suggestions for which the writer is grateful: 

 Robert S. Arthur, John A. Knauss, Joseph L. 

 Reid, Gunnar I. Roden, and Warren S. Wooster 

 of the Scripps Institution of Oceanography; 

 Richard A. Barkley, Gerald V. Howard, J, F, 

 T. Saur, and Oscar E, Sette of the Bureau of 

 Commercial Fisheries; and John Lyman of 

 the National Science Foundation. 



Most of the oceanographic data used in the 

 paper have been listed and explained else- 

 where in papers containing descriptions of 

 equipment and methods (Holmes and Black- 

 burn, I960; Blackburn, Griffiths, Holmes, and 

 Thomas'. 



Sections showing vertical distributions of 

 physical and chemical properties were pre- 

 pared from station property curves drawn 

 by the Data Processing Section of the Scripps 

 Institution according to the method ofKlein*, 

 from bathythermograms, and from curves of 

 phosphate concentration against depth. The 

 depths corresponding to preselected property 

 levels were read from the curves. An attempt 

 was made to secure consistency between the 

 upper parts of sections and charts showing 

 horizontal distributions of properties at or 

 near the surface. The surface level was the 

 most convenient for charts of physical and 

 chemical properties except phosphate con- 

 centration, for which the 30-m. level was 

 chosen in order to present contrasts. The 

 reference level selected for dynamic height 

 anomalies was 500 decibars, because any 

 lower level would have eliminated too many 

 stations. Zooplankton data are displacement 

 volumes of small organisms (<5 cm. long) in 

 day or night oblique hauls made at about 

 2 knots through the uppermost 200-400 m.; 

 micronekton' data are displacement volumes 



3 Blackburn, M., R. C. Griffiths, R. W. Holmes, and 

 W. H. Thomas. MS. Physical, chemical and biological 

 observations in the eastern tropical Pacific Ocean: 

 three cruises to the Gulf of Tehuantepec, 1958-1959. 

 U. S. Fish and Wildlife Service, Special Scientific 

 Report- -Fisheries. 



* Klein, H. T., MS. A new technique for processing 

 physical oceanographic data. Scripps Institution of 

 Oceanography, La Jolla, Calif. 



' Micronekton, a term occasionally found in marine 

 biological literature (e.g., Marshall 1954), is here de- 

 fined as the assemblage of actively swimming fishes, 

 cephalopods, and crustaceans ranging from about 1 cm. 

 to 10 cm. in length. It was collected in a net 19 ft. long, 

 with mouth-aperature 5 ft. x 5 ft., of Marion Textiles 

 pattern 467 nylon throughout (mesh size about 5.5 mm. 

 X 2.5 mm.), and filtering area 7.6 times aperture area. 



of organisms in night oblique hauls made at 

 about 5 knots through the uppermost 90 m. 

 (approximately); surface chlorophyll a. meas- 

 urements were made in the daytime, generally 

 at local noon; and surface productivity esti- 

 mates were obtained by the C-14 method by 

 incubating the inoculated water samples in 

 sunlit sea surface water for half a solar 

 day.* 



TOPOGRAPHY AND CLIMATOLOGY 

 OF THE TEHUANTEPEC REGION 



The Gulf of Tehuantepec is a large bight 

 that lies between Port Angeles and Suchiate 

 Bar, on the Pacific coast of Mexico adjacent 

 to Guatemala (fig. 1). The head of the Gulf is 

 only about 120 miles south of the southern 

 shore of the Gulf of Mexico. On this narrow 

 isthmus the mountain chain of the American 

 continent becomes lower, and there is a pass 

 (Chivela Pass, 735 feet above mean sea level) 

 through which a survey for an interoceanic 

 ship canal has been made, A railroad and a 

 telegraph line cross the isthmus through this 

 pass from Salina Cruz to Coatzacoalcos. 



For the purpose of this paper the region is 

 defined to include all waters north of latitude 

 13° N. between longitude 98° and 92° W. The 

 95° meridian crosses the Isthmus of Tehuan- 

 tepec a few miles east of the port of Salina 

 Cruz through Chivela Pass. 



There is practically no Continental Shelf 

 west of the 96° meridian, but there is a broad 

 one east of 95° where most of the northeastern 



Footnote 5--Cont. 



The displacement volumes given in the illustrations are 

 the actual volumes of animals caught (excluding coelen- 

 terates, tunicates, and heteropods) per estimated 1,000 

 m.2 of water strained; the estimate of water strained 

 is the product of mouth-aperture in m.2 and length of 

 track in m., multiplied by a filtration coefficient of 

 0.757 which was obtained in an experiment in which 

 flowmeter readings were compared in identical tows 

 with and without the net. The method of hauling is 

 given in the manuscript by Blackburn, Griffiths, Holmes, 

 and Thomas (see footnote 3). 



6 The method of incubation was changed from time to 

 time. At some stations, bottles were suspended from 

 a drifting buoy in the ocean at the place where the water 

 samples were collected (in situ method); another method 

 was to tow bottles astern (trailing bottle method); and 

 another was to place them in a sunlit shipboard tank of 

 circulating surface seawater (deck incubator method). 

 Details are given in the manuscript by Blackburn, 

 Griffiths. Holmes, and Thomas (see footnote 3). 



