v<r)>*; 



On 



100- 



200- 



B 



T 



200tish/IO''m3 



S leucopsarus D theta 



f 



p. thompsont 



Figure 12. — Vertical distribution of three species of myctophids 

 relative to a sound-scattering layer recorded with the 12-kHz 

 echosounder. A. Midday distribution of fish and an echogram 

 showing the position of the scattering layer at the time of sam- 

 pling (Day 3 at Station Q). B. Nighttime distribution offish and 

 an echogram showing the position of the scattering layer at the 

 time of sampling (Nights 3, 4 at Station Q). 



gration of D. theta closely parallel the behavior of 

 the migratory sound-scattering layer (Figures 2, 

 6). To examine this relationship more closely, at 

 Station Q two series of three horizontal samples 

 each were collected at 125 m in the periods preced- 

 ing, during, and after ascent of the migratory 

 sound-scattering layer past that depth. In the first 

 series (Table 11, 17 July) bothS. leucopsarus and 

 D. theta were most abundant in the sample col- 

 lected as the scattering layer was passing 125 m. 

 Euphausiids (predominantly E. pacifica) and 

 Sergestes similis were also abundant in the sam- 

 ples; however, maximum concentrations of each 

 were obtained either in the sample collected before 

 or after the scattering layer had passed 125 m 

 (Table 11). Results from the second series (Table 

 11, 18 July) were similar except that euphausiids 

 were not as abundant in the first sample of the 

 series, and Stenobrachius leucopsarus was most 

 abundant in the sample collected after the scatter- 

 ing layer had passed 125 m. The results, therefore, 

 indicate that both migratory myctophids and 

 euphausiids are associated with the migratory 

 sound-scattering layer, whereas sergestid 

 shrimps are not. 



Table ll. — Occurrence of migratory myctophids and crusta- 

 ceans (number/10,000 m'') in two series of three samples col- 

 lected in horizontal hauls at 125 m depth before, during, and 

 after ascent of the migratory sound-scattering layer past that 

 depth. 



Position of the nonmigratory portion of the deep 

 sound-scattering layer which was present at night 

 was strongly correlated with the distribution of P. 

 thompsoni, particularly the small size class. The 

 scattering layer was broader and more diffuse at 

 night, and so was the distribution of P. thompsoni 

 (Figures 2, 8, 12B). Over the 200-300 m stratum, 

 the average concentration of P. thompsoni was 

 16.3 fish/10,000 m^ for the profile shown in Figure 

 12B. The day-to-night persistence of the nonmi- 

 gratory scattering layer (Figure 2) cannot be 

 explained by reference to the distribution of either 

 S. leucopsarus or D. theta. The two smaller size 

 classes of S. leucopsarus and all D. theta have 

 migrated into the surface layers at night, and the 

 largest S. leucopsarus are not only rare but 

 broadly distributed over 50-450 m. There are no 

 other abundant potential sound-scattering or- 

 ganisms concentrated in the 200-300 m stratum at 

 night. 



DISCUSSION 



Previous work on myctophids in open waters of 

 the subarctic Pacific dealt chiefly with systematics 

 and biogeography (Wisner 1976). However, Aron 

 (1962) and Taylor (1968) considered aspects of the 

 distribution of myctophids in eastern subarctic 

 waters. Aron's (1962) results are qualitative due 

 to the nature of the sampling gear used (unme- 

 tered, nonclosing nets of variable mesh size). Dif- 

 ferences between results of our study and those of 

 Taylor's (1968) comprehensive investigation are 

 probably attributable to the different sampling 

 gear employed rather than to fundamental varia- 

 tions in behavior of fish in different parts of the 

 subarctic Pacific. For example, Taylor's use of very 

 course-meshed nets probably accounts for both his 

 finding of different relative abundances of myc- 

 tophid species and for somewhat different patterns 



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