382 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



interval between the first and fourth crossings of 

 the Equator was 16 days and from the start of sec- 

 tion 1 to the end of section 4 was 23 days. 

 Austin (1954b) summarizes the hydrographic 

 changes during this period as follows : 



1. The slope of the isotherms associated with the 

 Countercurrent is greater in the fourth than in the 

 first leg, suggesting an increased easterly flow of 

 the Countercurrent. This was substantiated in 

 the calculated velocities, 60 cm. /sec. on the first 

 leg and 120 cm. /sec. on the fourth leg. 



2. The 80° isotherm-surface intercepts for the 

 fourth section have moved to the north and south 

 of those for the first section. 



3. The 70° isotherm shows considerably more 

 doming at the Equator in the fourth section. 



4. Between the first and fourth sections there is a 

 generally southerly shift in selected isohalines. 

 There is a similar change in the slope of sigma-t 

 isopleths in the region of the Countercurrent. 

 Selected sigma-t surfaces show a general displace- 

 ment to the south when comparing the first and 

 fourth sections. 



5. The most apparent change in the phosphate 

 sections is the deepening of the 0.8 and 2.0 ng at/L 

 isopleths in the region of the Countercurrent which 

 is associated with the suggested change in flow, 

 and the change in configuration of the 0.8 ^gat/L 

 isopleth near to and south of the Equator. 



Observations of wind speed and velocity along 

 the four section lines, as diagramed by Austin 



400 

 



1 — I — I — \ I I \ rp I r 



SECTION 4 I A 



80- / \ 



0° r 2° 3° 

 LATITUDE 



FiGiRE 18. — Variation in (adjusted) zooplaiikton voluiiK's 

 and in the configuration of the 70° and 80° F. i.sothernis 

 on section.s I and 4 of Hugh M. Smith cruise 15, along 

 140° W.. longitude in May-June 1952. (Temperature 

 sections adapted from .\ustin 1954b.] 



(1954b), show this was a period of moderate and 

 variable winds. Since we did not have observa- 

 tions simultaneously to the north and south of the 

 section lines, the changes with time are complex 

 and difficult to summarize. In the region of the 

 Countercurrent there appears to have been a re- 

 duction in the northeast trade winds and an exten- 

 sion to the northward of the moderate southeast 

 trades. South of the Equator there was first a 

 slackening in tlie winds followed by an increase, 

 with the strongest winds of the cruise being 

 recorded on the southern ends of the third and 

 fourth sections. 



When the adjusted zooplankton volumes (table 

 9, appendix B) from the four series of stations 

 along 140° W. are subjected to an analysis of 

 variance with two-way classification, we find there 

 are no significant (P>0.05) differences among the 

 four sections but highly significant (P<0.01) 

 differences among stations (latitudes). The latter 

 significance results from the wide difference be- 

 tween the high volumes obtained in the Counter- 

 current and at the Equator and the low volumes 

 from about 3° S. to 7° S. latitude. 



When we examine differences in zooplankton 

 distribution between tlie first and fourth legs in 

 relation to changes in the temperature structure 

 at the Equator (fig. 18), we find that the increased 

 distance between the 80° isotherm-surface inter- 

 cepts (an indication of an increase in width of the 

 mixing zone) was accompanied by a broadening 

 of the zooplankton "rich zone." On the first 

 section there was a single peak of abinidance 

 directly on the Equator; on the fourth section 

 there were two peaks, at about 1° S. and 1° N. 

 latitude, with a trough at the Equator. In the 

 Countercurrent the zooplankton catch was high 

 in volume on all four sections. The suggested 

 change in rate of flow in the Countercurrent was 

 not reflected in any noticeable change in zooplank- 

 ton abundance or distribution. 



It is difficult to explain or to draw conclusions 

 from these events. In one instance (cruise 11) a 

 change in rate of flow of the Cotuitercurrent was 

 accompanied by a change in zooplankton distribu- 

 tion; in the second instance (cruise 15) changes ni 

 the CountorciuTent were not evidenced by any 

 noticeable change hi the volume of zooplankton. 

 On both cruises an increase in breadtii of tlie zone 

 of divergence or mixing at tlie Etiuator was fol- 

 lowed by a corresponding broadening in the plank- 



