FISHERY BULLETIN: VOL. 87, NO. 3. 1989 



heterogeneity. The study followed a routine sur- 

 vey by the California Cooperative Fisheries 

 Investigations (CalCOFI Cruise 8502) of the 

 northern anchovy spawning domain. Visual in- 

 spection of plankton catches made on this survey 

 yielded estimates of northern anchovy egg 

 abundance at 4-10 nmi intervals. Within areas of 

 high egg abundance, the ship's survey records of 

 3 m temperature and in vivo phytoplankton pig- 

 ment fluorescence were scanned for evidence of 

 local gradients. Low variations of pigments and 

 temperature in the vicinity of possible sites were 

 verified by inspection of available satellite 

 images of sea surface temperature and surface 

 layer color, and several sites were then targeted 

 for this study. 



At sea, we verified that our criteria held at 

 each site by inspection of plankton tows to con- 

 firm recent (and therefore sustained) spawning 

 activity, and by inspection of underway records 

 for local homogeneity of temperature and pig- 

 ment fluorescence at 3 m depth. We occupied 

 sites shown in Figure 1. 



Procedures on Site 



Upon verification that a site met the three 

 criteria, we launched a radio-transmitting sur- 

 face drifter. The ship then moved several nmi to 

 launch a second drifter. Thereafter, we alter- 

 nated our station pattern between the two drift- 

 ers, locating each drifter by radio direction-find- 

 ing equipment and visually. Each site was thus 

 defined, in the Lagrangian sense, to be the 



water corpus that was moving along with the 

 two drifters. We assumed that both drifters at 

 each site were implanted in macroscopically 

 homogeneous water with respect to physical and 

 biological character. We examine the limits of 

 this assumption by contrasting variations within 

 and between drifters at the same site. 



Stations were patterned around each drifter, 

 in turn, at cardinal points 2 km from the drifter. 

 Vertical plankton tows, "CalVETs" (Smith et al. 

 1985), were made with 150 |xm mesh nets from 

 50 m depth at all four stations to catch fish eggs 

 and larvae to determine age-specific anchovy 

 production and mortality. Obhque bongo tows 

 were made to and from 50 m depth with 333 ixm 

 mesh nets and, after we lost the 333 |xm mesh 

 nets, with 505 |xm mesh nets to catch northern 

 anchovy larvae for estimation of starvation inci- 

 dence and recent growth rate. At two of the four 

 stations, paired vertical tows similar to 

 CalVETs were made from 50 m depth with 75 

 |xm mesh and 333 |xm mesh nets to estimate 

 composition and quantity of larval food rations 

 and of other small plankton, respectively. CTD/ 

 Niskin casts were made to 100 m depth at one 

 station, per drifter visit, to measure physical 

 structure of the water column and to get water 

 samples at 10 depths to determine concentra- 

 tions of particulates, chlorophyll-a and phaeopig- 

 ments. Secchi depth was determined at one sta- 

 tion (daylight permitting) at each drifter to 

 estimate thickness of the euphotic layer. Stan- 

 dard weather observations were made once per 

 drifter visit. 



120"W 119- 118' 117" 



Figure 1. — Study site locations in the Southern Cahfornia Bight. 



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