124 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 9. — Percentage composition, by number, of the six major constituents of the zooplankton collections, by cruise and 



longitude 



Major constituents 



Cruise 2 

 Jan.-Feb. 1950 



172° W.i 



158° W.' 



Total 



Cruise 5 

 June-Aug. 1950 



172° W.i 



158° W. 



Total 



Cruise 7 



Oct.-N T ov. 



1950 



158° W. 



Cruise 8 

 Jan.-Mar. 1951 



172° W.< 



158° W.s 



Total' 



Copepoda 



Chaetognatha. 



Tunicata 



Euphausiacea. 

 Siphonoptaora. 

 Foraminifera_. 

 Miscellaneous.. 



58.9 

 16.7 

 5.6 

 3.9 

 4.2 

 2.5 

 8.2 



56.2 

 14.0 

 8.2 

 9.1 

 2.9 

 1.3 

 8.3 



57.4 

 15.2 

 7.1 

 6.8 

 3.5 

 1.8 

 8.2 



56.3 

 8.3 

 5.6 

 4.9 

 5.0 

 7.7 



12.2 



53.0 

 10.0 

 5.6 

 3.5 

 3.4 

 9.9 

 14.6 



54.9 

 9.0 

 5.6 

 4.3 

 4.3 

 8.6 



13.3 



60.2 

 12.0 

 4.7 

 3.0 

 4.3 

 3.0 

 12.8 



58.6 

 12.4 

 6.1 

 4.6 

 5.2 

 1.6 

 11.5 



58.0 

 12.2 

 5.3 

 5.9 

 4.0 

 4.8 

 9.8 



58.4 

 13.5 

 5.3 

 5.6 

 4.3 

 2.5 

 10.4 



1 Includes stations 1 to 26. 

 ! Includes stations 30 to 53. 

 3 Includes stations 1 to 27. 



* Includes stations 77 to 106. 

 s Includes stations 1 to 26. 



• Includes stations 1 to 106, all longitudes. 



The uniform composition of the zooplankton, 

 as shown by our measurements, indicates the ex- 

 istence of a stable ecological balance among the 

 various components of the plankton and between 

 them and their environment. This uniformity, 

 or stability, is doubtless the result of a complex 

 interaction of factors. The physical and chemi- 

 cal factors of the environment throughout the 

 range sampled are rather homogeneous and, per- 

 haps while capable of effecting differences in total 

 abundance of organisms, do not provide sufficient 

 variations to promote major differences in the 

 composition of populations. The relative absence 

 of irregularly occurring swarms of larval and 

 adult forms may account for some of the uni- 

 formity. It is reasonable to assume that, despite 

 the great species differences that may occur (Wil- 

 son 1942) , there is a particular niche in this eco- 

 system of the tropical and subtropical Pacific 

 which wil) support a certain number of copepods, 

 and another which will accommodate a certain 

 number of chaetognaths, and so forth. The uni- 

 formity of the data on plankton composition also 

 argues that the collecting method and its uni- 

 formity of application were appropriate for pro- 

 ducing repeatable results. 



ZOOPLANKTON AS FOOD 



It is generally recognized that neither an enu- 

 meration of organisms present nor a total-volume 

 measurement shows the actual food value of a 

 plankton sample. Food value could be estimated 

 by chemical analysis of each sample, but this pro- 

 cedure is hardly practical when large numbers of 

 samples must be examined. Sufficient work has 

 been done on the chemical composition of the ma- 

 jor zooplankton types to show that they vary 



widely in nutritive value among types and even 

 within types for different localities. 



According to Bigelow and Sears (1939), the 

 separation of the crustaceans and chaetognaths 

 from the other types of zooplankton permits an 

 approximate division of the zooplankton into (1) 

 more nutritive forms which may be important as 

 fish food and (2) forms of little or no nutritive 

 value, such as the tunicates and siphonophores. 

 In our collections, the crustacean-chaetognath 

 group averages 70 to 80 percent by number. 



Nakai (1942) has shown that plankton animals 

 from the southern part of the Sea of Japan gen- 

 erally contain less fat than those of northern areas. 

 The inference is made that in the warmer waters 

 to the south, the scarcity of phytoplankton, par- 

 ticularly of diatoms with their rich oil reserves, 

 prevents the accumulation of fat by the zooplank- 

 ton. Clarke (1940) found that in the western 

 Atlantic the plankton of coastal water had a 

 higher percentage of organic matter than that of 

 continental-slope or Sargasso-Sea water. Sub- 

 tropical plankton in general had a low organic 

 content. It is possible, therefore, that while 

 three-fourths by number, and a smaller fraction 

 by volume, of the zooplankton of the central Pa- 

 cific is theoretically nutritious, its actual food 

 value in calories may be less than for similar or- 

 ganisms of higher latitudes. 



The Pacific Oceanic Fishery Investigations is 

 conducting a study of the food of tunas. The 

 results to date show that a variety of zooplankton 

 forms are utilized directly as food by these fish. 

 For example, representatives of the following 

 groups have been captured in our plankton nets 

 and have also been found among the stomach con- 



