BRINTON: POPULATION BIOLOGY OF EUPHAUSIA PACIFIC A 



were within a somewhat narrow range of tem- 

 perature, 12°-15.5°C, as compared with the weak- 

 upwelling period, and most spawners were within 

 a broader range, 11.5°-17.5°C. 



By years, during 1953, 1955, and 1956 the mean 

 maxima of larvae were at 14°-15°C. In 1954 there 

 were maxima at both 13°-14°C and 14.5°-16°C. 

 Occurrences of larvae and spawners at tempera- 

 tures >18.5°C during January-March and August- 

 December were only during 1955, the year in which 

 spawning extended on into September and 

 November. Occurrences of April-July larvae at 

 temperatures >18°C were all during 1954, the year 

 of weakest upwelling, except for a single record in 

 1953. 



Overall frequency of spawners did not differ 

 between the periods of strong and weak up- 

 welling, in contrast to large differences in the 

 frequency of recruits. This implies that factors 

 other than temperature are important to 

 recruitment— probably the production of food 

 associated with the upwelling period. Patchy local 

 increases in surface nutrients associated with the 

 upwelling season of 1969 are described in the 

 Discussion. 



It is also noteworthy that during periods of both 

 strong and weak upwelling, mean maxima of 

 spawners occurred at or just outside the limits of 

 the optimal temperature range for recruits: dur- 

 ing weak upwelling months, at 12°-12.5°C and 

 15.5°-16°C; and during strong upwelling months, 

 at 11.5°-12°C and 17°-17.5°C. This implies that 

 stations showing maximum densities of larvae 

 and those showing maxima of spawners were 

 mutually exclusive— an impression gained earlier 

 during counting. Removal of adults from the 

 region where they might fortuitously feed upon 

 their young could be brought about by the vertical 

 migration of the adults and their consequent 

 differential transport at greater daytime depths, 

 in accordance with the hypothesis of Hardy (1956). 



Growth 



Monthly L-F polygons for E. pacifica consis- 

 tently peak at larvae 3-4 mm in length (Figures 9, 

 10). In the stream of continuous recruitment, a 

 month-class is first distinguishable as high 

 numbers of larvae relative to those produced in the 

 months before and after. Subsequent growth can 

 be traced through successive months as an L-F 

 mode, either in the form of a crest, irregularity in 

 slope, or change in height relative to the month 



before. Observations of growth and survival ap- 

 pear most reliable when cohorts are traced that 

 begin as densities in excess of 2,000 larvae per 1 

 mm length increment per 1,000 m^. 



A cohort is designated by the year-month (e.g., 

 5303) in which its larva maximum is observed. 

 Presumed relationships of egg maxima (Figure 4c) 

 to subsequent recruitment are indicated in Figure 

 12. 



When presented in terms of biomass (Figures 9, 

 10), population composition differs from that 

 indicated by length frequency. For example, 

 biomass modes may increase in height with time 

 owing to growth, while corresponding L-F modes 

 decrease in height because of mortality. As a 

 consequence, cohorts are often more conspicuous 

 when plotted as biomass. Biomass is plotted on a 

 linear scale while abundances (length frequency) 

 were plotted on a logarithmic scale to accom- 

 modate fluctuations in the many larvae and the 

 few large adults. The biomass of larvae was 

 generally low but periods of heavy recruitment are 

 conspicuous. 



1953 Cohorts 



A small February 1953 cohort (Figures 9, 12) was 

 tentatively traced through April as 10-11 mm 

 adolescents. More substantial recruitment oc- 

 curred in March from the February egg maximum, 

 followed by little recruitment in April; growth 

 appears to have been to 7-8 mm in April, 10 mm in 

 May, 13 mm in June, 15-17 mm in July, 18 mm in 

 August, and 18-19 mm in September. 



Production of larvae first became intense during 

 May-July 1953 (cuises 5305-5307), resulting in a 

 broad mode recognizable as 3-7 mm in June 

 (enclosed by a pair of dashed lines in Figure 11, one 

 originating at 3 mm in May and the other at 3 mm 

 in June). July larvae appeared to show poor sur- 

 vival, as shown in the reduced 5-7 mm component 

 of the population in August. This is interpreted as 

 leading to graphic separation of the May-June 

 cohort as a conspicuous L-F mode, first observed in 

 August (5308) as 8-13 mm juveniles and young 

 adults (Figures 9, 11a), persisting into September 

 at 12-15 mm, and perhaps surviving without 

 growth into October, though decimated. Develop- 

 ment of this cohort is even more conspicuous 

 through the sequence of biomass modes. 



An increase in recruitment in September (5309) 

 over August, followed by low production in Oc- 

 tober, yielded a particularly conspicuous cohort 



745 



