142 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



relation between concentrations of phytoplankton 

 and zooplankton (Riley and Bumpus 1946). In 

 conducting a fish-farming experiment, Raymont 

 ( 1947) found that in both enclosed and unenclosed 

 small sea areas the addition of a nitrate and phos- 

 phate fertilizer stimulated phytoplankton growth, 

 which in turn maintained a high density of zoo- 

 plankton which promoted a rapid growth of 

 flatfish. 



From tuna-catch records and observations on 

 occurrence of surface schools, the Pacific Oceanic 

 Fishery Investigations is accumulating evidence 

 on the distribution of tunas which indicates quite 

 definitely that areas of the greatest zooplankton 

 abundance in the central Pacific are also areas of 

 greatest tuna abundance. 



We are fully aware that the data we have pre- 

 sented on the variation of zooplankton abundance 

 with latitude does not in any way reflect the "rate 

 of turnover," the most difficult element to deter- 

 mine in estimates of productivity. In this area 

 of the tropical Pacific, with temperatures very uni- 

 form in time and space and zooplankton very uni- 

 form in composition, the rate of turnover should 

 not be a disturbing feature in the comparison of 

 the several parts of our area, i. e., for our data 

 standing crop should be proportional to produc- 

 tivity. 



SUMMARY AND CONCLUSIONS 



1. This report presents the results of 210 quanti- 

 tative zooplankton collections made in the central 

 Pacific in 1950 and 1951, between 27° N. and 14° 

 S. latitude, and 155° and 175° W. longitude. 



2. Most of the collections were obtained by 

 oblique hauls to 200 meters' depth, employing 

 1-meter nets of 30xxx grit gauze with aperture 

 widths of 0.65 mm. 



3. A method of sampling was developed that 

 harmonized with hydrographic and fishing opera- 

 tions, required little of ship's time, and involved 

 no particularly elaborate treatment of samples in 

 the laboratory. 



4. The zooplankton taken by our collecting 

 method was composed on the average by number, 

 of 57 percent Copepoda, with the other chief com- 

 ponents ranking as follows : Chaetognatha, 12 per- 

 cent; Tunicata, 6 percent; Euphausiacea, 5 per- 

 cent; Siphonophora, 4 percent; and Foraminifera, 

 4 percent. 



5. The composition of the collections was re- 

 markably uniform when longitudes and cruises 

 were compared. 



6. Despite the use of oblique tows to 200 meters' 

 depth, we found significant differences between 

 day hauls and night hauls. 



7. Since distinctly larger catches were obtained 

 on the "summer"' cruise (cruise 5) than on the 

 "winter" cruise (cruise 8), a seasonal difference in 

 zooplankton abundance is indicated. 



8. Within the range of latitudes sampled, the 

 greatest abundance, both by number and volume, 

 of zooplankton occurred in the region of the Equa- 

 tor; sometimes the greatest concentrations were 

 found north of the Equator, when related to a 

 convergence, and to the south when no marked 

 convergence existed. 



9. The abundance of zooplankton is correlated 

 with such chemical and physical environmental 

 factors as inorganic phosphate, water tempera- 

 ture, dissolved oxygen, and thermocline depth, 

 which are influenced by the upwelling resulting 

 from the equatorial divergence. 



10. Upwelling along the Equator replenishes 

 the supply of nutrients in the euphoric zone, thus 

 providing a favorable environment for the growth 

 of phytoplankton. Since animal life fluctuates 

 with its food supply, conditions in this region 

 are favorable for the development of a zooplank- 

 ton population. 



11. While our observations on the standing 

 crop of zooplankton do not give a measure of the 

 rate of production in its strict sense, we believe 

 that they do provide a useful index to the relative 

 productivity of different areas of the central 

 Pacific. 



LITERATURE CITED 



Agas.sk, Alexander, and A. G. Mayer. 

 1902. Report on the scientific results of the expedition 

 to the Tropical Pacific. III. Medusae. Mus. Comp. 

 Zool., Mem. 26 (3) : 139-176. 

 ARRHENIUS, Gustaf. 



1950. Late cenozoic climatic changes as recorded by 

 I he Equatorial Current System. Tellus 2 (2): 

 S3-88. 

 Beebe, William. 



1926. The Arcturus Adventure, xix, 439 p. New 



York: G. P. Putnam's Sons. 

 1936. Food of the Bermuda and West Indian tunas 

 of the genera Parathunnus and Neothunnus. Zoo- 

 logica 21 (15) : l9<5-205. 



