372 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 3. — Differences in the average volumes of day, night, 

 and twilight hauls and in the night/day ratios for six 

 cruises of the Hugh M. Smith in the equatorial Pacific 



' Sections A and C only (King and Demond, 1953, table 1). 

 ' Northbound section only. 



On tlie majority of cruises sampling was con- 

 ducted around the clock so that there were about 

 equal numbers of night and day stations. Under 

 this S5'stem there rarely were more than two day 

 stations or two night stations occupied consecu- 

 tively. On certain cruises, however, such as 

 cruise 18 of the Hugh M. Smith, and cruise 15 of 

 the Charles H. Gilbert, hauls were made at about 

 the same hour throughout the cruise; e. g., on 

 cruise 18 all hauls were made near midnight, on 

 cruise 15 between 1900 and 2000. The resulting 

 data are most useful for within-cruise comparisons, 

 but some modification is necessary if they are to be 

 compared or combined with the results of the 

 other cruises. 



An adjustment to remove the effect of diurnal 

 change in zooplankton catch was described by 

 King and Hida (1954). The method is based on 

 the similaritj^ of diurnal variation in zooplankton 

 abundance to the curve of the sine function when 

 midnight is equated to the angle whose sine is 

 + 1.0. The zooplankton volumes are increased 

 or lowered dependent on the hour of hauling and 

 adjusted to 0600 or 1800 hours, when the sine = 0. 

 Since illumination is the major factor controlling 

 the diurnal migration of plankton (Kikuchi, 1930; 

 Gushing, 1951), solar time is used in the calcula- 

 tions. 



The method as originally designed was applied 

 to zooplankton volumes from the Hawaiian 

 Islands area, where the geographical variation was 

 slight and the night/day ratio rather uniform from 

 cruise to cruise. On the long sections crossing the 

 Equator we found considerable variation in the 

 night/day ratio associated with latitude and the 

 current system (p. 380) . and the geographical varia- 

 tion is much greater than in the Hawaiian area. 



Although these factors lessen the accuracy and 

 effectiveness of the method, it still provides a 

 reasonably good correction for day-night differ- 

 ences as judged by the significance of the "t" 

 values and the night/day ratios for the adjusted 

 volumes (table 4), and has therefore been applied 

 to the equatorial data. 



Table 4. — Regression coefficients (b), "t" values and 

 probability values for the sine transformation method of 

 adjustment for 5 cruises of the Hugh M. Smith in the 

 equatorial Pacific. 



I A comparison of the night/day ratios for the zooplankton volumes before and 

 after adjustment indicates the general validity of the method). 



Throughout this report we have employed the 

 adjusted volumes in examining the variation in 

 zooplankton abundance with respect to special 

 features of the current system, with longitude, 

 and with season. The data from cruises 5, 8, 

 11, 14, and 15 of the Hugh M. Smith were ad- 

 justed by individual cruise. A pooled regression 

 coefficient (b = 0.1 248) calculated from the com- 

 bined data of these 5 cruises that covered large 

 areas of the equatorial Pacific during which the 

 stations were visited consecutively regardless of 

 the time of day or night, was used in adjusting 

 the volumes of cruises 2, 7, 9, 16, 18, and 19 of 

 the Hugh M. Smith and of cruise 1 5 of the Charles 

 H. Gilbert. On the latter cruises, sampling was 

 not conducted around the clock, or there were too 

 few data to be adjusted by individual cruises. 

 Unadjusted volumes for the Hugh M. Smith 

 cruises 2, 5, 7, and 8 have previously been pub- 

 lished (King and Demond, 1953). The adjusted 

 volumes for these cruises are provided herewith 

 in table 14 (appendix B). 



DESCRIPTION OF THE ENVIRONMENT 



The general pattern of the Pacific equatorial 

 current system has been described by Sverdrup 

 and others (1942:708-712). In brief, the major 

 surface currents of this region are the North and 

 South Equatorial Currents flowing toward the 

 west, and the eastward flowing Equatorial Coun- 



