RELATIVE AREAL ZOOPLANKTON ABUNDANCE OFF THE 



PACIFIC COAST 



Zooplankton volumes for the period 1949-1955 have been reported in a 

 series of six Special Scientific Reports- -Fisheries, which are listed at the end 

 of the text of this report. Also included in this report are monthly and yearly 

 distribution charts contouring the plankton volumes found in the area surveyed 

 on cruises of the California Cooperative Oceanic Fisheries Investigations. These 

 charts graphically present the data contained in the previous reports. 



The volumes of plankton shown in the charts are "wet" volumes of the 

 smaller organisms . In the reports on plankton volumes, two volumes are given 

 for each station: (1) the total wet volumes standardized to the amount in 1,000 

 cubic meters (cm.) of water strained, and (2) the volume of smaller organisms 

 after the removal of larger organisms, such as jellies, fish, larger molluscs, 

 larger crustaceans, squid, pyrosomes, and salps. Our criterion of a larger organ- 

 ism (except in the case of juvenile or adult fish) is one whose volume exceeds 5 cc. 



In determining the volume of a sample, the plankton was separated from its 

 preserving liquid by filtering and was allowed to drain. Then, the volume of 

 plankton was determined by the displacement process. 



In the accompanying charts, the abundance of plankton has been divided 

 into five categories, as follows: (1) light, to 33 cc . of plankton per 1,000 cubic 

 meters of water strained; (2) moderate, 34 to 100 cc. of plankton per 1,000 cubic 

 meters of water strained; (3) average, 101 to 300 cc. of plankton per 1,000 cubic 

 meters of water strained; (4) fairly heavy, 301 to 900 cc. of plankton per 1,000 

 cubic meters of water strained; and (5) heavy, more than 900 cc . of plankton per 

 1,000 cubic meters of water strained. These categories have been chosen in order 

 to obtain good contrast between light and heavy concentrations of plankton. Other 

 numerical groupings were tried, such as multiples of 10, but they were found to be 

 less satisfactory than the categories employed. 



Throughout the period covered by the charts, the same type of plankton net 

 was used. This type of net is constructed of No. 30xxx grit gauze (an extra heavy 

 type of silk bolting cloth) and is 1 meter wide at the mouth and about 5 meters long. 

 The anterior section of the net, approximately 1 meter wide, is cylindrical, and the 

 posterior section is conical. The last 40 centimeters of the cod-end bag is made 

 of No . 56xxx silk grit gauze . 



