VERTICAL DISTRIBUTION AND DIEL MIGRATION OF EUPHAUSIIDS 

 IN THE CENTRAL REGION OF THE CALIFORNIA CURRENT 



Marsh J. Youngbluth^ 



ABSTRACT 



The density, vertical range, and diel movement of total zooplankton and euphausiid populations in the 

 central region of the California Current were determined during a period of coastal upwelling, 

 July-August 1970. Collections were made along four transects with opening-closing Bongo nets towed 

 through 50- to 100-m intervals in the upper 800 m. Four- to nine-depth intervals at 13 day-night stations 

 were sampled. Twenty euphausiid species from seven genera were identified from 124 hauls. 



Zooplankton assemblages in the nearshore regions differed from those farther offshore in having a 

 larger biomass as well as a smaller number and higher density of several species. Diel vertical 

 movement among euphausiid populations, particularly Euphansia pacifica, tended to be more 

 pronounced in offshore waters. This behavior suggests that, although assemblages of zooplankton are 

 strongly structured by physical factors, some species alter their vertical distribution and diel migration, 

 presumably in response to the prevailing food supply. 



Since 1949 an intensive plankton sampling pro- 

 gram has been conducted in the California Cur- 

 rent under the auspices of the California Coop- 

 erative Oceanic Fisheries Investigations 

 (CalCOFI). These surveys, concentrating on the 

 distribution and density of pelagic organisms in 

 the upper 150 m, have revealed abundance and 

 dispersion patterns of zooplankton which are 

 related to annual and seasonal changes in hydro- 

 graphic conditions (Brinton 1960, 1962a; Thrailkill 

 1963; Fleminger 1964, 1967; Alvarino 1965; 

 McGowan 1967; Berner 1967; Isaacs et al. 1969). 

 Within a given year, varying proportions of zoo- 

 plankton assemblages typical of any one of several 

 water masses are likely to be present (Berner 1957; 

 Bieri 1959; Brinton 1962b; Johnson and Brinton 

 1963; Cushing 1971). Seasonal hydrographic 

 fluctuations near the coastal boundary of the 

 current act to further transform the numbers and 

 types of pelagic species that develop. For example, 

 the eutrophic environment produced by coastal 

 upwelling during the spring and summer months 

 is characterized by a much higher biomass and 

 lower species diversity than the more oligotrophic, 

 off"shore portion of the current (Frolander 1962; 

 Hebard 1966; Laurs 1967; Longhurst 1967; Pieper 

 1967). 

 Considerably less attention has been given to 



'Hopkins Marine Station of Stanford University, Pacific 

 Grove, CA 93950; present address: Harbor Branch Foundation 

 Laboratory, RFD 1, Box 196, Fort Pierce, FL 33450. 



Manuscript accepted March 1976. 



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



the vertical distribution of zooplankton in this 

 current principally because it is difficult, time 

 consuming, and costly to repeatedly sample dis- 

 crete depths. The scope of this study was to 

 describe the vertical distribution and diel migra- 

 tion of zooplankton, particularly euphausiids, in 

 nearshore and offshore oceanic regions of the 

 central California Current during a period when 

 coastal upwelling was well developed. The samples 

 were collected in the summer of 1970 on two 

 cruises, Stanford Oceanographic Expedition 

 (SOE) cruise 22 and CalCOFI cruise 7008. 



DESCRIPTION OF 

 THE ENVIRONMENT 



The California Current is a blend of water 

 masses (Subarctic, North Pacific Drift, Central, 

 and Equatorial) and is therefore an extremely 

 variable environment (Reid et al. 1958; McGowan 

 1971). It flows southward throughout the year with 

 an average velocity of less than 0.5 knot. The 

 boundaries of this transitional water are between 

 lat. 48° and 23°N and extend to 700 km (long. 

 130° W) from the coast. Between depths of 200 and 

 400 m, a subsurface countercurrent flows north- 

 ward at about 0.5 knot from Baja California to 

 Cape Mendocino (Kin'dyushov 1970). 



Near the coast, hydrographic fluctuations in this 

 current have been separated into seasonal periods 

 of divergence (upwelling), relaxation (oceanic), 

 and convergence (downwelling) (Bolin and Abbott 



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