BRINTON: POPULATION BIOLOGY OF EUPHAUSIA PACIFICA 



E pacifica SURVIVORSHIP 



RAPID INOtEASE 

 IN % FERTUTY 



E 

 O 

 O 

 O— 



S§ 



10 C M 16 



BODY LENGTH (mm) 



Figure 14. -Catch cur\-es for all Euphausia pacifica sampled, 

 densities shown on logarithmic and linear (adults only) scales. 

 Periods of changing slope (changing sun'ivorship, net avoidance 

 and/or growth rates) indicated as related to life phases. Scale 

 used for density of sexed adults (right) is doubled for lumped 

 immatures (left). 



differences in survivorship between males and 

 females (Figure 14) are discussed below under Sex 

 Ratio. 



Survival rates for individual cohorts were ap- 

 proximated from relative amplitudes of month- 

 to-month modes in the sequences used to trace 

 growth (e.g.. Figure 11). Percent survivorship 

 plotted against estimated age shows cohort cun-es 

 to be similar (Figure 15a). A positive change in 

 slope consistently occurs within the range of 8-12 

 mm body length encompassing adolescence. How- 

 ever, regressions of logio density on age take two 

 forms: 



1) Mean life-span survival rate calculated as a 

 single linear regression for individual cohorts is 

 highest among those recruited during June- 

 December (06-12). For example, it is 51%/mo for 

 the 5512 cohort, 58^c for 5610, and o9^c for 5309. In 

 such late-year cohorts most of the juvenile-adult 

 phase is during August-March, the period of 

 reduced food and slowed growth. For example, the 

 cohort 5507 attained adolescence (9-10 mm) in 

 September and large adulthood (17 mm) in March 

 (Figure 15b), having an estimated life span of 10 

 mo. (Egg stage to 3 or 4 mm length is considered 

 the first month.) The cohort 5406 (Figure 15c) 

 attained adolescence at 9 mm in August, appeared 

 to show strong survival through 15-16 mm in 

 February-, and was distinguishable at 20 mm size 

 in June-a life span of 13 mo. Thus those cohorts 

 which attained 15-16 mm with densities >50/l,(X)0 



100 



80- 



g. 



X 

 CO 



geo 



> 



> 



<« 40 



20- ||-l2mm ^^ 



*("' \ \V9-I0rrm^ 

 - 8- IOn»n^\J<''J<''e 

 August 



0-l2n¥n, Januory 



2 34 5 67 89 10 II 12 



2 3 4 5 6 7 8 9 10 II 12 13 

 ESTtMATED AGE (MONTHS) 



FiGiniE 15.-Survivorship of cohorts of Euphausia pacifica, from 

 amplitudes of length-frequency modes, a, Percent survivorship 

 showing rapid decline until adolescence, ca. 9-11 mm. b, c, 

 Age-frequency distributions of 06-12 cohorts smoothed for 

 apparent piling up at times of slowed growth, d, e, Age- 

 frequency distributions of 02-05 cohorts, f, Curves seen in b-e, 

 clustered, g. Average slopes (from straight line regressions) for 

 02-05 cohorts seen as steeper than for 06-12 cohorts. 



751 



