ZOOPLANKTON ABUNDANCE IN THE CENTRAL PACIFIC 



137 



basic productivity, depth of thermocline, or other 

 factors of the environment. 



In summary, through the foregoing analyses, 

 we have demonstrated that the rich zooplankton 

 catches from near the Equator were significantly 

 greater than those of higher latitudes. We have 

 learned also that there was little difference be- 

 tween the populations along the two longitudes, 

 172° W. and 158° W., over the range of latitudes 

 sampled. We can state with assurance that there 

 was a distinct difference between the amount of 

 zooplankton taken on the two cruises, 5 and 8. 

 The mean for cruise 5 (0.0366 cc./m. 3 ) conducted 

 during the northern summer, was greater than that 

 for cruise 8 (0.0244 cc./m. 3 ) conducted during the 

 winter; therefore, a seasonal difference is sug- 

 gested. 



CORRELATIONS WITH ENVIRONMENTAL 

 FACTORS 



Some of the hydrographic data obtained on 

 cruises of the Hugh M. Smith have been pub- 

 lished (Cromwell 1951), and since the Pacific 

 Oceanic Fishery Investigations plans to publish 

 the hydrographic data on which these correlations 

 are based, we have not included the data in this 

 paper. Variations in certain features of the sur- 

 face layer as measured on cruise 5, and graphi- 

 cally portrayed in figure 9, may be summarized a 1 - 

 follows : 



1. Surface Inorganic phosphates — high concentra- 

 tions of 0.80 to O.itO mK at./L. were found in the immedi- 

 ate vicinity of the Equator ; values decreased to the 

 northward and southward to lows of 0.30 to 0.40 n?, 

 at./L. 



2. Surface temperature — highest water tempera- 

 tures, about 28° C, were recorded for latitudes 6° to 8° 

 N. ; temperature decreased gradually to the northward 

 and to the southward, reaching 24° to 25° O. 



3. Dissolved oxygen (percent saturation at sur- 

 face) — ranged from 94 percent near the Equator to 102 

 percent at higher latitudes. 



4. Thermocline depth — varied from about 120 feet 

 in the north to over 500 feet in the region of the con- 

 vergence. 



Employing cruise-5 data, 6 a method of correla- 

 tion analysis (following Snedecor 1946, p. 138) 

 was used to examine the relation between zoo- 

 plankton volumes and these environmental fac- 



• Since different methods of towing and nets of different mesh 

 size were used on cruise 2, and since the hydrographic data of 

 cruise 8 are not as yet entirely processed, we have used cruise-5 

 data for the correlation analyses. 



tors. The results, summarized in table 17, point 

 out the following : 



1. Statistically significant (P<0.01) positive cor- 

 relations between zooplankton volume and inorganic 

 phosphates at the surface, at the 100-meter depth, and 

 at depths midway to the top of the thermocline ; 



2. Statistically nonsignificant (P>0.05) correla- 

 tions between zooplankton volume and temperature, 

 whether at the surface, the 100-meter depth, or at 

 depths midway to the top of the thermocline; 



3. Statistically significant negative correlations be- 

 tween zooplankton volume and oxygen (as percent 

 saturation) at the surface (jP<0.0.">), at the 100-rneter 

 depth (P<0.01), and at depths midway to the top of 

 the thermocline (i»<0.01) ; 



4. Statistically significant (i > <0.01 ) correlations be- 

 tween zooplankton volume and depth to the top of the 

 thermocline. 



In most ecological investigations it has been 

 found that all factors examined are more or less 

 interrelated, variations in one factor having an 

 influence, either direct or indirect, on all the other 

 factors (Riley 1939a). The multiple-regression 

 method makes corrections for such interactions, 

 and in a three-variate analysis tests the relation 

 of a pair of variates irrespective of the influence of 

 a third. 



Tabt.e 17. — Correlations of zooplankton abundance, an iret 

 volumes, and certain environmental factors, cruise 5 



'• Indicates a highly significant correlation. 

 • Indicates a significant correlation. 



In the correlation analyses a positive relation 

 between zooplankton volumes and temperature at 

 the 100-meter depth was indicated (table 17), but, 

 being below the 0.05 level of probability, it was 

 not considered statistically significant. It seemed 

 of interest to examine the relation of zooplankton 



