VERTICAL DISTRIBUTION OF ZOOPLANKTON IN THE CENTRAL 

 EQUATORIAL PACIFIC, JULY-AUGUST 1952 



Longline -fishing surveys by the Pacific Oceanic Fishery Investigations (POFI) have 

 shown a concentration of deep-swimming yellowfin tuna between the Equator and 5 N. latitude 

 from 140° to 160°W. longitude (Sette 1954). This area is also relatively high in zooplankton 

 abundance (King and Demond 1953), and it is believed that the concentration of both zooplankton 

 and tuna within these latitudes is influenced by the increased fertility of the surface layer result- 

 ing from the equatorial upwelling of nutrient-rich water (Cromwell 1951, Sette 1954, and King_'). 



In longline fishing in this region there has been a tendency for the deeper hooks, fish- 

 ing at depths of about 300 to possibly 500 feet, to catch more tunaL' than the shallower hooks 

 fishing at 150 to 250 feet (Murphy and Shomura, 1953a, 1953b). In the area of best catch, the 

 deeper hooks apparently fish at about the level of the thermocline. One hypothesis immediately 

 suggests itself: that these deep-swimming tunas are concentrating at the level which provides the 

 most available food. It is known that the sharp temperature gradient associated with the thermo- 

 cline may have a concentrating effect on certain plankton forms (Sverdrup et al. 1942, p. 794) and 

 may restrict the migration of others (Moore et al. 1953). 



The food of the yellowfin tuna consists of a great variety of organisms, both fish and 

 invertebrates, varying widely in size (Reintjes and King 1953). Although zooplankton comprises 

 on the average a very small percentage of adult tuna food, it is essential food of the forage fish, 

 squid, and shrimp which are utilized directly by the tuna. Therefore, in sampling the zooplankton 

 we believe we are obtaining a reliable index to the basic fish-food present in an area whether uti- 

 lized directly or indirectly by the tunas. 



Since the vertical distribution of zooplankton cannot be determined by the 200-meter 

 oblique tow which has been used by POFI in surveying the relative abundance of zooplankton, a 

 series of horizontal closing-net hauls was made with Clarke-Bumpus samplers (Clarke and 

 Bumpus 1940) to investigate the vertical distribution of zooplankton in relation to the thermocline. 

 Townsend Cromwell was field party chief on cruise 16 of the Hugh M. Smith on which these col- 

 lections were made, and Heeny Yuen, field party member, was largely responsible for making 

 the hauls. The figures were prepared by Tamotsu Nakata. The oceanographic data resulting 

 fronn cruise 16 have been published (Austin 1954). 



AREA AND METHODS 



o o 



The sampling was done at 30 stations along 150 W. longitude extending from 12 N. to 



7 S. latitude in the 9-day period, July 27 to August 4, 1952, on cruise 16 of the Fish and Wildlife 

 Service vessel Hugh M. Smith . The approximate position of each station is shown in figure 1 and 

 given more exactly together with the date, hour, and depth of hauling in table 1, Of the 90 hauls 

 nnade, 68 are quantitatively usable. Improper functioning of the gear vitiated the remaining 22 

 hauls. 



At each station, horizontal hauls were nnade sinnultaneously at three levels with 

 Clarke-Bumpus aannplers equipped with nets of 56XXX grit gauze having mesh apertures averag- 

 ing 0.31 mm. in width. These were clamped on 5/32-inch (diameter) wire cable at intervals in- 

 tended to place one sampler at a depth of about 200 naeters during the haul, one at about the 70 F. 



1/ MS. Variations in zooplankton abundance in the central equatorial Pacific, 1950-52. To be 

 published in Proceedings of the Fifth Meeting of the Indo-Pacific Fisheries Council. 



2/ Yellowfin, Neothunnus macropterus (Temnninck and Schlegel); bigeye, Parathunnus sibi 

 (Temnninck and Schlegel); and albacore, Germo alalunga (Bonnaterre). 



