FISHERY BULLETIN: VOL. 74. NO. 4 



South of the Aleutian Islands in September, he 

 found a 6-12 mm group interpreted as having 

 hatched in the spring or early summer. Maximum 

 numbers of adult females were 16-19 mm in May, 

 17-21 mm in June, and 18-22 mm in September. 



Thus growth of E. pacifica is inferred to be 

 slower and of longer duration in the Subarctic seas 

 than off Oregon and California (Figure 22). 

 Nemoto's (1957) estimate from south of the Aleu- 

 tians was intermediate between Ponomareva's 

 (1963) from the western Pacific and those from the 

 American coast. Ponomareva's finding that sexual 

 maturity is attained by 15-17 mm, with some 

 mature at only 11-12 mm, agreed with the obser- 

 vations off southern California. 



During E. pacifica's main reproductive season 

 there is similarity in surface water temperatures 

 (Sverdrup et al. 1942; Anonymous 1963) among the 

 five North Pacific areas from which information on 

 life history comes; there is less agreement in 

 winter temperatures: 



Sea of Okhotsk 

 Off Kamchatka 

 South of Aleutians 

 Off Oregon 



10-13°C (Aug.), 



9-ll°C (Aug.), 



10-12°C (Aug.), 



10-14°C (Sept.), 



Off southern Califor- 10-18°C (June), 

 nia 



0°C (Feb.) 



0°- l°C(Feb.) 



2°- 4°C (Feb.) 



9°-ll°C(Feb.) 



12°-15°C (Feb.) 



The intense densities of E. pacifica at 8-12 mm, 

 also appearing as conspicuous biomass peaks, are 

 the rule rather than the exception. Therefore, such 

 regular concentrating at the adolescence inter- 

 phase, particularly in fall-winter, may be other 

 than an incidental consequence of reduced food. It 

 appears as a means of increasing size uniformity 

 in the population, hence improved breeding 

 efficiency, by the time of the spring bloom-a 

 condition fulfilled by stricter seasonality in the 

 high-latitude populations of E. pacifica. 



ACKNOWLEDGMENTS 



I thank E. W. Fager, J. D. Isaacs, and R. Lasker 

 for providing insight into aspects of this problem; 

 E. L. Venrick and M. M. Mullin for reading the 

 manuscript and offering suggestions; A. Town- 

 send, S. Drais, and T. Stewart for assistance in 

 many phases of the work; and F. Crowe and B. 

 Thomas for drafting most of the figures. Support 

 was provided by the Marine Life Research Pro- 

 gram, the Scripps Institution of Oceanography's 

 component of the California Cooperative Oceanic 

 Fisheries Investigations, a project under sponsor- 

 ship of the Marine Research Committee of 

 California, and by the National Science 

 Foundation. 



Winter temperatures in the three subarctic 

 areas are near 0°C whereas off Oregon and 

 California they differ little from spring-summer 

 temperatures influenced by upwelling. An overall 

 temperature regime for E. pacifica. is thereby 

 described in which low temperature does not limit 

 occupancy but in which 9°-16°C is suitable for 

 reproduction, food permitting. In the subarctic 

 region reproduction takes place at 9-13°C, the 

 highest annual temperatures there. To the south 

 of the California Current off mainland Mexico, 

 food seems to be abundant, but other factors 

 (temperatures >20°C, oxygen concentrations <0.1 

 ml/liter, different current systems) appear there 

 to curtail the species' range. 



The serial biomass representations included 

 here clearly show rise and decline of cohorts, but 

 are less exact than length frequency in determin- 

 ing growth and do not serve in estimating sur- 

 vivorship. It is evident that biomass of the species 

 fluctuates month-to-month, with recruitment and 

 growth not balancing mortality in any regular 

 way. However in 34 of the 48 mo, the biomass was 

 within the range of 8-22 g/ 1,000 m^. 



LITERATURE CITED 



Ahlstro.m, E. H. 



1948. A record of pilchard eggs and larvae collected during 

 surveys made in 1939 to 1941. U.S. Fish. Wild!. Serv., Spec. 

 Sci. Rep. Fish. 54. 75 p. 

 Anonymous 



1963. CalCOFI atlas of 10-meter temperatures and salini- 

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 Baker, A. de C. 



1959. The distribution and life history of Euphausia 

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Bakun, a. 



1973. Coastal upwelling indices, west coast of North Amer- 

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 Bargmann, H. E. 



1945. The development and life-history of adolescent and 

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1957. The ecology of sonic scattering layers in the Monterey 

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 BODEN. B. P. 



1950. The post-naupliar stages of the crustacean Euphausia 

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 Brinton, E. 



1960. Changes in the distribution of euphausiid crustaceans 



760 



