ZOOPLANKTON OF CENTRAL PACIFIC 



385 



phate again showed an inverse correlation, par- 

 ticularly with the yellowfin catch. Figure 14B 

 demonstrates tlie difference in yellowfin catch for 

 the two 6-month periods: (1) January-June, a 

 period of generally light, variable or northeast 

 winds, and (2) July-December, a period of strong 

 southeast trades. With the change in winds during 

 the latter half of the year there was apparently a 

 shift to the northward in the area of best catch. 

 The zooplankton exhibited a general increase 

 during this period, especially in the convergent 

 zone. 



SUMMARY AND CONCLUSIONS 



1. This is the second report of the Pacific 

 Oceanic Fishery Investigations on variations in 

 zooplankton abundance in the central Pacific; it 

 presents the results of 270 quantitative hauls 

 made on eight cruises during the years 1951 to 

 1954. Data from earlier cruises, included in a 

 previous report (King and Demond, 1953), were 

 also utilized in this study. 



2. The majority of the collections were ob- 

 tained with 1-meter nets of 30XXX grit gauze 

 (aperture widths 0.65 mm.). For comparison, a 

 few haids were made with 56XXX nets (aperture 

 widths 0.31 mm.). Oblique hauls to 200 meters' 

 depth were employed at most stations. The 

 results from a short series of horizontal hauls are 

 included. 



3. The displacement volumes of all samples 

 were measured in the laboratory. For each 

 sample there was calculated the volume of the 

 more nutritious zooplankton per unit of water 

 strained. Counts were made on six samples to 

 examine the composition of the catch from nets 

 of different mesh size. 



4. The catch of 56XXX grit gauze nets (aper- 

 ture widths 0.31 mm.) was about IK to 1^4 times 

 greater in volume than that of the catch of the 

 30XXX nets (aperture widths 0.65 mm.). The 

 number of plankters retained by the finer-meshed 

 net was 3 to 5 times that retained by the coarser- 

 meshed net. At three stations, two rich and one 

 poor, the catches for the two nets were generally 

 proportional. 



5. Horizontal hauls made simultaneously at 

 three levels showed that the greatest bulk of 

 zooplankton was near the surface even in the 

 daytime, rather than at depths just above or 

 l)elow the thermocline. 



6. The night hauls yielded volumes averaging 

 1 .57 times the volumes of day hauls; twilight hauls 

 were intermediate in volume. To reduce these 

 differences associated with hour of hauling, a 

 method of adjustment was employed based upon 

 the similarity between the diurnal variation in 

 zooplankton abundance in the upper 200 meters 

 and the curve of the sine function, with midnight 

 equated to the angle whose sine is +1.0. 



7. When the adjusted zooplankton volumes were 

 combined according to natural subdivisions of the 

 equatorial current system, disregarding differences 

 associated with longitude and season, we found 

 the greatest concentration of zooplankton occur- 

 ring at the Equator in the region of upwelling and 

 divergence. Average volumes for the convergent 

 zone and the Countercurrent were greater than for 

 the South Equatorial Current south of the Equa- 

 tor. This asATnmetrical distribution of zooplank- 

 ton in respect to the Equator may result from 

 the prevalence of southeast trade winds in this 

 part of the Pacific. 



8. As determined from exploratory longline fish- 

 ing conducted by POFI, in the central equatorial 

 Pacific the greatest abundance of yellowfin tuna 

 occurred in the convergent zone just to the north 

 of the area of highest zooplankton abundance, and 

 although the peaks did not exactly coincide, there 

 was a high degree of co-variation in yellowfin and 

 zooplankton in respect to the current system. 



9. Oceangraphic fronts occurring in the transi- 

 tion zone between the Equator and the southern 

 boundary of the Countercurrent appeared to de- 

 marcate areas of high zooplankton abundance on 

 the south from areas of poor to moderate abund- 

 ance on the north. 



10. The Countercurrent in the east-central Pa- 

 cific produced unusually high zooplankton vol- 

 umes. As this is an area of shallow thermocline, 

 with high-phosphate water within the photosyn- 

 thetic zone and within reach of wind-induced tur- 

 bulence, conditions are more favorable for plank- 

 ton production than fartlier to the westward where 

 the thermocline deepens. 



11. Within the equatorial region there was a 

 west-east gradient of increasing zooplankton abun- 

 dance from 180° to 150° W. longitude which was 

 correlated positively with average wind velocity 

 and inversely with thermocline depth, and was 

 closely paralleled by a gradient of increasing yel- 

 lowfin catch. East of 140° W. the vellowfin catch 



