ZOOPLANKTON ABUNDANCE IN THE CENTRAL PACIFIC, Part II 



By Joseph E. King, Fishery Research Biologist, and Thomas S. Hida, Fishery Aid 



As a result of tlie last cruise of the Carnegie in 

 1929 and the recent surveys of the Swedish Deep- 

 Sea Expedition, Scripps Institution of Ocea- 

 nography, and the Pacific Oceanic Fishery Investi- 

 gations (POFI) of the United States Fish and 

 Wildlife Service, there has developed a general 

 understanding of the vertical and horizontal 

 currents in the equatorial Pacific and their relation 

 to marine life (Graham, 1941; CVomwell, 1953; 

 Jerlov, 1953; Sette and others, 1954, Sette, 1955.' 

 In brief, the moderate to strong east and southeast 

 winds which prevail throughout most of the year 

 in the eastern and central equatorial Pacific, 

 together with the Coriolis force of the earth's 

 rotation, produce a divergence of the surface 

 waters at the Equator. Upwelling associated 

 with this equatorial divergence replenishes the 

 supply of nutrients in the surface water and pro- 

 vides a suitable environment for the growtli of 

 phytoplankton and consequently for zooplank- 

 ton. Convergence and sinking of the surface 

 waters, occurring between the Equator and 

 the southern boundary of the Countercurrent, 

 may phj'sically tend to concentrate the zooplank- 

 ton into a rich pasturage for small fish, squid, and 

 other forage organisms. These in turn serve as 

 food for the larger fishes such as the tunas, the 

 group of fish presently under study in these in- 

 vestigations. Many aspects of this complex 

 succession of events, such as the actual rates of 

 production at the different eutrophic levels and 

 the causes of variation in the system, are still to 

 be determined. 



This is the second POFI report concerned with 

 zooplankton abundance in the central equatorial 

 Pacific. The first report (King and Demond, 

 1953) was based on four cruises in 1950 and 1951; 

 the present paper contains an analysis of plankton 



' Ako paper by O. E. Sette entitled. Nourishment of Central Pacific 

 Stocks of Tuna by the Erjuatorial Current System, to be published in the 

 ProceedinRS of the Eighth Pacific Science Congress (Manila); and unpub- 

 lished manuscript of T. S. Austin entitled. Review of Central Equatorial 

 Pacific Oceanography, igSO-.W. 



Note —Approved for publication October 11, 195fi. Fishery llulletin 118. 



data resulting from eight cruises in the period 1951 

 to 1954 and utilizes some of the observations from 

 the earlier publication. With these extensive ob- 

 servations, we are now able to show more clearly 

 how the abundance of zooplankton is a function 

 of such environmental factors as the equatorial 

 divergence, convergence, depth of the isothermal 

 layer, and other features of the surface waters of 

 the equatorial current system. Variations in the 

 zooplankton are also shown to be related to hour 

 of hauling, season, area (longitude), and direction 

 and velocity of the trade winds. 



The chief purpose of these studies has been to 

 obtain a quantitative measure of the standing 

 crop of zooplankton, or basic fish food, which may 

 be used as an index to the relative productivity of 

 different areas of the sea. It is hoped that this 

 information together with other oceanographic 

 observations made simultaneously by these in- 

 vestigations will help explain variations in the 

 distribution and abundance of tunas as determined 

 by experimental and commercial fishing. 



AREA AND METHODS 



This study is based primarily on 270 collections 

 obtained on 7 cruises (cruises 9, 11, 14, 15, 16, 18, 

 and 19) of the motor vessel Hugh M. Smith and 

 one cruise (cruise 15) of the motor vessel Charles 

 H. Gilbert during the years 1951 to 1954. The 

 approximate locations of the plankton stations are 

 shown in figures 1 to 4. More exact positions 

 together with date and hour of hauling, amount of 

 water strained, and the zooplankton volumes for 

 each cruise are given in appendix B, tables 6 to 13. 

 Data collected on 4 earUer cruises in 1950 and 

 1951 {Hugh M. Smith cruises 2, 5, 7, and 8), and 

 published in an earlier report (King and Demond, 

 1953), are also utilized in this study. Appendix 

 A presents the results of a special study on varia- 

 tions in zooplankton abundance about nn oceanic 

 island. Hydrographic data collected on certain 

 of these cruises have appeared in other POFI 

 reports (Cromwell, 1951, 1954; Austin, 1954a, 



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