The EASTROPAC field work was divided into seven, 2-month cruise periods. During each period there was a single- or multi-ship cruise. Each cruise was planned to last about 50 days; in practice some lasted longer. There were two types of cruises - survey and monitor. Survey cruises were multi-ship operations designed to provide simultaneous coverage of as much of the study area as possible. The study area was defined as latitude 20° N to latitude 20° S and from the coast of the American continents westward to longitude 119° W. There were three such surveys, 6 months apart. In each interval between survey cruises there were two, single-ship monitor cruises, making a total of four. The area covered by monitor cruises was determined partly by oceanographic considerations and partly by time and logistic constraints placed on a vessel operating from San Diego, but the purpose was to monitor changes in oceanographic conditions taking place between survey cruises.
          Cruises by Latin American cooperating vessels were usually timed to coincide with survey cruises, and their coverage designed to supplement the survey cruises.
           In addition to the major EASTROPAC cruises and Latin American cooperating vessels, certain ships of opportunity presented themselves. In some cases, such as Te Vega, the ship was operating in the same area and at the same time, and the data were made available to EASTROPAC. In other cases, such as Oceanographer and Charles H. Davis, the observations made during a portion of each of their cruises were planned to be part of EASTROPAC. EASTROPAC personnel were aboard Oceanographer during that portion of her cruise.
          The cruises are designated in the atlas by a two-digit numbering system. The first number, from 1 through 7, designates one of the seven, 2-month cruise periods beginning with February-March 1967. The second number indicates the track followed or area covered within that cruise period. The monitor cruise track is designated by 0. For the first survey cruise (4 ships) the tracks are designated 1 to 4 with the westernmost track having the lowest number. For the second and third survey cruises (3 ships each) the tracks are numbered 5, 6, and 7 with, again, the westernmost track carrying the lowest number. Thus, cruise 12 represents track 2 of the first survey cruise; 30 designates the second monitor cruise; 77 refers to track 7 (the easternmost track) of the third survey cruise. Local practice has been to include the name of the ship when referring to a cruise, e.g., Jordan 12, but it is not necessary for complete identification of the cruise. There has also been some use of the expressions "10-series cruises," "40-series cruises," etc., to refer to cruises made during the first survey cruise period, second survey cruise period, etc. Cruises of foreign cooperating vessels or ships of opportunity have been identified by the expedition names or cruise numbers assigned by the operating agency.
          Figure 1 is a graphical summary of EASTROPAC cruises. The affiliations of the participating ships are shown in the accompanying table (Table 1).

TABLE 1. Affiliations of ships participating in EASTROPAC (1967-68).

Category and ship                    Country Agency or institution

Principal participating ships:
Alaminos …………………….. U.S.A. Department of Oceanography, Texas A &M University
Argo …………………………. U.S.A. Scripps Institution of Oceanography, University of California
David Starr Jordan …………. U.S.A. Bureau of Commercial Fisheries, La Jolla
Rockaway …………………… U.S.A. U.S. Coast Guard
Undaunted …………………... U.S.A. Bureau of Commercial Fisheries, Miami
Thomas Washington ………… U.S.A. Scripps Institution of Oceanography, University of California

Latin American cooperating ships:
Defiance …………………….. Mexico & U.S.A. Dirección General de Pesca e Industrial Conexas, Mexico, and U.S. Bureau of Sport Fisheries and Wildlife
Huayaipe ……………………. Ecuador Instituto Nacional de Pesca
Tuxpan ………………………. Mexico Dirección General de Pesca e Industrial Conexas
Unanue ……………………… Peru Instituto del Mar
Yelcho ……………………….. Chile Instituto Hidrográfico de la Armada
Yolanda ……………………... Mexico Dirección General de Pesca e Industrial Conexas

Ships of opportunity:
Charles H. Davis ……………. U.S.A. U.S. Naval Oceanographic Office
Esmeralda …………………… Chile Instituto Hidrográfico de la Armada
Oceanographer ……………… U.S.A. Environmental Science Services Administration, U.S. Coast & Geodetic Survey
Te Vega ……………………… U.S.A. Stanford Oceanographic Expeditions, Stanford University

          The field work began with the first survey cruise (10-series cruises) in February-March 1967 (figs. 10-TC-a, 10-TC-b). Major coverage was by four ships: Argo of the Scripps Institution of Oceanography, University of California, San Diego, and David Starr Jordan of the Bureau of Commercial Fisheries, Fishery-Oceanography Center in La Jolla, both operating from San Diego, and the U.S. Coast Guard Cutter Rockaway from New York, and Alaminos from the Department of Oceanography, Texas A & M University, which began their EASTROPAC operations after entering the Pacific from the Canal Zone. The area covered was from 20° N to 20° S and from the coast westward to longitude 126° W. It should be noted that this was the only cruise in which there was coverage on 126° W. Subsequent cruises only extended westward to 119° W. Additional coverage along the coast of cooperating nations was provided by Yolanda of the Direccion General de Pesca e Industrial Conexas, Mexico, (DGPIC) operating out of Mazatlan in a joint project with the Inter-American Tropical Tuna Commission (IATTC), Unanue of the Institute del Mar of Peru (IMP) from Callao, and the MARCHILE V cruise run by Yelcho of the Instituto Hidrográfico de la Armada, Chile, (IHA) from Valparaiso. Ships of opportunity operating during the period were the USNS Charles H. Davis of the U.S. Naval Oceanographic Office from San Diego, which ran a north-south section on 85° W, and Te Vega of Stanford Oceanographic Expeditions, Stanford University, from Pacific Grove, which operated on a line from Cape San Lucas to the Galapagos.
          Two monitor cruises by David Starr Jordan, cruise 20 in April-May (fig. 20-TC) and cruise 30 in June-July (fig. 30-TC), followed. During the first monitor period, Yolanda again operated from Mazatlan and Te Vega was working off the coast of Mexico between 13° and 24° N. During the second monitor cruise, Defiance, a chartered U.S. fishing vessel, operated out of Mazatlan instead of Yolanda for a joint cruise sponsored by DGPIC, the U.S. Bureau of Sport Fisheries and Wildlife, and the IATTC; Huayaipe of the Instituto Nacional de Pesca, Ecuador (INPE), in conjunction with IATTC, operated in the waters between Guayaquil and the Galapagos ; and Te Vega occupied stations in the entrance to the Gulf of California.
          The second survey cruise (40-series) took place in August-September 1967 (figs. 40-TC-a, 40-TC-b). Only three ships were available for the principal coverage: Thomas Washington, SIO, Undaunted from the BCF Tropical Atlantic Biological Laboratory in Miami, and Rockaway. It was necessary to rearrange the cruise tracks, especially in the eastern portion of the area, and the extent of the southward coverage was reduced. Other vessels operating during this period were Tuxpan of DGPIC and IATTC operating from Mazatlan in the area south of the entrance to the Gulf of California, Unanue running a network of stations off the coast of Peru, and Yelcho (MARCHILE VI) running five lines off northern and central Chile.
          The third and fourth monitor cruises were run by David Starr Jordan, cruise 50 in October-November (fig. 50-TC) and cruise 60 in December 1967-January 1968 (fig. 60-TC). During the third monitor period, the U.S. Coast and Geodetic Survey Ship Oceanographer made observations during part of her track between Callao and San Diego. This portion of her inaugural, round-the-world voyage had been designated for EASTROPAC participation. Also during this period Huayaipe made a second cruise in the area between Guayaquil and the Galapagos. Te Vega ran a north-south line on 100° W and some stations in the vicinity of the Galapagos in a time period which overlapped between the fourth monitor and third survey cruise periods.
          The principal observations during the third survey cruise (70-series) in February-March 1968 (figs. 70-TC-a, 70-TC-b) were made by three ships: Thomas Washington, David Starr Jordan and Rockaway. The cruise tracks were similar to those of the second survey except that some of the north-south lines extended to 20° S and there was no coverage along 92° W. Latin American cooperating ships operating at the same time were Huayaipe, Unanue, Yelcho (MARCHILE VII). The Esmeralda, a training ship of the Chilean Navy, has been listed as a ship of opportunity. Her personnel made a series of mechanical bathythermograph observations during a run from Valparaiso to Panama.