FISHERY BULLETIN: VOL. 74, NO. 4 



500 



60r 



a 02 03 04 05 06 07 



CRUISE 



10 II 12 



Figure 20.— Mean densities of presumed mature (>10.5 mm) male 

 and female Euphausia pacifica, by month. 



low reproductive activity. Furthermore, in 1954 

 the sex ratios for 13, 14, and 16 mm body length 

 were 1:1, as compared with other years (Figure 

 21a). However, no relationship was seen (Figure 

 13) between numbers of gravid females of a given 

 size and the difference between numbers of males 

 and total females of the same size. Therefore, the 

 observed increase with body length (at least to 

 15-16 mm) in the ratio of gravid to nongravid 

 females appears natural, attributable either to 



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 1956 



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11-12 13-14 15-16 17-18 19-21 



SIZE OF ADULT Euphausia pacifica ( mm ) 



Figure 21.— a, Sex ratios for Euphausia pacifica by 1 mm body 

 length, all data for each year averaged and 4-yr average, b, 

 Annual gravidity ratios, by body length. 



higher frequency or longer duration of egg 

 production with increasing body length. 



4) An increase with body length in female/male 

 ratio may be due to their differing growth rates. 

 Both sexes tend to mature at the same size, ca. 11 

 mm. Thereafter, females grow slower, appearing 

 increasingly numerous relative to males at suc- 

 cessive body-length increments (Figures 14, 21a). 

 Nemoto's (1957) data suggested that the adult 

 male of E. pacifica tends to be smaller than the 

 female of the same age, and Mauchline (1960) 

 stated this to be the case in Meganyctiphanes 

 norvegica. Slower growth rate in females indicates 

 shorter life span for males, probably by 1 or 2 mo, 

 since females grow to 21 mm length off southern 

 California (rarely more) compared with 20 mm for 

 males. 



In summary, reasons were sought for a) un- 

 derestimation of spawning, b) paired spermato- 

 phores in males, and c) apparent imbalance in sex 

 ratio. These explanations were considered: eggs 

 can ripen and females can spawn more often than 

 the frequency of the surveys, applicable to a) and 

 b); the bias in sex ratio favoring females is real 

 and develops either with higher male mortality at 



756 



