The method of determining the 'Vret" plankton volume of a sanq)le is 

 as follows: The plankton is separated tron^ its preserving liquid (^ for- 

 malin) by filtering, allowed to drain, and then the displacement volume is 

 obtained by placing the drained wet plankton in a graduated cylinder with 

 a known volume of 3% formalin. Two volumes are obtained for each sample: 

 the total wet volume of all plankton material, and the volume of smaller 

 organisms. Larger organisms are arbitrarily limited to those whose vol- 

 umes per individual exceed five cubic centimeters, and to juvenile and 

 adult fishes irrespective of volume. These volumes are then standardized 

 to the amount in one thousand cubic meters of water strained. 



The distribution charts included in this report are based on the 

 volume of smaller organisms only. In order to obtain a good contrast 

 between light and heavy concentrations of plankton, five categories of 

 abundance are used: (1) light, to 33 cc, of plankton, (2) moderate, 

 34 to 100 cc, of plankton, (3) average, 101 to 300 cc, of plankton, (4) 

 fairly heavy, 301 to 900 cc, of plankton, and (5) heavy, more than 900 

 cc, of plankton. The distribution charts are of two kinds: a chart based 

 on the average plankton volumes at each station during the year (fig, 3), 

 and a series of monthly charts (figs, 4-15) which accompany the plankton 

 volume data. 



Plankton nets used during 1957 were constructed of either No, 30xxx 

 grit gauze (an extra heavy type of silk bolting cloth) or of nylon bolting 

 cloth of fairly similar mesh size. The nets were 1 meter in diameter at 

 the mouth, and approximately 5 meters in overall length. 



Each haul was an oblique tow made from approximately 140 meters 

 deep to the surface (depth of water permitting). Since 1951, this has been 

 our standard method of taking net tows; therefore, in this report, the 1957 

 plankton volumes are coiq}ared only with th06e taken from 1951 through 1956 

 (fig, 2), During 1949 and 1950, the tows were taken from depths of about 

 70 meters to the surface. 



Comparison of 1957 plankton volumes with those obtained in 1951-1956 



The portion of the CC(FI survey area that has been most consistently 

 ocetq)ied lies between Point Conception, California, and Point San Juanico, 

 Baja California (station lines 80-137), In other studies (Ahlstrom 1954), 

 this area has been divided into two roughly equal parts: a northern por- 

 tion comprising the station lines off southern California and northern Baja 

 California (lines 80-107), and a southern portion made up of the station 

 lines off central Baja California (lines 110-137). This division is fol- 

 lowed in comparing 1957 plankton volumes with those of previous years. 



Coverage of these areas usually has been obtained at monthly inter- 

 vals during the period of heavy sardine spawning, the first seven months 

 of the year, and less frequently and less con9>letely during the remaining 

 five months of each year. 



