378 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



inverse correlation (r =—0.688, P <C0.01) from 

 east to west between zooplankton volume and 

 depth of the 70° isotherm which lies within the 

 thermocline. We believe this relation results be- 

 cause of differences in depth of high-phosphate 

 water and amount of wind-induced enrichment, 

 although there is some evidence (Moore and 

 others, 1953) that a shallow thermocline may act 

 as a thermal floor in controlling the vertical dis- 

 tribution of zooplankton. Probably the correct 

 explanation cannot be obtained from our 200- 

 meter hauls but would require a detailed studj' 

 employing horizontal hauls with closing nets. 



In general, zooplankton volumes from the 

 eastern and central longitudes were higher than 

 in the west. When the volumes are combined by 

 10-degree intervals of longitude for the Counter- 

 current with boundaries at about 5° N. and 10° N. 

 latitude, and for the South Equatorial Current 

 from about 5° N. to 5° S. latitude, we obtain the 

 results shown in figure 12. In the Countercurrent 

 there was a sharp peak in abundance at 140° W. 

 longitude and a marked reduction both to the 

 east and west. In the South Equatorial Current, 

 bracketing the Equator, the highest average vol- 

 ume occurred at 150° W., but there was actually 

 little variation with longitude between 120° W. 

 and 170° W. The few collections taken along 

 110° W. were omitted from this comparison. We 

 should point out that the means shown for 120° 



100 



100 200 300 400 



DEPTH TO 70°, IN FEET 



FiGiRE 11. — Variation of zooplankton volume (adjusted) 

 with depth to the 70° F. isotherm for the latitudes 8° 

 N.-ll° N. on longitudes 140° W. to 170° W. (Number 

 of stations providing zooplankton and temperature 

 observations are shown in parentheses.] 



s 

 o 

 o 

 o 



40 



o 

 o 



(6) (8) 



(23) (48) 



(50) (59) 



(14) (45) 



(18) (36) 



o COUNTER CURRENT 



SOUTH EQUATORIAL CURRENT 



(3) (7) 



(3) (9) 



t 



lag 



160° 150° 140° 



WEST LONGITUDE 



120° 



Figure 12. — Longitudmal variation in zooplankton vol- 

 umes (adjusted) for the Countercurrent, extending from 

 about 5° to 10° N. latitude, and for the South Equa- 

 torial Current from about 5° N. to 5° S. latitude. The 

 limits of the 0.95 fiducial interval are shown for each 

 mean. [The number of samples for each area is indi- 

 cated in parentheses.! 



W., 130° W., and 180° are based on few samples 

 with poor seasonal coverage. 



From an analysis of variance we conclude that 

 differences between the two subdivisions of the 

 current system are highly significant (F = 5.57, 

 P<]0.01), but that differences associated with 

 longitudes are not significant (F = 0.76, P>0.05). 

 Despite the statistical evidence that the differences 

 among longitudes are not significant (with the ex- 

 ception of that between 140° W. and 150° W. in 

 the Countercurrent, as indicated by lack of over- 

 lap of the 0.95 fiducial intervals of the means), 

 the general picture of decreasing zooplankton 

 abundance from 140° W. to 180° parallels certain 

 changes in the environment. Along the Equator, 

 with decrease in wind velocity from east to west, 

 we may expect a corresponding decrease in up- 

 welling and enrichment of the surface waters; in 

 the region of the Countercurrent, the possibility 

 of enrichment through wind-induced turbulence 

 decreases from east to west with the deepening in 

 thermocline. 



DIFFERENCES AMONG SEASONS AND 

 YEARS 



It was pointed out by King and Demond (1953) 

 that the zooplankton volumes taken in January 

 and February in the equatorial Pacific averaged 

 significantly less than those obtained in June and 



