FISHERY BULLETIN: VOL. 76. NO. 4 



n 



Nl Dl N2 D2 N3 D3 N4 



B 



1 D 2 N 1 .2 D 3 N 3.4 D 4 



c 



N 1 



N 2 



D 1 



D 2 



FIGURE 6.— Vertical distribution oWiaphus theta. A. 1973, Sta- 

 tion P. B. 1974, Station Q. C. 1975, Station P. Scales represent 25 

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



variation in vertical distribution and vertical mi- 

 gration. The smallest sizes ( 35-44 mm ) offish were 

 consistently shallower than larger sizes both dur- 

 ing the day and at night (Table 6); although the 

 numbers offish are small, they do indicate a possi- 

 ble trend. Diaphus theta was not captured in the 

 very deep (782-440 m) daytime vertical series at 

 Station P in 1975. 



The second most abundant myctophid, Pro- 

 tomyctophum thompsoni , did not perform an ex- 

 tensive die! vertical migration similar to that of S. 

 leucopsarus or D. theta; it remained below about 

 200 m both day and night (Figure 8). Neverthe- 

 less, the species tended to be somewhat more shal- 

 lowly distributed at night than in the daytime. 

 This is best demonstrated by the data from Station 

 Q where the largest catches of this species were 

 made. At Station Q,P. thompsoni ranged from 16 



30-, 



25- 



20- 



15 - 



jR 



A 



30 



1 ' r 



50 60 



LENGTH ( mm ) 



80 



70 -1 



50 - 



30 - 



20 - 



3 



10 - 



LENGTH (mm ) 



Figure 7. — Length-frequency distributions of Diaphus theta 

 {A),N = 304, andProtomyctophum thompsoni {B),N = 413, from 

 all vertical series at Station Q, 1974. 



to 53 mm SL and the length-frequency distribu- 

 tion of the population was bimodal (Figure 7B). 

 Calculations of mean depths of the two size classes 

 showed that the smaller fish were always slightly 

 more shallowly distributed that the larger fish 

 (Table 6). Moreover, both size classes tended to be 

 deeper in the daytime than at night, although the 

 average change in depth (30-40 m for both size 

 classes) was relatively small (Table 6). Protomyc- 

 tophum thompsoni was rare below 440 m at Sta- 

 tion P in 1975 (Table 4). The size range of the 

 species at Station P was 18-51 mm (1973) and 

 16-50 mm ( 1975), and the size-frequency distribu- 

 tion was similar to that of Station Q. 



The above three species of myctophids had ver- 

 tical distributions which were, with the possible 

 exception of the rare large specimens of S. leucop- 

 sarus, well bracketed by our vertical series of 

 samples. Two other relatively abundant species of 

 fish seemed to have vertical distributions which 



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