FISHERY BULLETIN: VOL. 74, NO. 4 



themselves to time-series studies of biological and 

 environmental developments. The CalCOFI 

 (California Cooperative Oceanic Fisheries Inves- 

 tigations) Atlas series (Numbers 1-24) presented 

 varied material, including euphausiid distribu- 

 tions derived from the program. Charts of dis- 

 tributions of E. pacifica based on the data that are 

 the subject of the present analysis are included in 

 Brinton and Wyllie (in press). Smith (1971) de- 

 scribed the distribution of zooplankton biomass. 



Description of the Study Area 



The southern California eddy is the southern- 

 most area in which E. pacifica is still both abun- 

 dant (commonly 10-1,000 individuals beneath 1 

 m^ of sea = 10-1,000 mg wet weight) and domi- 

 nant among the larger zooplankters (Brinton 1967a, 

 b). The eddy may be considered bounded on the 

 north by Point Conception, lat. 34°N, and on the 

 south by about lat. 30°N. Its east-west extent is 

 about 250 km; beyond its western limits, flow is 

 consistently from the north and apparently con- 

 tributes relatively little water and biota to the 

 eddy. 



The sluggish circulation off southern California 

 evidently permits substantial autonomy for the 

 resident populations. The currents are commonly 

 5-10 cm/s and rarely as much as 25 cm/s, both at 

 the surface and at 200 m depth (Wyllie 1966). 

 Direction of flow sometimes reverses between 

 these two levels. These are, respectively, the night 

 and day depth levels occupied by vertically mi- 

 grating E. pacifica (juvenile and adult) in the area; 

 larvae remain near the surface day and night 

 (Brinton 1967a). 



Circulation of the eddy is cyclonic. Within it, 

 therefore, there is upward transport of enriched 

 water. The center of the eddy (no surface flow) is, 

 on the average, near San Nicholas Island (lat. 

 33°15'N, long. 119°30'W), 100 km off the midpoint 

 of the southern California coast. The study area 

 was centered here. Farther east, mean flow is 

 northwesterly along the coast. To the west, flow is 

 southeasterly, angling toward the coast near lat. 

 30°N. 



About 150 km south of Point Conception, mean 

 geostrophic flow approaches 135°, averaging 10 

 cm/s. A parcel of water entering the eddy from the 

 northwest would, at that speed, take 100 days 

 to move around the eddy back to Point Conception, 

 flow permitting. Average velocities within the 

 eddy are much less. Places where substantial 



734 



advection takes place across margins of the area 

 are determinable from the flow diagrams in a 

 relative sense. Northerly surface flow into and out 

 of the area is characteristic of winter months 

 when the Davidson Countercurrent is developed. 

 Southerly flow into or through the western part of 

 the area is usually strongest in April-July. The 

 eddy persisted in almost all of the months studied. 



Upwelling enhances the temperate character of 

 the area during spring and summer, usually in- 

 tensifying during April-June (Bakun 1973) when 

 annual temperature minima are usually found. It 

 is responsible for much of the local nutrient 

 enrichment (Reid et al. 1958). Seasonal periodicity 

 is evident when water temperature is averaged for 

 the area of the eddy as a whole: August-October is 

 generally warmest and January-April coolest 

 (Anonymous 1963). The area contains a scatter of 

 islands which provide substantial shoal grounds, 

 regarded off Oregon to be areas best suited for E. 

 pacifica (Smiles and Pearcy 1971). Such islands 

 also provide topography for the formation of 

 downstream eddies which are enrichment centers 

 (Uda and Ishino 1958). They also serve as centers 

 of upwelling. Here, upwelling is less dependent on 

 the direction of the wind than on its intensity. 

 However, the coast from Point Conception 

 eastward remains the main focus of upwelling 

 during the period of prevailing northwest 

 winds, February-June. According to the indices 

 derived by Bakun from extrapolated atmospheric 

 pressure gradients at the sea surface, upwelling 

 off southern California is the most intense to be 

 found in the California Current. 



For this initial life-history study, the period 

 chosen (1953-56) was one of generally stable 

 oceanic climate and hydrographic conditions, 

 compared with the years immediately following, 

 which included times of more extreme fluctuations 

 in temperature and flow characteristics. During 2 

 of the 4 yr, 1955 and 1956, upwelling was inferred 

 by Bakun (1973) to be more intense than the 

 1946-71 mean; however, during 1954 it was less, 

 and during 1953 upwelling commenced early but 

 barely achieved the June peak of mean intensity 

 and was greatly diminished in the summer 

 months. 



Thus it was anticipated that the study period 

 would yield observations of low annual variability 

 in the population of E. pacifica, thereby providing 

 a baseline against which eventually to measure 

 events in years of known extremes in ocean 

 climate, e.g., 1957-59 (Brinton 1960). 



