BRINTON: POPULATION BIOLOGY OF EUPHAUSIA PACIFICA 



precise trends in development of presumed cohorts 

 provide growtli rates which corroborate the more 

 obvious trends. Some pathways of development 

 indicated in Figures 9 and 10 may appear imagi- 

 nary unless the shapes and amplitudes of the 

 related L-F distributions, adjacent in time, are 

 closely compared. When such indistinct modes are 

 followed, precision and accuracy in recognizing 

 rates of development are reduced. Graphical 

 procedures for mathematically defining cohorts 

 composing irregular L-F polygons (e.g., Harding 

 1949) required some subjectivity in recognizing 

 modes and were employed only in an exploratory 

 way. 



There can be important inaccuracies in field 

 estimates of growth rate when reliance is upon 

 time-sequences in L-F modes. Even with steady, 

 uniform recruitment, peaks or troughs would 

 appear in the L-F distribution owing to differing 

 growth rates and survivorship among life phases 

 or between sexes. With unsteady recruitment, 

 such peaks may sometimes lie in phase with the 

 cohort being traced, but the cohort nevertheless 

 becomes compounded by younger individuals 

 when its growth is differentially slowed or by older 

 individuals when accelerated. 



It is possible that the individuals composing a 

 mode could be totally replaced in the course of its 

 time progression, although the modal assemblage 

 persists as a size group, presumably feeding and 

 mating as a unit. I have noted above that spring- 

 summer cohorts tend to "pile up" in fall-winter 

 when growth of adolescents appears to be food 

 limited. 



In tracing growth, reliance is therefore upon the 

 more substantial cohorts. Although these can be 

 masked, their frequent appearance as modes at 

 sizes not associated with life-phase changes gives 

 credence to the method. 



Growth rates of E. pacifica off southern 

 California appear similar to those off Oregon 

 (Smiles and Pearcy 1971). Figure 22 shows gen- 

 eralized growth cun^es for this species from four 

 areas in the North Pacific. The Oregon population 

 showed steady growth after September recruit- 

 ment. The juvenile and adolescent phases were 

 during the winter and 13 mm was reached by 

 February. About 22 mm was attained after 1 yr. 

 This parallels development of a winter cohort 

 (5512) off southern California which grew to 12 mm 

 in 3 mo and was traced to about 21 mm after 8 mo. 



Spring (5406) and summer (5309) cohorts off 

 southern California grew at rates similar to the 



NORTHEAST 

 PACIFIC 



OREGON- >.'^'' ^(Nemoto) 



(Smiles a Pearcy)/^ 



OREGON 

 (Smiles a Pearcy) 



SO CALIF 

 5512 



>■ I I I I I 1,1 I I I I I I I I I I I I I Ill 



6 8 10 12 2 4 6 8 10 12 2 4 6 8 10 

 I MONTHS I 



Figure 22.-Representative growth curves from southern 

 California area compared with curves previously derived for 

 Euphausia pacifica and illustrated by Smiles and Pearcy (1971). 



winter cohort, except for slowing during October- 

 December-5406 during adolescence and 5309 dur- 

 ing the juvenile phase. 



Here, life expectancy appears to be about 8 mo 

 for winter and early-spring cohorts, to sizes of 

 18-20 mm by August-October. December-January 

 populations never included individuals larger than 

 19 mm. Life expectancy is up to 12 mo for late- 

 spring and summer cohorts, which grew to 21-22 

 mm by the following April-July. This agrees with 

 estimates of 12 mo for September cohorts off 

 Oregon. 



Growth in other euphausiid species, mostly 

 summarized in Smiles and Pearcy (1971), is 

 similar. Several reach about 22 mm after 1 yr: E. 

 superba (Ruud 1932; Bargmann 1945; Marr 1962), 

 E. triacantha (Baker 1959), Thysanoessa raschii 

 (Mauchline 1966), Meganyctiphanes norvegica 

 (Ruud 1936; Einarsson 1945; Mauchline 1960; 

 Matthews 1973), and Thysanopoda acutifrons 

 (Einarsson 1945). Most of these species have a life 

 expectancy of 2 yr, reproducing in each and grow- 

 ing slowly or not at all in winter. 



During winter in the westernmost North Pacific 

 (Sea of Okhotsk), Ponomareva (1963) found E. 

 pacifica to be 8 mm (considered to have hatched 

 the previous summer) and 14-15 mm (considered 2 

 yr old). In the spring it was 12-13 mm (1 yr old) and 

 19 mm (2 yr old). Both groups bred in June. Off 

 nearby Kamchatka in the summer, Nemoto (1957) 

 found a size range of 12-22 mm, much like that 

 found by Ponomareva, but with most at 14-20 mm. 

 There were no larvae, but females with spermato- 

 phores were present in September, as off Oregon. 



759 



