FISHERY BULLETIN; VOL. 74, NO. 4 



1963; Dodimead et al. 1963). From March to 

 August, the force of the prevailing northerly 

 winds and the earth's rotation cause the southerly 

 flowing surface waters, within 100 km of the shore, 

 to move away from the coast (Yoshida 1955). This 

 displaced water is replaced by cooler, more saline, 

 nutrient-rich water upwelled from deeper regions, 

 providing favorable conditions for high rates of 

 primary production. In the central part of this 

 current, the upwelling period is considered to 

 begin and end with the shifting of the 9°C iso- 

 therm above and below the 100-m level (Barham 

 1957). 



The northerly winds subside from September to 

 November and surface water temperatures in- 

 crease, resulting in the formation of a strong 

 thermocline in the upper 50 m. During this sur- 

 face-warmed period, in the absence of upwelling, 

 tongues of offshore, oceanic water of the Califor- 

 nia Current may reach the coast. Where this occurs 

 there is probably considerable mixing of oceanic 

 and neritic planktonic communities (e.g., see 

 Longhurst 1967). 



When southerly winds prevail, in the period 

 from December to February, a northerly flowing, 

 coastal countercurrent (Davidson Current) may 

 develop. Surface water converges toward the coast 

 and disrupts the stratification characteristic of the 

 surface-warmed water period. Vertical eddy cir- 

 culation results, promoting the overturn, mixing, 

 and downwelling of warm, lower salinity, nu- 

 trient-poor surface waters. This mixed water 

 period can be characterized by a temperature 

 gradient of less than 1°C in the upper 50 m. 



The environments sampled at the shoreward 

 and seaward stations in the summer of 1970 

 differed in several ways. Physical and chemical 

 features relating to phytoplankton studies during 

 the SOE cruise are presented in Malone (1971). 

 These and other hydrographic features in the 

 upper 800 m at each station are tabulated and 

 discussed in Youngbluth (1973). By way of sum- 

 mary, it was clear from the low temperatures and 

 high salinities and the shoreward elevation of 

 nitrate isopleths that upwelling conditions 

 prevailed near the coast. Chlorophyll-a values in 

 the upper 150 m decreased with increasing dis- 

 tances from shore, 2.1-0.5 mg/m\ The photic zone 

 was usually deeper at the seaward stations, rang- 

 ing from 55 m in coastal regions to 105 m at the 

 western edge of the transects. The depth of the 

 thermocline was shallower nearshore and deeper 

 offshore, ranging from 5 to 40 m, respectively. The 



largest temperature difference between the ther- 

 mocline and 150 m was about 4°C. At depths below 

 150 m, temperatures differed by 2°C or less among 

 stations. 



Temperature-salinity (T-S) curves from each 

 station were compared to two different schemes 

 (Youngbluth 1973). First, the data, when plotted 

 with T-S relationships that characterize the per- 

 cent mixing between waters near the northern 

 and southern limits of the California Current 

 (Okutani and McGowan 1969), indicated that 

 between 150 and 800 m 70-100% northern water 

 was present. The small percentage of southern 

 water was most noticeable at the intermediate and 

 nearshore stations of the southern transect. 

 Second, the data, when contrasted with T-S curves 

 that distinguish water masses, revealed that 

 samples below 250 m were collected in North 

 Pacific Intermediate water. 



MATERIALS AND METHODS 



Zooplankton samples were collected with open- 

 ing-closing Bongo nets of 0.333-mm mesh and cod 

 ends with 0.222-mm mesh (McGowan and Brown 

 1966). At nearly all stations, shallow and mid- 

 water casts were made, within 3 h of midday and 

 midnight at nearly the same location (Table 1). 

 Shallow tows were taken with a single frame 

 (SOE) or with four frames (CalCOFI) in the upper 

 150-200 m. Three (SOE) or four (CalCOFI) frames 

 were used on mid-water hauls between 200 and 600 



Table l.-The date and position of Bongo net tows. 



926 



