Abstract. We used canonical 



correspondence analysis (CCA), an 

 eigenvector ordination technique 

 that includes direct gradient analy- 

 sis, to investigate habitat use by 

 spotted, Stenella attenuata, spin- 

 ner, S.longirostris, striped, S. 

 coeruleoalba, and common, Del- 

 phinis delphis, dolphins in the east- 

 ern tropical Pacific during 1986- 

 90. Data were collected during an- 

 nual research vessel cruises con- 

 ducted in August-November of 

 each year. Environmental vari- 

 ables included in the analyses were 

 surface temperature, salinity, 

 sigma-t, and chlorophyll, and ther- 

 mocline depth and thickness. The 

 dominant pattern in the species-en- 

 vironment relationship ( 1st canoni- 

 cal axis) separated common dol- 

 phins from spotted and spinner 

 dolphins, based on their associa- 

 tions with cool upwelling habitat 

 and warm tropical habitat, respec- 

 tively. The second axis separated 

 whitebelly spinners from eastern 

 spinner dolphins. Both occurred in 

 tropical water, but were separated 

 primarily by thermocline topogra- 

 phy. The species-environment cor- 

 relations were 0.67 on the first axis, 

 0.42 on the second. Overall, the 

 environmental data explained 15% 

 of the variance in the species data. 

 For individual school types this 

 ranged from 36% for common dol- 

 phins to 6% for striped dolphins. 

 Interannual variability in the spe- 

 cies data was small but was judged 

 significant by a Monte Carlo ran- 

 domization test. Residual inter- 

 annual variance was insignificant 

 after removing variance associated 

 with environmental variables. 



Interannual variability of dolphin 

 habitats in the eastern tropical 

 Pacific. I: Research vessel surveys, 

 1986-1990 



Stephen B. Reilly 

 Paul C. Fiedler 



Southwest Fisheries Science Center. National Marine Fisheries Service. NOAA 

 RO. Box 27 1 , La Jolla. CA 92038 



Manuscript accepted 27 September 1993 

 Fishery Bulletin 92:434-450 (1994) 



434 



The eastern tropical Pacific (ETP) 

 supports a diverse and abundant 

 cetacean fauna of over 25 species 

 (Leatherwood et al., 1982; Au and 

 Perryman, 1985). It is a vast area, 

 larger than the entire North Atlan- 

 tic. Its waters are truly pelagic, ex- 

 cept near a few islands and over a 

 narrow continental shelf. Prior to 

 the 1960s almost nothing was 

 known of the distribution and ecol- 

 ogy of the region's cetaceans beyond 

 summaries of catch localities from 

 nineteenth century fisheries for 

 large whales (Townsend, 1935). 



By the late 1960s, it became clear 

 that large numbers of dolphins were 

 being killed in the ETP in tuna 

 purse seine operations (Perrin, 

 1969). The U.S. government initi- 

 ated a program at that time to place 

 scientific observers on purse sein- 

 ers to monitor dolphin mortality 

 (Smith, 1983). Beginning in 1974, 

 research vessels were dispatched to 

 the region to supplement the ob- 

 server data (Holt et al., 1987). The 

 combination of these efforts has pro- 

 duced unprecedented amounts of 

 information on pelagic cetaceans. In 

 this study we focused on the dolphin 

 species affected by the tuna fishery: 

 spotted dolphins, Stenella atten- 

 uata, two subspecies of spinner dol- 

 phins, S. longirostris, "whitebelly 

 spinners" and "eastern spinners" 

 (Perrin, 1990), striped dolphins, S. 

 coeruleoalba, and common dolphins, 

 Delphinus delphis. 



Dolphin distribution patterns 

 have been described from sighting 

 and collection localities (e.g. Evans, 

 1975; Perrin, 1975; Perrin et al., 

 1983, 1985; Au and Perryman, 

 1985). Au and Perryman (1985) 

 studied cetacean habitats of the 

 eastern tropical Pacific from sight- 

 ings and oceanographic data col- 

 lected during the northern winter. 

 They described two complementary 

 patterns, both with apparent links 

 to regional oceanography. One pat- 

 tern, shown by common and striped 

 dolphins, coincided with "upwelling- 

 modified" conditions found along 

 the equator and at the tropical ter- 

 minations of eastern boundary cur- 

 rents off Baja California and the 

 coasts of Ecuador and northern 

 Peru (described in more detail be- 

 low in Study Area). The second pat- 

 tern, shown by spotted and spinner 

 dolphins, coincided with largely 

 tropical waters off Mexico, where 

 warm, low-salinity surface waters 

 occur over a strong, relatively shal- 

 low thermocline. 



Reilly ( 1990) examined ETP dol- 

 phin distributions during the north- 

 ern summer, and quantitatively 

 tested Au and Perryman's habitat 

 hypotheses. 1 le found an apparent 

 offshore shift in spotted, spinner, 

 and striped (but not common) dol- 

 phins coincident with seasonal 

 shoaling of a thermocline ridge 

 along 10°N (Fig. 1). This seasonal 

 shoaling is part of the dominant 



