ZOOPLANKTON ABUNDANCE IN THE CENTRAL PACIFIC 



By Joseph E. King, Fishery Research Biologist, and 

 Joan Demond, Fishery Aid 



The Pacific Oceanic Fishery Investigations is 

 authorized and directed to gather information 

 which will ensure the maximum development and 

 utilization of the high-seas fishery resources of 

 the United States territories and island posses- 

 sions in the tropical and subtropical Pacific. One 

 project of fundamental importance in the research 

 program concerns the relative productivity of 

 different areas of the sea. 



Productivity has been defined by Ivlev (1945) as 

 the "capacity of a body of water to produce a 

 given quantity of organic matter in some particu- 

 lar form.*' A direct measurement of the "rate 

 of production" (Clarke 1946) would require that 

 all processes by which organic matter is built 

 up and destroyed be known and that the rates 

 of these processes be determined. This is difficult 

 to do and seldom has been done even for enclosed 

 bodies of water. In mid-ocean the difficulties are 

 vastly greater. We believe, however, that rela- 

 tive productivity, or productivity as defined by 

 Ivlev, may be estimated indirectly by measuring 

 the amounts of basic chemical nutrients in the 

 water and the standing crops of plankton and 

 fish. This report considers the quantity of 

 zooplankton, 1 of the 2 main constituents of the 

 total plankton crop, and its relation to certain 

 physical and chemical factors in the central Pacific 

 environment. 



Zooplankton is essential food for much of the 

 vertebrate fauna of the sea. It is utilized both 

 directly and indirectly by tunas (the group of 

 fish presently under study by these investiga- 

 tions). Kishinouye (1924) and Imamura (1949) 

 have shown that zooplankton is prominent in the 

 food of juvenile tunas. A variety of zooplankton 

 organisms has also been observed in the food of 

 adult tunas (Kishinouye 1917; Beebe 1936; 

 Suyehiro 1942 ; Clemens and Wilby 1946 ; Reintjes 

 and King 1953). The bulk of the zooplankton, 

 however, reaches the tuna indirectly, being uti- 



lized by plankton-feeding animals which are in 

 turn eaten by the tunas. 



Potential food-fish resources are likely to exist 

 in proportion to the amount of substance available 

 for their nutriment. When vast areas of the sea 

 are to be investigated, the several physical, chem- 

 ical, and biotic properties of water associated with 

 the production of nutriment for fish can be more 

 readily and reliably surveyed than the abundance 

 of the fish themselves. This report is concerned 

 with the zooplankton from the particular view- 

 point of its usefulness as an indicator of the 

 relative productivity of the various portions of 

 the area covered. 



The literature includes a number of papers 

 dealing witli the plankton of the tropical and sub- 

 tropical Pacific. One of the most valuable of 

 these is the report by Graham (1941) on plankton 

 collections taken by the Carnegie in the eastern 

 and central Pacific. Kramer (1906) reported on 

 a series of collections extending from Samoa to 

 the Marshall Islands. Jesperson (1935) described 

 results obtained by the Dana while traversing a 

 series of stations reaching from Panama to the 

 western Pacific, south of the Equator. For the 

 western Pacific there are tin- publications of 

 Matsuya (1937), Motoda (1940), Haneda (1942), 

 and Tokioka (1942), which deal mainly with 

 the plankton of lagoon, bay, and coastal waters 

 but also provide some data on offshore plankton. 

 The several papers of Marshall (1933), Russell 

 (1934), and Russell and Colman (1931, 1934, 

 1935) supply a wealth of information on the 

 plankton of the Great Barrier Reef Lagoon, but 

 little on oceanic plankton. The papers of Johnson 

 ( 1!>49) on the plankton of Bikini, and Sargent 

 and Austin (1949) on the productivity of an atoll 

 in the northern Marshalls, also deal primarily 

 with the lagoon environment. The California 

 Cooperative Sardine Research Program — a coop- 

 erative undertaking of the Scripps Institution of 

 Oceanography, the United States Fish and Wild- 

 Ill 



