POPULATION BIOLOGY OF EUPHAUSIA PACIFIC A 

 OFF SOUTHERN CALIFORNIA 



Edward Brinton^ 



ABSTRACT 



Euphausia pacifica was observed with respect to reproduction, growth and development of cohorts, and 

 successions in population structure and biomass during 4 yr, 1953-56. The southern California eddy and 

 its upwelling regime serve as a reproduction refuge for a warm-temperate population of this 

 euphausiid. Three size classes spawn there during a year-the largest in April-June, an intermediate in 

 June-February, and small, newly mature females usually in August-January. There were year-to-year 

 differences. 



The largest densities of larvae were observed about a month after egg peaks (one survey later) or 

 appeared coincident with them. In 1953 there was strong spring recruitment, abruptly subsiding with 

 an early decline in upwelling-the index of environmental enrichment used. During 1954 only one 

 substantial cohort was recorded, in June at the height of a poor upwelling season. In 1955 repeated 

 spawning occurred during the long upwelling season, but recruitment after July was poor. The year of 

 most intense upwelling, 1956, yielded three strong cohorts— the last, July-October, being exceptionally 

 strong. Smallest larvae were usually in 12°-16°C waters. Ripe females were concentrated at high 

 densities at these same temperatures during August-March but were distributed over a broader range 

 at 10.5°-19°C during April-July. 



Growth was estimated to be about 3 mm body length per month, slowing during September-January 

 or after about 17 mm. Females appeared to grow slower in breeding seasons. Maturity can be at 11 mm, 

 but reproduction is not general until 15-16 mm. Here, maximum size was 21 mm after about 7 mo for 

 early-year recruits and a year for summer recruits. Survival rates appeared higher in the latter. Growth 

 rates were similar to those reported for E. pacifica off Oregon and higher than in the subarctic Pacific. 

 Survivorship was lowest for furcilia larvae, increased in juvenile and young adult phases, then 

 decreased after reproduction became regular. Slowed growth and increased survivorship at life 

 interphases appeared to cause regular frequency and biomass maxima at lengths of 7, 10-12, and 15 

 mm. Sex ratio favored females. Males apparently accomplished multiple fertilizations. 



Euphausia pacifica Hansen is a temperate North 

 Pacific euphausiid crustacean, composing a sub- 

 stantial part of the zooplankton of the North 

 Pacific Drift, lat. 40°-50°N, and ranging south- 

 ward along the coast of North America as far as 

 lat. 25°N (Brinton 1962a). In the cooler part of the 

 California Current, it occurs in association with 

 the euphausiids Nematoscelis difficilis and Thy- 

 sanoessa gregaria. Depth ranges of the three 

 species overlap daily as E. pacifica and A'', difiicilis 

 engage in distinctive vertical migrations while T. 

 gregaria does not migrate (Brinton 1967a). Hor- 

 izontal ranges are sufficiently similar so that 

 these species, together with E. gibboides, were 

 considered the euphausiids of a California Cur- 

 rent-Transition Zone plankton assemblage 

 (Brinton 1962a). 



Euphausia pacifica performs extensive vertical 

 migrations. Off California it lives at daytime 

 depths of 200-400 m, entering the surface layer at 



'Scripps Institution of Oceanography, La Jolla, CA 98093. 



Manuscript accepted May 1976. 



FISHERY BULLETIN: VOL. 74, NO. 4, 1976. 



?-5? ' ^i Z- 



night. It is an omnivore (Lasker 1966) and pos- 

 sesses thoracic food-gathering limbs which are 

 nearly uniform in length and in setation of the 

 filtering screens. 



Euphausia pacifica is usually the most abun- 

 dant euphausiid. Its maximum densities are often 

 centered relatively near to the coast of California. 

 The low-latitude part of the population of E. 

 pacifica is the object of this study. Aspects of its 

 life history have been observed in the more 

 typically temperate regime to the north of lat. 

 40°N (Nemoto 1957; Ponomareva 1963; Smiles and 

 Pearcy 1971) where environmental characteristics 

 show stronger seasonality than to the south. The 

 extent to which the downstream portion of this 

 distributional range is maintained by local 

 processes has not been previously investigated. 



The study was organized in relation to existing 

 knowledge of the physical-chemical characteris- 

 tics of the area and of the species distributions. 



Surveys of the region of the California Current 

 since 1949 have provided a reservoir of hydro- 

 graphic data and plankton samples that lend 



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