BRINTON: POPULATION BIOLOGY OF EUPHAUSIA PACIFIC A 



traceable for 10 mo through July 1954 (5407) when 

 it had achieved large-adult size, 17-20 mm (Figures 

 9, lib, 12). Separate L-F curves for males and 

 females (Figure 13), commencing at the onset of 

 maturity ca. 11 mm, show that the modes for the 

 5309 cohort illustrated in Figures 9 and lib ac- 

 tually are made up of paired overlapping peaks, 

 for females regularly at a larger body-length 

 increment by about 1 mm and for males where the 

 difference in absolute frequency between males 

 and females is greatest. 



It is not likely that females, upon maturity, have 

 undergone sudden, relatively rapid growth so as to 

 exceed males in size. The curves (Figure 13) show 

 larger females to be at a relatively greater 

 frequency than males and the converse would be 

 expected with increased female growth-rate. 

 (Average male/female ratio is probably 1:1 at on- 

 set of adulthood, discussed under Sex Ratio.) 

 Rather, the most mature females— those at the 

 leading edge of the mode-cohort at the onset of 

 February-March breeding— are growing slower 

 than before, thereby appearing more numerous. 

 At the same time, decreasing relative male sur- 

 vivorship could contribute to the increasing in- 

 equality in sex ratio. At body lengths >16 mm, 

 females tend to dominate by 2:1 or more, indicat- 

 ing that they then spend twice as long as males at 

 given sizes, at least while breeding, or that their 

 survivorship is then greater, or that males remain 

 below sampling depths at night. These alterna- 

 tives are considered in the discussion of Sex Ratio, 

 below. 



GROWTH CURVES OF THE COHORTS CONSIDERED TRACEABLE 



% 



J FMAMJ J ASONOJ FMAMJJ A 

 1953 1954 



D|JFM>MJJ SONDJFUAMJJ OND 



I9S5 1956 



Figure 12.-Growth curves of Euphausia pacijica inferred from 

 length-frequency modes. Clear (solid lines) and unclear (dashed 

 lines) sequences as in Figures 9 and 10. Times of egg production 

 are extrapolated, see Figure 4c. Fall-winter period of slowed 

 growth is crosshatched. 



1954 Cohorts 



The single intense spawn of 1954 (June) led to 

 strong June-July recruitment, establishing a 



cohort (5406) that was followed through a 10-mo 

 period to 17-19 mm in April 1955 and, with less 

 certainty, to 20 mm in June (Figures 9, 10, lie, 13). 



1955 Cohorts 



Conspicuous 1955 cohorts arose in February 

 (5502) and July (5507). The former appeared to 

 attain 18 mm after 7 mo (September) and the 

 latter reached 17-18 mm after 8 mo, following 

 slowed growth during October-January (Figures 

 lld-e, 12). This cohort appeared at too-low density 

 in October (5510) relative to a month later. This 

 may be due either to sampling variability or to 

 "piling up" at the 11-12 mm increment in 

 November owing to growth being faster into the 

 newly adult phase than out of it, energy then 

 being diverted to gonad development. Neverthe- 

 less, it is noteworthy that the 5502 and 5507 

 cohorts appeared to be distinguishable in October 

 (5510) as modes of 10-12 mm and 15-16 mm, Figure 

 le, discussed earlier when the southern California 

 area was compared with the California Current as 

 a whole. 



The December 1955 cohort was the only distinct 

 year-end cohort observed during 1953-56 (Figures 

 10, 12, 13). It grew rapidly at 4 to 5 mm/mo during 

 December-February and 3 to 4 mm /mo during 

 February-April, apparently attaining 18 to 20 mm 

 length by June 1956. 



1956 Cohorts 



These were scarcely traceable except for that 

 appearing as 8-11 mm individuals in early 

 November and as 8-12 mm in December. This 

 mode doubtless derives from extremely dense 

 larvae sampled during 5507 and 5509-10, its crest 

 appearing to relate mainly to the latter. The small 

 biomass peak at 10-11 mm during 29 September-5 

 October is clearly derived from the very heavy July 

 recruitment. It subsequently becomes indistin- 

 guishable during November and December from 

 the biomass of 8-12 mm juvenile-adults considered 

 to have grown from 5509-10 larvae. The 29 October 

 -4 November peak appears most likely to have 

 derived from the 5509-10 larvae. 



Survivorship 



The average L-F distribution for all samples 

 (Figure 14) shows that decline in density with 

 body length is roughly exponential. The decline is 



749 



