ZOOPLANKTON ABUNDANCE IN THE CENTRAL PACIFIC 



125 



tents of maturing and adult yellowfin tuna (Re- 

 intjes and King 1953) : 



emphasizing the importance of plankton in the 

 food of invenile tunas. 



food of juvenile tunas. 



mysids 

 euphausiids 

 amphipods 

 stomatopod larvae 

 shrimps 

 palinurid larvae 



brachyuran larvae 



heteropods 



pteropods 



cephalopod young 



tunicates 



fish young 



References have been previously cited (p. Ill) 



ABUNDANCE OF THE ZOOPLANKTON 



Estimated numbers and volumes of zooplank- 

 ton, times of sampling, and amounts of water 

 strained, are given in tables 1, 2, 3, and 4. Varia- 

 tions in abundance with latitude are demonstrated 

 by the histograms of figures 4, 5, 6, 7, and 8. The 

 results are briefly summarized in tables 10 and 11. 



' Sample at 19° N. latitude was greatest in volume but was atypical, as it contained a highly unusual number and volume of amphipods. (See table 3.) 



Tap.le 11. — Numbers and volumes of zooplankton organ- 

 isms as related to equatorial currents and latitudes 



Iii general, at the time and through the range of 

 latitudes sampled (-27° N. to 14° S.), the greatest 

 abundance of zooplankton was found in the re- 

 gion of the Equator, between 6° N. and 5° S. lati- 

 tude. The latitude of peak abundance varied with 

 longitude and cruise. Although not evaluated 

 statistically, there appears in all sections a strik- 

 ingly parallel variation between the estimated 

 number for each sample and the displacement vol- 

 ume. Because of the disparity in size among the 

 different kinds of zooplankton organisms, num- 

 bers are much less meaningful indicators of pro- 

 ductivity, or available food, than sample volumes ; 

 therefore, our statistical studies have been based 

 solely on volume determinations. 



When the varying abundance of zooplankton is 

 reviewed in respect t<> the presence or absence of 

 a convergence north of the Equator, certain inter- 

 esting relationships are indicated : 



1. When a well-marked convergence is present, 

 the rich zone of zooplankton appears to lie be- 

 tween the Equator and the convergence to the 

 northward (fig. 4. left panel; Iilt- 5, right panel). 

 The eastern section, cruise 2 (tig. 4. right panel) 

 on which only surface tows were employed, does 

 not conform to this generality. Much of the ir- 

 regularity in this section, however, is due to dif- 

 ferences between samples taken by day and by 

 night. 



2. The northern boundary of the rich zone is 

 practically demarcated by the position of the con- 

 vergence — when it occurs. 



3. When a well-marked convergence is lacking, 

 the peaks of abundance — both in number and vol- 

 ume — occur south of the Equator (fig. 5, left 

 panel; tig. 7, right panel). 



4. The cruise-8 hydrographic data are not as yet 

 completely processed and the presence or absence 

 of convergences has not been determined for all 

 sections; however, there is no evidence of a con- 



vergence along 158 c 



W. longitude (fig. 7 



, right 



panel). 



