FROST and McCRONE: MESOPKLACMf FISHES IN THE EASTERN SUBARCTR- PACIFIC 



Table 6. — Mean depth (meters) of size classes ofDiaphus theta 

 and Protomyctoph urn Ihompsoni in day (D1-D4) and night (Nl. 

 N2) vertical series at Station Q in the northeastern Pacific. Mean 

 depth/) was calculated from the equation D = i;!,Z,/i/!, , where 

 n I is the population density (number/ 10,000 m^) of a size class in 

 sample; andZ, is the midpoint of the depth range of sample i. 



Size class 

 (mm) 



D1 



D2 



D3 



D4 



Nl 



N2 



Total no 

 captured 



Protorrtyctophum thompsoni: 



16-35 332 316 330 301 294 257 



36-53 381 338 340 340 307 309 



354 

 59 



fl 



Nl Dl N2 D2 N3 D3 N4 



B 



2 N 1.2 D 3 N 3.4 4 



E 200 



c 



300 



400 



500 



N 2 



N 1 



D 1 



D 2 



Figure 8. — Vertical distribution of Protomyctophum 

 thompsoni. A. 1973, Station P. B. 1974, Station Q. C. 1975, 

 Station P. Scales represent 25 individuals/10^ m^. Sequence of 

 vertical series as in Figure 3. 



extended deeper than our usual range of sampling. 

 Stenobrachius nannochir was only captured below 

 275 m in the routine vertical series at Stations Q 

 (1974) and P (1975). As noted earlier, due to the 



poor condition of the catch, small specimens of S. 

 nannochir and S. leiaupsarus were not distin- 

 guished in samples from Station P ( 1973). Half of 

 the total catch ofS. nannochir was from below 400 

 m, and all of the specimens caught above 440 m 

 were <35 mm SL. It is for this reason that we 

 think that the species must have been extremely 

 rare in the 0-440 m layer at Station P in 1973, for 

 we caught almost no small Stenobrachius in the 

 deep samples at night (Figure 5A). The virtual 

 restriction of catches of S. nannochir to our 

 deepest samples, day and night, indicates that its 

 distribution probably extended below our range of 

 sampling. Indeed, it was the most abundant fish in 

 the one very deep daytime vertical series at Sta- 

 tion P ( 1975); it occurred down to 782 m and was 

 concentrated in the 440-540 m layer (Table 4). 

 Furthermore, an interesting vertical stratifica- 

 tion by size was evident in this series, with the 

 smallest fish dominating the shallowest sample 

 and largest fish dominating the deepest two sam- 

 ples (Table 7). Note that we captured only small 

 specimens ( <35 mm) in all of the other, shallower 

 vertical series. Stenobrachius leucopsarus and S. 

 nannochir of similar body size tended to be verti- 

 cally well separated in the water column at all 

 times (Tables 4, 7; Figure 5). 



Table 7. — Vertical distribution of size classes of Stenobrachius 

 nannochir in the deep daytime vertical series at Station P ( 1975) 

 in the northeastern Pacific, as number/10,000 m^. 



Depth (m) 



22-37 mm 



38-70 mm 



85-113 mm 



The only other moderately abundant fish was 

 the chauliodontid Chauliodus macouni, and only 

 at Station Q was it captured in sufficient numbers 

 to warrant description. Chauliodus macouni al- 

 ways occurred below 150 m, and there was no 

 conclusive evidence of change in its vertical dis- 

 tribution during the day-night cycle (Figure 9, 

 Table 3). However, in contrast to P. thompsoni, 

 whose range of vertical distribution apparently 

 was well sampled day and night ( Figure 8B, Table 

 4), it appears from the abrupt truncation of histo- 

 grams in Figure 9 that the deepest portion of the 

 population of C. macouni was not sampled either 

 in the daytime or at night. Indeed, in the very deep 

 vertical series at Station P ( 1975), a number of C. 

 macouni were captured in the 440-540 m layer 



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