FISHERY BULLETIN; VOL 76, NO, 4 



At Station P in 1973, S. Icmopsarus occurred in 

 largest numbers in the surface layer (0-55 m) at 

 night and at 275-330 m during the day (Figure 

 3A). This pattern could occur if most specimens 

 undertook a diel vertical migration over a depth 

 range of 250-300 m. To be certain that the data 

 reflect a vertical migration and not simply light- 

 aided avoidance of the net by fish in the daytime, it 

 was necessary to compare day and night total 

 catches of fish integrated over the water column 

 sampled (Table 3 ). Assuming that the entire verti- 



cal range of S. leiicopsarus was sampled (this as- 

 sumption is qualified below), then it is clear that, 

 since the total catches for day and night series 

 were statistically indistinguishable (Table 3), 

 there was no evidence of daytime avoidance of the 

 trawl by fish. Further, judging from the results of 

 replicate sampling of zooplankton with nets 

 (Wiebe and Holland 1968), the day and night to- 

 tals in Table 3 are well within the range of vari- 

 ability expected for repeated samples from a 

 pelagic population. Thus, the observed diel differ- 



n 



Nl 01 N2 D2 N3 D3 N4 



c 



B 



FIGLIRE 3. — Vertical distribution of Stenobrachius leucopsarus 

 at Stations P and Q in the northeastern Pacific Ocean. A. 1973, 

 Station P: four night (Nl and three day (D) vertical series. B. 

 1974, Station Q: four day and two composite night vertical series. 

 C. 1975, Station P: two composite night and two day vertical 

 series. The profiles for each year are presented in the chronologi- 

 cal order in which they were taken. Scales represent 100 

 individuals/10'' m^. 



D 1 D 2 N 1.2 D 3 N 3,4 D 4 



Table 3. — Day and night total catches for the water column sampled, of selected fish and crustacean 

 species at Stations P and Q in the northeastern Pacific; means and ranges (parentheses) as number/100 

 m^. Ratios given of largest to smallest estimate of abundance for a station. 



Species 



Station 



Day 



Nigtit 



Ratio 



Stenobrachius leucopsarus 



Diaphus Iheta 



Protomyctophum thompsoni 



Chauliodus macouni 



Euphausia pacifica 



Sergestes similis 



'Day and night abundance significantly different (P- 

 ^Estimate based on smallest nonzero catcfi. 



0.1) by rank test (Tate and Clelland 1957). 



756 



