FROST and McCRONE: MESOPELAGIC FISHES IN THE EASTERN SUBARCTIC PACIFIC 



fl 



Nl 01 N2 D2 N3 D3 N4 



B 



Nl 01 N2 02 N3 03 N4 



c 



Nl 01 N2.D2 N3 03 N4 



Figure 5. — Vertical distribution of three sizes of Steno- 

 brachius leucopsarus at Station P, 1973. A. 19-35 mm, scale 

 represents 100 individuals/lO^m^. B. 38-82 mm, scale represents 

 50 individuals/lO'' m^. C. 90-112 mm, scale represents 25 

 individuals/10'* m^. Sequence of vertical series as in Figure 3A. 



The one shallow night vertical series (60-0 m) at 

 Station P indicated that S. leucopsarus were dis- 

 tributed throughout the surface layer but were 

 concentrated between 15 and 30 m (Figure 3C). 

 Examination of sizes of fish caught in this series 



suggested very fine-scale vertical stratification by 

 age or size (Table 5). Recall that in all other verti- 

 cal series taken at night very large fish ( >80 mm) 

 were always captured (except for one fish) below 

 50 m. Because we took only one such shallow ver- 

 tical series, we cannot evaluate the frequency of 

 occurrence or temporal persistence of this appar- 

 ent stratification offish by age in the surface layer 

 at night. 



The third most abundant myctophid in our sam- 

 ples, Diaphus theta, also performed a diel vertical 

 migration (Figure 6); there was no consistent dif- 

 ference between day and night catches (Table 3). 

 At night, D. thcta ranged over the upper 1 65 m but 

 was concentrated near the surface (0-55 m), while 

 during the day most of these fish were collected 

 below 275 m. As stated above for 5. leucopsarus, 

 the occurrence of D. theta at 275-330 m the first 

 night at Station P ( 1973 ) is misleading because the 

 sample was probably collected after the downward 

 vertical migration of myctophids had begun. 



The size range for the total catch of D. theta was 

 36-88 mm in 1973 and 46-84 mm in 1975 at Station 

 P, and 33-76 mm at Station Q. The size-frequency 

 distributions were similar in all 3 yr. Considering 

 only Station Q, for which we have the largest col- 

 lection, the size-frequency distribution (Figure 

 7A) was quite different from S. leucopsarus (Fig- 

 ure 4B). Small and large fish were rare and the 

 samples contained primarily intermediate sizes 

 (45-58 mm). Distinguishing, somewhat arbi- 

 trarily, three classes in the size-frequency dis- 

 tribution, there is indication of size-dependent 



Table 5. — Vertical distribution of size classes ofStenobrachius 

 leucopsarus in the shallow night vertical series at Station P 

 (1975) in the northeastern Pacific, as number/10,000 m^. Data 

 based on a single haul with a single sample at each depth. 



Table 4. — Deep daytime vertical distribution of selected species of micronekton at Station P (1975) 

 in the northeastern Pacific, as number/10,000 m^. For Stenobrachius leucopsarus, the numbers in 

 parentheses are abundances of large fish (91- 1 12 mm SL). Data based on a single haul with a single 

 sample at each depth. 



Stenobrachius Stenobrachius Prolomyctophum Lampanyctus Chauliodus Sergestes 

 Depth (m) leucopsarus nannochir thompsoni ritten macouni similis 



759 



