FISHERY BULLETIN: VOL. 83, NO. 4 



ature from June through December. These waters 

 support a different cetacean community (Ikble 5), 

 though school densities there can be as high as in 

 areas off Mexico and Central America (Au et al. 

 1980). 



Other relationships between distribution and 

 movements of dolphins and water masses, conver- 

 gences, and thermal conditions have been described 

 by Fraser (1934), Gaskin (1968), Kasuya (1971), 

 Nishiwaki (1975), Evans (1975), and Miyazaki and 

 Nishiwaki (1978). Hui (1979) found that common 

 dolphins off California tended to occur over promi- 

 nent features of bottom topography. The deep depths 

 of such areas suggest that surface eddies and conver- 

 gences caused by topography-induced accelerations 

 to deep reaching currents (Sverdrup et al. 1942; 

 Neumann 1960) may have concentrated food and at- 

 tracted the dolphins. 



The distributions of dolphin species as seen from 

 the January-March cruises (Figs. 3-7) are similar to 

 the all-season school distributions derived from data 

 of scientific observers aboard tuna seiners. These 

 data, consisting of thousands of sightings per 

 species, were recently summarized by Scott (1981) 

 and Perrin et al. (1983). The same major distribu- 

 tional patterns as presented here for the January- 

 March cruises were apparent, including, for spotted 

 and spinner dolphins, the relative unimportance of 

 equatorial latitudes and the secondary band of in- 

 creased concentration of schools 2°5° north of the 

 Equator. The latter may be related to the Equatorial 

 Front and increased food concentration and possibly 

 production in the convergence zone there (Sette 

 1955; King and Iverson 1962; Blackburn and Laurs 

 1972; Murphy and Shomura 1972; Pak and Zaneveld 

 1974; Blackburn and Williams 1975; Greenblatt 

 1979). Increased abundance of micronekton occurs 

 at least sometime in this zone (Love 1971, 1972; 

 Blackburn and Laurs 1972). The purse seiner data, 

 like those of this paper also showed spotted and spin- 

 ner dolphins more concentrated in the tropical 

 waters off Mexico and along lat. 10°N, while striped 

 and common dolphins tended to be found in the Cen- 

 tral American Bight and along the Equator. This 

 complementary type of distribution was less ap- 

 parent with the more broadly distributed striped 

 dolphin. It should be noted that our southern 

 distributional lobe for spotted and spinner dolphins, 

 at ca. lat. 5°S, may in part be due to the sampling 

 pattern. However, the density of these dolphins along 

 the Equator is definitely reduced, and we know of 

 no information that does not suggest a rapid decline 

 in density south of our lobe 



Our January-March data differs from the all- 



season data in indicating fewer schools in the area 

 around the Revilla Gigedo Islands (at ca. lat. 19°N, 

 long. 111°W) and between long. 90 °W and 100°W 

 along lat. 10°N for spotted and spinner dolphins. 

 Also our data suggested that striped and common 

 dolphins had a more localized distribution near the 

 region of the Costa Rica Dome, and were relatively 

 infrequent between long. 105°W and 120°W, along 

 lat. 10°N. These differences may be due to seasonal 

 changes in distribution. 



The relative densities of these dolphins, as school 

 encounter rates in the tuna purse seine fishery, were 

 recently calculated by Polacheck (1983). The patterns 

 he derived were fragmentary, but not inconsistent 

 with those of this paper. He showed, for example, 

 higher densities of spotted and spinner dolphins ex- 

 tending to the southwest from off southern Mexico 

 and reduced densities in the Central American 

 Bight. For striped and common dolphins, he also 

 described a three-lobed distribution pattern as in this 

 paper. However his equatorial lobe was centered just 

 south of the Equator. 



It seems likely that the dolphin community of the 

 Upwelling-Modified Water differs from the Tropical 

 Water community because of water-mass specific dif- 

 ferences in the distribution and availability of food. 

 This is supported by the different biotic features of 

 Equatorial and Subtropical Waters relative to 

 Tropical Waters. The distinction is clearly shown by 

 the surface distribution of nutrients and primary 

 production in these waters as measured during the 

 EASTROPAC cruises (Love 1971, 1972). The 

 equatorial waters of the eastern Pacific in particular 

 are different. They support abundant plankton- 

 feeding storm petrels rather than fish and 

 cephalopod-feeding flocking birds that are usually 

 abundant both north and south of the Equator (see 

 also Love 1971, 1972 and King 1974). Dolphin species 

 along the Equator tend not to be with fish or birds 

 (Figs. 11, 12), and the species composition of the 

 cetacean community appears to be distinct (Tkble 5; 

 Au and Pitman 1981). Of course it has previously 

 been known that equatorial waters are notable in 

 being important sperm whale grounds (Ibwnsend 

 1935) and have a zooplankton community distinct 

 from other parts of the eastern tropical Pacific 

 (McGowan 1972). Finally the fact that the common 

 dolphin, a species characteristic of coastal upwell- 

 ing waters from California to Peru, occurs with 

 greater frequency in the equatorial waters and near 

 upwelling areas in the Central American Bight, sug- 

 gests that the shorter and different food chains of 

 the upwelling environments (Parsons et al. 1977) may 

 be the basis of the community difference 



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