With only two exceptions (stations 1 and 24, at latitudes 12 N. and 3 S. ) the largest 

 volumes were found among the surface samples. There was no evidence of a concentration of zoo- 

 plankton in the region of the thermocline. The apparent greater abundance of deep-swimming tuna 

 at this depth cannot, therefore, be explained on this basis. Information on the abundance of the 

 intermediary forage organisms, which have not as yet been sampled quantitatively, and their graz- 

 ing effect on the zooplankton are needed if we are to fully understand the complex interrelationship 

 that exists. 



The plankton counts dennonstrated the same general variation with latitude and depth as 

 was found for the volumes (fig. 7B). Figure 8 illustrates the variation in numbers of organisms 

 with latitude and depth for eight of the major zooplankton groups. Although there is considerable 

 station-to-station variation — partly due to differences in the hour of sampling--the figure shows 

 that, for most groups, the largest numbers were found at the surface and in the general region of 

 the Equator. This is particularly well demonstrated by the Copepoda, Foraminifera, eggs (mostly 

 invertebrate), and Tunicata. The other groups shown on figure 8 were present in relatively small 

 numbers and do not provide as definite conclusions. 



Another variation related to depth and hour of hauling is that of size of organism. A 

 rough estimate of average size (volume) for the constituents of each sample was obtained by di- 

 viding the displacement volume of the sample by the estimated nunnber of organisms. The results, 

 summarized in figure 9, show an increase in average size with depth. Disregarding time of haul- 

 ing, the means for the three sampling depths--surface, internnediate, and deep--were 2. 0, 2. 7, 

 and 3.0 X 10"'*cc. , respectively. These were found to be significantly different (P < 0. 05) when 

 examined by means of an analysis of variance. 



When the data are segregated into day, night, and twilight hauls, we find some sugges- 

 tions that the mean size of the zooplankton was greater at night than in the day at the surface and 

 intermediate levels, but the opposite was true at the deep level (fig, 9). It is possible that the re- 

 latively large organisms captured in the day hauls at the deep level swam upward at night to be 

 taken in the night hauls at the intermediate level, but most of this group, apparently, never reached 

 the surface. The presence of larger organisms in the night hauls than in day hauls at the upper two 

 levels is possibly the result of upward movement of larger organisms from deeper layers in the 

 case of the intermediate depth, but dodging of the net in the daytime could also be involved, es- 

 pecially at the surface level. 



COMPOSITION OF THE ZOOPLANKTON 



The great variety of organisms making up the collections is characteristic of tropical 

 zooplankton populations as contrasted with the larger volumes but relatively few species which 

 are typical of temperate and boreal waters. The Copepoda were the most abundant group in actual 

 numbers in all samples (table 3). The similar innportance of copepods in 1-meter net collections 

 from the central Pacific was previously noted by King and Demond (1953). Next in order of numer- 

 ical abundance were the Foraminifera (mostly Globigerina and Globorotalia), eggs (mostly inverte- 

 brate), Tunicata (mostly Appendicularia), Gastropoda (mostly Pteropoda and Heteropoda), 

 Chaetognatha, Radiolaria, crustacean larvae, Ostracoda, Euphausiacea, Siphonophora, and 

 Amphipoda. Average numbers per cubic meter and percentage composition, by major constituents, 

 of the zooplankton are summarized by depth for the entire cruise in table 8. While most groups 

 decreased in absolute number with depth of sannpling, the Ostracoda showed a small but consistent 

 increase with depth. The Radiolaria also averaged greater in number in the deep samples than at 

 the surface and intermediate levels. With respect to percentage composition, the Copepoda and 

 Tunicata became relatively less important with increased depth of sampling, while Ostracoda, 

 Euphausiacea, and Annphipoda consistently gained in relative importance with depth. The per- 

 centages for the remainder of the groups varied in irregular manner or were approxinnately the 

 same at all three depths. 



14 



