ZOOPLANKTON OF CKNTRAL PACIFIC 



371 



purpose of the sampling was to investigate the 

 abundance and vertical distribution of tuna eggs 

 and larvae in this area, some information was ob- 

 tained on the variation in zooplankton volumes 

 witli deptli and with time over a 14-hour i)eriod. 

 According to the results shown in table 8B 

 (appendix B) and figure 5, at each of the 7 stations 

 (SI to S7) the largest volume of zooplankton was 

 taken in the surface net. At station S5, hyperid 

 amphipods were apparently swarming at the sur- 

 face and resulted in an unusually large catch. 



O 40 

 O 



o 



1400 1600 1800 2000 2200 0000 0200 

 HOUR (ZT) 



FiouRE 5. — Variation in zooplankton volumes with hour 

 of hauling, as obtained with horizontal hauls at the 

 3 depths: surface, intermediate (10.5-120 meters), and 

 deep (210-240 meters): Hugh M. Smith crviise 14 

 February 9-10, 1952. 



The other samples were of mixed composition, 

 typical of tiiis area. At four of the 7 stations 

 the intermediate net, fishing just above the 

 thermocline at a depth of 105 to 120 meters, 

 caught more than the deep net fishing below the 

 thermocline at 210 to 240 meters. These results 

 arc generally similar to those obtained with 

 Clarke-Bumpus samplers (employing 56XXX 

 nets) on a series of 150 stations extending from 

 12° N. to 7° S. latitude along 150° N. longitude 

 (Hida and King, 1955). The surface samples of 

 the latter series averaged 60.7 cc./l,000m.^ the 

 intermediate samples (from within the thermo- 

 cline) averaged 29.2, and the deep samples (at 

 200-300 meters) averaged 16.6. 



Although this sampling period of about 14 hours 

 is not adequate to demonstrate the diurnal cycle, 

 there is some evidence of an "evening rise" 

 between 1400 and 2000 hours, followed by a drop 

 in catch at the intermediate and deep levels and 

 then what is possibly the start of a "morning 

 rise" at these levels. The parallel variation 

 (r= 0.837, P<0.05) in volume of catch at the 

 intermediate and deep levels is of interest and 

 suggests that the zooplankton at these depths 

 was behaving differently from that at the surface 

 in response to varying illumination. 



ADJUSTMENT FOR DIURNAL VARIATION 



The hour of hauling provides an important 

 source of knowledge of the variation in quantita- 

 tive measurements of zooplankton abundance. 

 Presumably, the difference between day and night 

 hauls is due either to an augmentation in the 

 upper strata of water by upward migration of the 

 plankton at night or to a reduction in catch in the 

 daytime owing to the greater ability of the plank- 

 ton to dodge the net when there is light, or to a 

 combination of the two. In some areas of the 

 tropical Pacific the day-night difference is suffi- 

 ciently great, if no correction is applied, to obscure 

 the geographical and seasonal differences which are 

 of primary interest in this study. 



Significant differences in zooplankton volume, 

 associated with latitude, were observed among 

 the night samples and not among the day samples 

 on cruises 5 and 8 of the Hugh M. Smith in the 

 central equatorial Pacific (King and Demond, 

 1953). In Hawaiian waters the volumes of night 

 hauls have averaged about l}i times the volumes 

 of da^' hauls (King and Hida, 1954). In the 

 present instance during the 6 cruises in the equa- 

 torial region on which sampling was conducted 

 around the clock, niglit hauls yielded volumes 

 about Uptimes the volumeof thedayhauls(table3), 

 while the twilight hauls were intermediate in 

 average volume.^ Some of the variation among 

 cruises, as shown in table 3, may be due to differ- 

 ences in season, longitude, and range of latitude 

 sampled. Variations in the night/day ratio asso- 

 ciated with the current system will be discussed 

 later. 



' Knr purposes of this comparison we deslRiiated the twiU^ht hours as 0430 

 to 073(1 and Ifi30 to 1930. which iHTioti.'i include suiiri.se and sunset and the 

 heginningnnd end of twilight asspecided by the .\merican Nautical .\linanaci 

 the day period was thereby limited to 0730-1630 and the night to 1930-04,30 

 hours. 



