ABSTRACT 
From September '2'A to December 17, lOoo, the U. S. Fish and Wildlife 
Service research vessel Hu(/h M. San7// participated in a multiple-vessel oceano¬ 
graphic survey of the eastern tropical racitic (Eastropic). The results of 
physical and chemical o(‘eanographic observations made from the Smith are 
described, witli emphasis on those observations and resnlts which are of signifi¬ 
cance to the distribution and abundance of the marine biota. 
IV 
OCEANOGRAPHY OF THE EAST CENTRAL EQUATORIAL PACIFIC AS 
OBSERVED DURING EXPEDITION EASTROPIC 
By Thomas S. Austin, Oceanographer 
Bureau of Commercial Fisheries 
Since early in 1950, the Pacific Oceanic Fishery 
Investigations (POFI),^ U. S. Fish and Wildlife 
Service, has been studying the oceanography of the 
central equatorial Pacific. These studies have been 
centered in an area between 140° W. longitude and 
the 180th meridian and between 12° N. and 10° S 
latitude. The work has been directed toward de¬ 
termining circulation features and the associated 
distribution of the marine chemical and physical 
factors in order to more adequately understand the 
variations in the distribution and abundance of the 
biota, especially the yellowfin tuna (Neothunnus 
macroptems Temminck and Schlegel). 
The earlier phases of the program were explora¬ 
tory. As the resulting data were analyzed, the 
need for specialized studies became evident. One 
such study involved the east-west variations in the 
ecology of the northern boundary of the Equa¬ 
torial Countercurrent; another, an extension of 
observations along the Equator to the east of 140° 
W. longitude. These two studies were carried out 
during a five-vessel, simultaneous survey of the 
eastern tropical Pacific (Eastropic). The survey 
extended from 160° W. longitude to the west coast 
of the Americas and was conducted during the fall 
and winter of 1955. 
Data and descriptions of the circulation features 
and distribution of the variables in the general 
area surveyed during expedition Eastropic are 
available from several sources. These include, in 
part, the reports of the Dana (Thomsen 1937), the 
Carnegie (Sverdrup et ah, 1944; Fleming et ah, 
1945), the Alhatross (Bruneau et ah, 1953), POFI 
vessels (see Sette et ah, 1954; Austin 1957), and 
unpublished data kindly furnished by the Scripps 
Institution of Oceanography (SIO) from their 
operations Shellback and Capricorn. 
^ Redesignated Bureau of Commercial Fisheries Biological Lab¬ 
oratory, Honolulu, effective Jan. 1, 1959. 
Note.— Approved for publication, Nov. 21, 1958. Fishery 
Bulletin 168. 
The currents in the east-central equatorial 
Pacific are generally zonal (east-west). They in¬ 
clude the westerly flowing North Equatorial Cur¬ 
rent with its southern boundary near 10° N. lati¬ 
tude, the westerly South Equatorial Current with 
its northern boundary near 5° N. latitude, and the 
easterly Equatorial Countercurrent in between. 
Centered about the Equator and beneath the South 
Equatorial Current is the easterly flowing Equa¬ 
torial Undercurrent (Cromwell et ah, 1954; 
Fofonoff and Montgomery, 1955). 
Five agencies collaborated in expedition 
Eastropic. Personnel from POFI aboard the re¬ 
search vessel Hugh M. Saiith (cruise 31) studied 
east-west variations in conditions along the north¬ 
ern boundary of the Equatorial Countercurrent 
and along the Equator. Representatives of SIO 
and the Inter-American Tropical Tuna Commis¬ 
sion aboard the Scripps' vessels, the Spencer F. 
Baird and Horizon^ operated southward from San 
Diego, Calif., to northern Peru and in the Gulf of 
Panama. The B, S. P. Bandy, assigned by the 
Peruvian Hydrographic Offlce, surveyed off the 
northwest coast of South America in the general 
area of the Peru Current, and the California De¬ 
partment of Fish and Game vessel, the N, B. Sco- 
fedd^ conducted longline Ashing in the area 
studied by the Horizon and the Batrd. The 
tracks of all the cooperating vessels are shown in 
figure 1. 
The Smith departed Honolulu, Hawaii, on 
September 23, 1955, on a southerly course, crossed 
the westerly flowing North Equatorial Current 
and proceeded into the easterly Equatorial Coun¬ 
tercurrent. The boundary between the two cur¬ 
rents was determined by the variations in the 
vertical distribution of temperature (ridge in the 
thermocline at the northern boundary of the 
Countercurrent) and the change in direction of 
flow as shown by tlie geomagnetic elect rokine- 
tograph (GEK) until it became inoperative on 
