BRINTON: POPULATION BIOLOGY OF EUPHAUSIA PACIFICA 



Sex Ratio 



Fifty percent of the estimates of prespawned 

 eggs were two to four times greater than the 

 estimates of larvae in the plankton a month later. 

 The other 50% of egg/larva ratios were even lower 

 than two (Figures 4d, 6). Further evidence that 

 spawning was underestimated is seen in an ex- 

 amination of ratio of the sexes and state of their 

 reproductive products. 



The ripe male E. pacifica stores two spermato- 

 phores in a pair of ducts. The fertilized female 

 possesses a single attached spermatophore 

 (Brinton in press). This discrepancy might be 

 attributed to a sex ratio in which females 

 predominate, or to a need for more than one 

 fertilization when spawning is protracted or in- 

 termittent across days, intermolt periods or 

 longer. If such multiple fertilizations take place, 

 males transfer one spermatophore to each of two 

 females, probably quickly because single ripe 

 spermatophores were not observed in males. The 

 paired spermatophores in males were observed 

 always to be of equal size, color, and readiness for 

 extrusion. (Ready spermatophores may be easily 

 expelled with gentle external pressure in the 

 laboratory.) A continuing preponderance of ripe 

 males, as shown in Figure 19, would tend to insure 

 fertilization of females whenever they ripen. 

 Mauchline and Fisher (1969) have explained, with 

 reference to Meganyctiphanes norvegica, that 

 fully formed spermatophores may be stored in the 

 ejaculatory ducts for some time. 



Here, ripe and unripe females outnumber males 

 by about 1.5 times at 15 mm, and 3 times at 20 mm 

 (See Figure 21a). Ponomareva's (1963) data on E. 

 pacifica from the Sea of Japan showed females to 

 be 56% of the adult population, and from the 

 Okhotsk Sea 63% in April, 62% in June-July, and 

 44% in October. Four factors may contribute to the 

 apparently greater number of females: 



1) In the present data, apparent dominance by 

 females (all body lengths lumped, Figure 20) is 

 partly due to periods in which the population 

 included late-maturing individuals of 10.5-11.5 

 mm length, some males of which were as yet 

 without petasmas and were therefore categorized 

 as females. (Secondary sexual characters of E. 

 pacifica are usually evident at this size.) For 

 example, this apparently happened during count- 

 ing of material from cruises 5401 and 5402 (Figure 

 13), and cruises 5610-12 (Figure 20) when 



MAXIMUM LARVAE 



FEa-APfi MAX 



MEDIUM ' SIZED 



SPAWNERS 



MAXIMUM EG6S 



APR.,JUN 



OOAL MAXIMA 



FOR 



LARGE SMWNERS 



SUMMER MAX. 



MEDIUM -SIZED 



SPAWNERS 



FALL MAX. 

 SMALL SMWNERS 



Figure 19.-a, Densities of ripe female Euphausia pacifica by 

 months, three body-length groups, 1953-56 data combined from 

 Figure 6d. b. Densities of males with ready spermatophores, 

 same body-length groups. 



"females" dominated the dense population of 8-12 

 mm individuals. 



2) Increasing mortality in males relative to that 

 in females may take place after 12 mm body 

 length. Since the ratio of males to females 

 decreases with body length, multiple fertilizations 

 by males would be increasingly important with 

 increasing size. (Mates are probably of similar 

 size, in view of large spermatophores being at- 

 tached to large females and small spermatophores 

 to small females.) 



3) Large males and unripe females may be more 

 underestimated than egg-bearing females if the 

 latter are less able to avoid net capture. For 

 anchovy larvae, Isaacs (1965) hypothesized that 

 avoidance of the 1-m net becomes significant after 

 15 mm body length. Similar differential avoidance 

 might contribute to the female/male bias here. 

 For 3 of the 4 yr, the average percentage of 

 females that are ripe crested at 15-16 mm (Figure 

 21b). It remained high, 40-60%, through the larger 

 size groups. The 1954 data differed in that the 

 proportion of ripe/unripe females remained low 

 through 16 mm body length. This is also seen in 

 Figure 4d in which the 12.6-16.5 mm group showed 



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