Standing Crops and Trophic Levels — B lackburn 
57 
about 140 m, for each of 22 stations near the 
Revillagigedo Islands and 2 near Alijos Rocks 
(Fig. 1). These data were gathered on the ex- 
pedition called "Tuna Spawning Survey” in July, 
1957. The net and hauling procedures were as 
described under "Material” in this paper. The 
two groups of stations were evenly divided be- 
tween day and night; so there were 12 of each. 
The published entry for station 7 gives 74.4 ml 
of zooplankton for the deeper haul and 52.3 ml 
for the shallower, but a check on the original 
collections showed that these entries had been 
reversed. 
The least-squares regression of logarithm of 
volume from the 300 m haul (Y) on logarithm 
of volume from the 140 m haul (X) was ob- 
tained for the 24 stations, after analysis of co- 
variance had revealed no significant difference 
between the day and night series. This regres- 
sion is 
Y = 0.0566 + 0.8009X 
with standard error of estimate 0.130, and has 
been used where necessary in this paper to esti- 
mate 300 m-haul volumes from 140 m-haul 
volumes of zooplankton and copepods (see Table 
1 and "Material”). The corresponding regression 
for estimating logarithm of volume of 140 m 
haul (Y) from logarithm of volume of 300 m 
haul (X) is 
Y = 0.2059 + 1.053X 
with standard error of estimate 0.128, and it was 
also used where necessary (see "Discussion”). 
These regressions can be used to obtain some 
information about the vertical distribution of 
zooplankton within the upper 300 m. For a 
range of selected values of concentration (ml/ 
10 3 m 3 ) in either the 0-140 m or the 0-300 m 
layer, concentrations for the other layer can be 
estimated. These values can be used to estimate 
actual volumes (milliliters) simultaneously pres- 
ent at 0-140 m and 0-300 m under 1 m 2 of sea 
surface, which may be called a and b. The ratio 
a/b is quite high even for very low concentra- 
tions ( e.g., 65% where the 0-300 m concentra- 
tion is 7 ml/10 3 m 3 ), and it becomes 100% 
where the 0-300 m concentration is 42 ml/ 
10 3 m 3 . These calculations are to some extent 
unrealistic, for obvious reasons; e.g., b-a, repre- 
senting actual volume at 140-300 m, becomes 
negative at concentrations higher than those 
above. They strongly suggest, however, that most 
of the zooplankton captured in 0-300 m hauls 
in the eastern tropical Pacific is located in the 
0-1 40 m layer, especially in areas of average and 
high concentrations, either by day or by night. 
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