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PACIFIC SCIENCE, Voi XII, April, 1958 
dional sections is given. King and Demond 
(1953) described the possible errors in regard 
to the total zooplankton, and a large part of 
the following review is based on their report. 
One source of error is from the fact that 
estimates of each plankton sample were made 
by doubling the figures obtained by direct 
measurement and counting of the half sam- 
ples. In addition to the halving of each plank- 
ton sample, another source of error comes 
from the removal of organisms larger than 5 
cm. from each of the samples. Undoubtedly 
this removal of large salps has considerably 
reduced the volume of capture at some 
stations. 
Day-night variation was shown by King 
and Demond to have had considerable in- 
fluence on the total zooplankton of cruises 
5 and 8, but there was no important effect of 
this variate on salps, at least in the 0-200 
meter layer. This is in agreement with the 
report of Apstein (1906), who considered that, 
in general, salps perform no diurnal vertical 
migrations. One species, Thalia democratka , 
however, was considered by Michael (1918) 
to migrate vertically as a result of temperature 
variations and reproductive behavior. Hardy 
(1936) and Fraser (1949) have given evidence 
that indicates a nightly movement toward the 
surface, but their tows were shallower than 
200 meters. 
Perhaps the most important source of error 
is the "patchy” distribution, both horizon- 
tally and vertically, of plankters in general. 
Nothing is known of the extent to which the 
variations in capture of the plankters is re- 
liable: i.e., there is doubt as to whether there 
is a true ecologic difference between the cap- 
ture at one station of 1 cc. of salps/ 1000m 3 
and the capture at another station of 1.5 or 
2 cc/1000m 3 . Perhaps the difference is due 
simply to variation that could be expected 
from the method of capture alone or from 
the patchiness of the salps. A better way to 
minimize this error would be to take a series 
of plankton tows at each station so that an 
average could be obtained. This was imprac- 
tical for the POFI cruises. In order to mini- 
mize patchiness, on cruises 5 and 8 only 
oblique tows were made, as the oblique tow 
has been regarded by Winsor and Clarke 
(1940) as well as by King and Demond (1953) 
as more efficient than either horizontal or 
vertical tows. By this oblique towing method, 
a large volume of water was strained: King 
and Demond assumed, therefore, that "varia- 
tion in catch due to the uneven distribution 
of organisms is minimized” (p. 119). How- 
ever, in view of the small volumes and num- 
bers of salps observed occasionally, the actual 
population density may have been quite 
different. 
The POFI cruises traversed three principal 
currents in the central Pacific: the westerly 
flowing North Equatorial Current which arises 
in part from waters of the California Current; 
the easterly Counter Current, with its source 
waters primarily from the neighborhood of 
the southern Philippine Islands and New 
Guinea; and the westerly South Equatorial 
Current fed from the Peru Current and the 
Gulf of Panama. Figures 1 and 2 show this 
region and the stations where the plankton 
samples were collected. For purposes of this 
report, the data from cruise 5, 172° W. are 
used as an example of the currents disclosed 
by a meridional section (cf. Fig. 4). The 
North Equatorial Current flows mainly west- 
erly from the Hawaiian Islands (27° N., sta- 
tion 1) to the northern limit of the Counter 
Current at about 8° N., station 14; the easterly 
flowing Equatorial Counter Current (from 
about 8° N., station 14, to 4° N., station 18); 
and the westerly flowing South Equatorial 
Current (from about 4° N., station 18) to the 
southern limit of the section. 
Since the surface phosphate concentration 
is associated with the stratification of the 
water, which is in turn primarily dependent 
on the temperature, the vertical sections for 
these two parameters are- closely related. Be- 
cause the vertical temperature sections are so 
revealing, they are used to describe the gen- 
eral ecologic features to be considered with 
