ZOOPLANKTON OF CENTRAL PACIFIC 



383 



ton rich zone. It does not seem likely that these 

 rather quick responses of zooplanktdn to varia- 

 tions in tlie pliysical environment are the result 

 of immediate changes in l)iological productivity 

 reflected in growth of the population, but are 

 simply a shifting and perhaps dispersal or con- 

 centration of the population associated with 

 changes in the water mass. 



PHOSPHATE, ZOOPLANKTON, AND TUNA 



The primary objective of our zooplankton 

 studies has been to obtain an estimate of the basic 

 fish food present in different areas of the sea with 

 the hope that this information would increase our 

 understanding of variations in the abundance and 

 distribution of the tunas. Where other factors, 

 temperature for example, are not of a limiting 

 nature, fast-swimming oceanic fishes such as the 

 tunas will occur, we believe, in proportion to the 

 amount of substance available for their nutriment. 

 This does not mean that we expect to find a high 

 positive correlation at all times and places between 

 the volume of food and the abundance of tunas. 

 In fact, it is probable that an inverse relation may 

 exist locally after a period of intensive feeding. 

 In general, however, when broad areas of the sea 

 are being compared, we believe that high abun- 

 dance of fish is most likely to occur in areas of high 

 concentration of zooplankton and other forage 

 organisms. 



The distribution of yellowfin tuna, A^eothunnux 

 macropterufi (Temminck and Schlegel), summa- 

 rized in figure 19, is derived from 12 cruises in the 

 central equatorial Pacific during the years 1950- 

 53.'° The highest average catch (5.3 yellowfin per 

 100 hooks) was obtained in the convergent zone, 

 with the second highest catch in the region of the 

 divergence. Although the peaks in abundance do 

 not exactly coincide, it is obvious that there is 

 more than a casual relation between zooplankton 

 and yellowfin. The best catches of bigcye, Para- 

 thuiinu.'f ■^ibi (Temminck and Schlegel), were made 

 in the North Equatorial Current and Counter- 

 current (fig. 19). This species appears to respond 

 in a different maimer than the yellowfin to the 

 better foraging conditions in the convergent and 

 divergent zones. A comparative study of the food 

 of the two species failed to show differences in the 



The tuna ratch records employed In this report have resulted from explor- 

 atory longline fishing conducted by POFI vessels and are analyzed in other 

 POFI reports (Murphy and Shomura ig.Wa, ig.Wb, 1955; Shomura and 

 Murphy 1955: Ivcrsen and Yoshida. 1956. 



o —• ZOOPLANKTON VOLUME 



o o SURFACE INORGANIC PHOSPHATE 



„ „ YELLOWFIN CATCH 



° ' BIGEYE CATCH 



O 

 O 

 O 



O 

 O 



60 « 

 O 



5.0? 



2.0 



14° 12° 10° 8° 



go 4= 2° 0° 2° 4° 6° 8° 10° 12° 14° 16° 18° 

 S — LATITUDE — N 



FioORE 19. — Variations with the current system in yellow- 

 fin and bigeye catch on longline gear, zooplankton 

 volumes (adjusted) and surface inorganic phosphate, for 

 the range of longitude 120° \V. to 180°. The tuna catch 

 data are derived from cruises 7, 11, and 18 of the Hugh 

 M. Smith, cruises 11, 12, 13, U. 15, 16, and 18 of the 

 John R. Manning, cruise 1 of the Charles H. Gilbert, and 

 cruise 1 of the Cavalieri. The phosphate data are from 

 cruises 2, ,5, 8, 11, 14, 15, 16, IS, and 19 of the Hugh M. 

 Smith. 



diet which might explain this marked difference in 

 distribution (King and Ikehara, 1956). 



Measurements of inorganic phosphate performed 

 on POFI hydrographic cruises during the years 

 1950-53 show that the zone of divergence and the 

 South Equatorial Current immediately south of 

 the Equator contained the highest concentrations 

 of this basic chemical nutrient while the North 

 Equatorial Current contained the lowest (fig. 19). 

 This variation may result from unequal utilization 

 of phosphate and/or the unequal mixing of high 

 and low phosphate water to the north and south of 

 the Equator as the result of the asynunetrical 

 effects of the southeast winds. As evidenced Viy 

 the zooplankton and yellowfin catch, the greatest 

 organic productivity occurred on, or to the north 

 of the Equator. The difference in degree of north- 

 ward displacement for the two eutropliic levels, 

 zooplankton and tuna, may to some extent be 

 indications of the lag periods in their development 

 and may also be related to the slow northward 

 drift in the surface cm-rents under the inlluence of 

 east and southeast winds. 



When long series of stations extending in a 

 north-south direction are examined, we usually 

 find a highly significant positive correlation be- 



