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Fishery Bulletin 92(2), 1994 



South Equatorial Current SURFACE WATER r.i*V_V.1"*»» /ECUADOR 



SUBTROPICAL 

 SURFACE WATER 



Figure 2 



Schematic representation of the surface circulation and water masses of the east- 

 ern tropical Pacific. 



3°N. Peru Current and California Current Waters 

 are found along the coasts of Peru-Ecuador and Baja 

 California, respectively. The Peru and California 

 Currents feed into the westward South and North 

 Equatorial Currents (SEC, NEC). The North Equa- 

 torial Countercurrent (NECC) flows eastward be- 

 tween the NEC and SEC into the center of the Tropi- 

 cal Surface Water mass. The NECC is strong during 

 September-December and weak or absent during 

 February-April . 



A permanent shallow thermocline underlies most 

 of the region, shoaling towards the coast (Wyrtki, 

 1966). Zonal thermocline ridges are found below sur- 

 face divergences in the SEC along the equator and 

 between the NEC and NECC along lat. 10°N (Fiedler, 

 1992). Upwelling driven by equatorward longshore 

 winds off Peru and Baja California and by trade 

 winds along the equator brings cold, nutrient-rich 

 water from below the shallow thermocline 

 (nutricline) into the surface layer. This nutrient in- 

 put maintains optimal (saturating) concentrations 

 of nitrogen at the surface and results in high levels 

 of new production in equatorial and eastern bound- 

 ary current systems (Chavez and Barber, 1987). Bio- 

 logical productivity is also enhanced by upwelling at 

 the Costa Rica Dome (a quasi-permanent cyclonic 

 eddy at the eastern end of the thermocline ridge along 

 lat. 10°N; Wyrtki, 1964; King, 1986), and by inter- 

 mittent, topographically induced offshore winds at 

 several points along the coast of Central America (e.g. 

 the Gulf of Tehuantepec, McCreary et al., 1989). Sec- 



ondary and higher level productivity and standing 

 stocks are generally high in areas of high primary 

 productivity (Blackburn et al., 1970). Within this 

 and other large oceanic regions, the abundance of 

 animals from plankton to large nekton is patchy on 

 a variety of spatial and temporal scales (Haury et 

 al., 1978), with major consequences for the ecology 

 of pelagic predators (e.g. Carr, 1987). 



Data and methods 



Field data collection 



Dolphin sightings and oceanographic data were col- 

 lected by two ships operating in different parts of 

 the ETP from 28 July through 6 December each year 

 from 1986 to 1990 (e.g. Holt et al., 1987). Track lines 

 are shown in Figure 3. The surveys were conducted 

 at 10 knots (18.5 km/hr) with three observers simul- 

 taneously on watch. One observer covered each side 

 of the ship with a pair of 25x binoculars to search an 

 arc from the bow to about 100° to the beam. The third 

 observer covered the track line with hand-held bin- 

 oculars and the naked eye. Sightings were ap- 

 proached when necessary to allow estimation of num- 

 bers within groups and species identification (Holt 

 and Sexton, 1990). 



While the ships were underway, surface tempera- 

 ture and salinity were recorded continuously by 

 thermosalinograph (ODEC Model TSG-102, Inter- 



