380 



FISHERY BULLETIN OP THE FISH AND WILDLIFE SERVICE 



O I50°W LONGITUDE 

 □ I60°W LONGITUDE 

 4 I70°W LONGITUDE 



S 80 - 



O 



o 

 o 



60 



20 - 



1950 



195! 



1952 



1953 



1954 



1 Only 1 sample 

 FiGUEE 15. — Annual variation in zooplankton abundance 

 for the most frequently sampled longitudes of (A) the 

 Countercurrent with boundaries at about 5° N. and 

 10° N. latitude, and (B) the South Equatorial Current 

 from about 5° N. to 5° S. latitude. 



differences among loitgitudes are not significant 

 (F = 0.178, P>0.05). It is obvious that the differ- 

 ences among years are derived principally from 

 variations within the South Equatorial Current. 

 The general agreement among longitudes is in 

 line with results from the previous tests. 



Along the Equator the volumes for longitudes 

 160° W. and 170° W. averaged considerably 

 higher in 1950 than in subsequent years. On 

 longitude 150° W., August-September 1951 provid- 

 ed much higher volumes than January and August 

 1952. In both the Countercurrent and the South 

 Equatorial Current there is some indication of a 

 rise in 1954. 



Possibly related changes are evidenced in other 

 environmental factors. From a study of the 

 rather sparse rainfall records available for the 

 central equatorial Pacific,' Austin concludes that 



• In unpublished manuscript entitled, Review of Central Equatorial 

 Pacific Oceanography, 1950-52. 



in the year 1950 the precipitation at Fanning 

 Island (located at about 4° N. latitude, 159° W. 

 longitude) was unusually low and Infers that 

 southeast winds predominated throughout the 

 year." On the other hand, judging by the climato- 

 logical summaries, the years 1951, 1952, and 1953 

 may be considered as normal years in respect to 

 rainfall and also, by inference, in respect to winds, 

 i. e., with northeast and variable winds during the 

 first 6 months and east to southeast winds during 

 the latter half of the year. Therefore the year 

 of highest apparent productivity in the zone of 

 interest coincided with the year in which the 

 southeast trades appear to have been unusually 

 vigorous, thus perhaps causing the upwelling 

 mechanism to operate more energetically. 



DIURNAL VARIATION AND THE CURRENT 

 SYSTEM 



Many physical and biotic conditions influence 

 the vertical movement of planktonic animals 

 (Kikuchi, 1930; Cushing, 1951). The diurnal 

 variation which we have observed in the zooplank- 

 ton catch from 200-meter oblique hauls probably 

 results from a combination of factors which in- 

 clude: (1) vertical migration of the organisms 

 in response to changes in illumination, and (2) 

 their increased ability to dodge the net during 

 daylight hours. In Hawaiian waters and in the 

 central equatorial Pacific, night hauls yield catches 

 about IK times the volume of day hauls (table 1). 

 When the average volumes of night, day, and 

 twilight hauls are segregated with respect to 

 subdivisions of the current system, as in figure 

 16, we find a marked variation in the night/day 

 ratio from north to south. In the North Equa- 

 torial Current, Countercurrent, and convergent 

 zone the ratios range from 1.31 to 1.43, while 

 in the divergent zone and the South Equatorial 

 Current to the southward the ratios are much 

 higher, ranging from 1.76 to 1.94. This trend 

 appears consistently in the individual cruises. 



The North Equatorial Current, an area of 

 relatively shallow thermocline within the latitudes 

 considered (fig. 7), has a very low night/day ratio; 

 the convergent zone, with a deep thermocline. 

 also has a low ratio, while the South Equatorial 

 Current south of 5° S. latitude, which is an area 



« A study of the records had shown that a period of "doldrums" or nortl 

 east winds bring heavy rains to the northern Line Islands. 



