386 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



dropped sharply, but zooplankton volumes re- 

 mained liigh. 



12. Largest zooplankton volumes occurred in 

 the quarter, July, August, and September, with 

 October, November, and December essentially 

 equal in rank, and the lowest in January, February, 

 and March. Best yellowfin catches were obtained 

 in July, August, and September. 



13. Zooplankton volumes averaged considerably 

 higher in the year 1950 than in 1951, 1952, and 

 1953. There was some indication of a rising trend 

 in 1954. 



14. The ratio of the volumes of night hauls to 

 day haids ranged from 1.31 to 1.43 in divisions of 

 the current system north of the Equator, and 

 from 1.76 to 1.94 at the Equator and in the South 

 Equatorial Current to the southward. The night/ 

 day ratio did not appear to be related to thermo- 

 cline depth or to general level of plankton abun- 

 dance. 



15. With an increase in breadth of the mixing 

 zone associated with the divergence at the Equa- 

 tor, there was a corresponding broadening in the 

 zooplankton rich zone. On one cruise an increase 

 in rate of flow of the Countercurrent was ac- 

 companied by a marked cliange in the distribution 

 of zooplankton within the cmrent. These obser- 

 vations indicate that the zooplankton was quick 

 to respond to physical changes in the environment 

 by dispersal or concentration of the population 

 following changes in the water mass. 



16. On long series of stations extending from 

 the phosphate-poor North Equatorial Current to 

 south of the Equator, we found highly significant 

 ])ositive correlations between zooplankton volume 

 and surface inorganic phosphate. On series 

 covering a sliort range of latitude the correlation 

 was insignificant or even negative. 



Although the highest concentration of phosphate 

 occurred in the divergent zone at the Equator, 

 agreeing in this respect with zooplankton, longi- 

 tudinally and seasonally there was some evidence 

 of an inverse relationship with zooplankton and 

 yellowfin; this may result from differences in 

 rate of utilization. 



17. Zooplankton distribution was rather uni- 

 form tiu'oughout the island waters of Palmyra, 

 an atoll lying in the Countercurrent at about 

 162° W. longitude. Sampling along four station 

 lines extending from a few hundred yards from 

 the outer reef to about 14 miles offshore revealed 



no significant change in zooplankton abimdance 

 with distance from land (appendix A). 



LITERATURE CITED 



Austin, T. S. 



1954a. Mid-Paeific oceanography III. Transequa- 

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 and Wildlife Service, Spec. Sci. Rept., Fisheries 

 No. 131, 50 p. 

 1954b. Mid-Pacific oceanography, Part V, Trans- 

 equatorial waters, May-June 1952, August 1952. 

 U. S. Fish and Wildlife Service. Spec. Sci. Rept.: 

 Fisheries No. 136, 86 p. 



Bir.ELow, H. B., and M.^ry Sears. 



1939. Studies of the waters of the Continental Shelf, 

 Cape Cod to Chesapealce Bay, III. A volumetric 

 study of the zooplankton. Mus. Comp. Zool., 

 Mem. 54(4):183-378. 



Cl.\rke, G. L. 



1940. Comparative richness of zooplanl^ton in 

 coastal and offshore areas of the Atlantic. Biol. 

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Cromwell, Townsend. 



1951. Mid-Pacific oceanography, January through 

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1953. Circulation in a meridional plane in the 

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 12(2):196-213. 



1954. Mid-Pacific oceanography II. Transequa- 

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 1951. U. S. Fish and Wildlife Service, Spec. Sci. 

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Cromwell, Townsend, R. B. Montcomery, and E. D. 

 Stroup. 



1954. p;quatorial Undercurrent in Pacific Ocean 

 revealed by new methods. Science 1 19(3097) :648- 

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Cromwell, Town.send, and J. L. Reid, Jr. 



1956. Astudy of oceanic fronts. Tellus 8(1) :94-101. 

 Crowe, P. R. 



1952. The seasonal variation in the strength of the 

 trades. Institute of British Geographers, Trans- 

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CUSHING, D. H. 



1951. The vertical migration of pianktonic Crus- 

 tacea. Biol. Rev. 26(2): 158-192. 



GR.'iH.^M, H. W. 



1941. Plankton production in relation to character 

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 4(3):139-197. 



HiD.\, T. S., and J. E. King. 



1955. Vertical distribution of zooplankton in central 

 equatorial Pacific, July-August 1952, U. S. Fish 

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Iversen, E. S., and H. O. Vo,shid.\. 



1956. Longline fishing for tuna in the central 

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