FROST and McCRONE: MESOPELAGIC FISHES IN THE EASTERN SUBARCTIC PACIFIC 



each vertical series of nekton samples at each sta- 

 tion. A slight (<l9c) bias toward underestimation 

 of weight was corrected using the approximation 

 for minimum variance unbiased estimator of the 

 mean given by Beauchamp and Olson (1973). 

 Mean standing stock, averaged over day and night 

 series, ranged from 0.27 to 0.53 g dry weight/m^ of 

 sea surface (Table 9). Variations in standing stock 

 did not closely follow variations in abundance be- 

 cause of large year-to-year variations in the size- 

 frequency distribution of the most abundant 

 species, S. leucopsarus (cf. Figure 4). For example, 

 the low standing stock at Station Q is due to the 

 relative scarcity of medium-sized (40-80 mm) S. 

 leucopsarus in the catch at that station (Figure 

 4B). This also accounts for differences between 

 years in the composition by species of the standing 

 stock of the three myctophids. At Station P in both 

 years, S. leucopsarus represented, on the average, 

 63.4-75.7'^ of the standing stock of myctophids, 

 but at Station Q it contributed only an average of 

 32. 4*^. At Station Q, the rarer, but relatively 

 larger, D. theta made up 59.3*7^ of the standing 

 stock; however, at Station P it made up only 21.2- 

 24.3%. Protomyctophum thompsoni , because of its 

 small body size (Figure 7B), averaged <13% of the 

 standing stock at all stations (range of means for 

 the three stations, 4.2-12.3%). Estimates of mean 

 standing stock based only on night vertical series 

 tended to average more than those based only on 

 day series (Table 9) because of the contribution 

 from the relatively rare (Figure 5), but very large 

 (>90 mm), specimens of S. leucopsarus which 

 were caught chiefly at night. 



Vertical Distribution of Crustaceans 



The most abundant organisms in the vertical 

 series of nekton samples were euphausiids, pre- 

 dominantly large individuals (>12 mm total 

 length). At Station P in both 1973 and 1975, 

 Euphausia pacifica made up more than 80% of the 

 euphausiid catch by number. At Station Q, 51% of 

 the total euphausiid catch was E. pacifica; other 

 species were Thysanoessa spinifera (30%), Tes- 

 sarabrachion occulatum (9%), Thysanoessa lon- 

 gipes (8% ), and Stylocheiron maximum (2% ). All of 

 these species also occurred at Station P, but were 

 rare. Consequently, only the data for E. pacifica 

 are presented here. 



During the day, large E. pacifica occurred in 

 greatest concentration between 275 and 400 m, 

 while at night they were usually concentrated in 



the upper 55 or 60 m (Figure 10). No consistent 

 difference between day and night total catches 

 was evident, but sporadic, extraordinarily large or 

 small catches of £. pacifica were obtained in both 

 1973 and 1974. Variations such as these are com- 

 mon in euphausiid catches (Brinton 1962b) and 

 are usually attributed to horizontal patchiness. 

 Our ranges of estimated abundances were con- 

 sequently very large (Table 3). The other four 

 species of euphausiids were too rare to draw 

 definite conclusions about their distributions. 

 The penaeid decapod shrimp, Sergestes similis, 

 was the only other abundant invertebrate in our 

 nekton samples. At Station P (1973) and Station 





 100- 



5 200- 



6 300 f 

 400 

 500 







100 



5 200 



n 



fc 300 



CD 



400 

 500 



n 



Nl Dl N2 02 N3 03 N4 



+ 



1 



B 



1 2 N 1.2 3 N 3.4 4 



500 



c 



N 1 



N 2 



1 



D 2 



Figure lO.— Vertical distribution of Euphausia pacifica. A. 

 1973, Station P. Scale represents 1,000 individuals/lO" m^. (The 

 0-55 m sample on the fourth night represents 10,447 

 individuals/lO-* m^*.) B. 1974, Station Q. Scale represents 500 

 individuals/ia» m^. C. 1975, Station P. Scale represents 500 

 individuals/10^ m^. (The 15-30 m sample on the first night repre- 

 sents 3,086 individuals/10^ m^*.) Sequence of vertical series as in 

 Figure 3. 



763 



