the organisms smaller than 2 cm., either 

 the entire sample (if the volume was less 

 than 10 cc.) or a fraction of about 10 cc. 

 (from larger samples) was distributed 

 evenly in a counting cell. The cell was a 

 plastic dish 15 by 20 by 1.5 cm., the bot- 

 tom of which was ruled into 300 squares, 

 each with an area of 1 cm. 2. 



2. The organisms in 10 randomly selected 

 squares were then identified and counted 

 under a dissecting microscope. Identifi- 

 cation was only to major taxonomic groups, 

 as shown in table 11. 



3. The estimated number of a particular 

 group of organisms was determined by 

 multiplying the count by 30, if the entire 

 sample had been placed in the plastic dish. 

 If the sample had been fractioned, the 

 count was multiplied by 30 and multiplied 

 again by the reciprocal of the fraction. 



Adjustment for Diel Variation 



The method of King and Hida (1954) for ad- 

 justing volumes of zooplankton for variation 

 with time of collection is based on the similar- 

 ity between diel variation in the volumes and 

 the curve of the sine function. Midnight was set 

 at 90°, and sine-time values were obtained for 

 the various hours of the day. Then adjusted 

 volumes were computed by regression analysis. 

 After adjustment the night/day ratios were much 

 closer to unity; much of the variation caused by 

 differences in time of sampling clearly had been 

 removed (table 2). 



Volumes of samples from the middle and 

 lower nets of the comprehensive series were 

 not adjusted because the regression coefficients 



Table 2 . --Comparison of night/day ratios of 

 unadjusted and adjusted volumes of zoo- 

 plankton 



Samples 



Night/day ratios 



Sample 

 volume 



Adjusted 

 volume 



Comprehensive series 

 CHG-24 



Surface to top of 1.26 0.89 



thermocline 



Surface to 200 m. 1.64 1, 



HMS-32 (upper nets) 1.42 1. 



HMS-34 (upper nets) 2.52 0, 



HMS-35 (upper nets) 2.05 1.06 



HMS-36 (upper nets) 1.87 1.01 



Monitor series 2.18 0.96 



Inshore-offshore series 3.11 1.25 



07 

 03 

 84 



did not yield significant values when subjected 

 to a "t" test (Snedecor, 1956). All volumes of 

 samples from the upper nets yielded highly sig- 

 nificant values when subjected to the same test. 

 (See table 3 for average depths sampled by the 

 three nets.) All volumes of samples from the 

 monitor series were pooled in making the ad- 

 justment, because the numbers of samples from 

 the individual cruises were insufficient for sep- 

 arate treatment. All data for the inshore- 

 offshore series also were combined. Data for 

 the comprehensive series were adjusted sep- 

 arately for each cruise. All later references to 

 "adjusted" and "unadjusted" samples concern 

 this time adjustment. 



Table 3. --Average corrected sampling depths 

 of the nets used in three-net hauls 



Net and depth limit 



Average corrected 

 sampling depth 



Upper net 

 Upper limit 

 Lower limit 



Middle net 

 Upper limit 

 Lower limit 



Lower net 

 Upper limit 

 Lower limit 





 49 



56 



118 



126 

 233 



Determinations and Corrections for Depth 



The spacing of nets on the towing cable for 

 three-net hauls was based on the assumption 

 that the cable was straight during the haul. This 

 assumption was proved wrong by a depth gage 

 (Miller, Moore, and Kvammen, 1953) used on 

 HMS-31^' and a subsequent test cruise. Cor- 

 rection factors were calculated, therefore, from 

 data obtained from the pressure gage. 



Approximate sampling depths were first com- 

 puted by multiplying the cosine of the cable 

 angle by the amount of cable that had been let 

 out. The values obtained were multiplied by the 

 appropriate correction factors to yield the cor- 

 rected depths. 



— 



— Cruises are identified by initials of the 

 ship and the cruise numbers. Examples are: CHG- 

 24 for Charles H. Gilbert cruise 24, JRM-30 for 

 John R. Manning cruise 30, HMS-32 for Hugh M. 

 Smith cruise 32 . 



