FISHERY BULLETIN: VOL. 74. NO. 4 



m^ by February-March continued to be evident on 

 into the spring bloom. 



Two exceptionally large cohorts, 5406 and 5507, 

 were initiated during late June-July. At first, 

 these survived poorly, 8-10'^/mo for 5406 through 

 August-September and 40%/mo for 5507 through 

 October (Figures lie, f; 15b, c). Growth apparently 

 then stopped after 9-11 mm body length, and the 

 density had declined to 100/1,000 m^ This took 

 place when the onset of maturity was in Sep- 

 tember-October. This is presumably the start of 

 the fall-winter period during which food supply is 

 inadequate to permit both gonad development and 

 size increase. During October-December, the 10-12 

 mm sizes increased in frequency, indicating con- 

 tinuing growth into that range by younger 

 elements of the overall population and much 

 reduced growth out of it. Therefore survivorship 

 of the 5406 and 5507 cohorts during September- 

 December could not be determined, but it appears 

 to have been high. By January, body-length 

 growth of these cohorts, now numerically en- 

 hanced, resumed. Survivorship of "5406" prevailed 

 at about 47'^/mo through June 1955 (21 mm), and 

 for "5507" at 40% /mo as before September. 



The large 5607 and 5610 cohorts appear to have 

 undegone similar development (Figure 10), ap- 

 pearing to coalesce at 9-12 mm during November- 

 December, with much increased frequencies at 

 those body lengths. 



2) Survival rate is poorer, 26-45%/mo, for 

 recruits produced earlier in the year, February- 

 June. Mean life-span survival was 43%/mo for the 

 5303 cohort, 26% for 5305, 37% for 5306, 30% for 

 5404, 44% for 5502, and 45% for 5605. Nonlinear 

 details of survivorship in these cohorts are depict- 

 ed in Figure 15d, e, while differences between 

 early-year and late-year cohorts in mean slope of 

 survivorship regressions are seen in Figure 15g. 

 Coincidence of the juvenile-adult phases of early- 

 year cohorts with the productive period May-Sep- 

 tember evidently accounts for the observed rapid 

 growth during this period, hence the poor survival 

 rate. These cohorts were traced to body lengths of 

 16-18 mm after 7.5-8 mo (5502, 5303, 5605) or to 

 13-15 mm after 4-5 mo (5404, 5305). Having de- 

 clined to densities <10/ 1,000 m-^ during summer- 

 fall, they were no longer recognizable in winter 

 sampling. 



Annual Biomass 



Annual biomass by body length shows year-to- 

 752 



Euphausia pacifica BIOMASS 



I4 4gm per lOCXJm' 



4 6 8 10 12 14 16 18 20 22 



BODY LENGTH ( mm ) 



Figure 16. -Biomass, annual mean values for Euphausia 

 pacifica and distributions per 1 mm body length, a, b, Uniform 

 distributions for 1953, 1954, with modes at 3-4 mm, 7 mm (onset 

 of juvenile phase), 10-12 mm (onset of adulthood) and 15-16 mm 

 (start of ma.ximum egg production, cf. Figure 21b). c, d. Dis- 

 tributions, strongly peaked at adolescence, biased by large 5507 

 and 5609-10 cohorts respectively. 



year similarities (Figure 16). Peaks are at 1) 3-4 

 mm, owing to consistent abundances of larvae in 

 early furcilia phase; 2) 7 mm (except 1956), the 

 onset of juvenile phase; 3) 10-12 mm (9-11 mm in 

 1956) the onset of adult phase; and 4) 15 mm, early 

 in the peak reproductive phase. It was noted 

 (Figure 5b) that monthly biomass peaks were 

 usually dominated by one or another of these four 

 body lengths. The larva peak occurs in spite of 

 rapid early growth. The other three peaks are at 

 ages when slowed body-length growth would be 



