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Fishery Bulletin 92(2), 1994 



Common dolphin habitat was centered in cool, 

 upwelling-modified water in three regions: off Baja 

 California, along 10°N with a maximum at the Costa 

 Rica Dome, and in the equatorial surface water 



To no too 



90 SO 



Figure 3 



Mean habitat quality (H) for spotted iStenello attenuate!), 

 eastern spinner (S. longirostris orientalis), whitebelly spin- 

 ner (S. longirostris), common (Delphinus delphis), and 

 striped (S. coeruleoalba) dolphins. H was calculated from cli- 

 matological fields of surface temperature, thermocline depth, 

 and thermocline thickness in the MOPS area. Heavy dashed 

 lines delimit SOPS population areas used by Anganuzzi and 

 Buckland (1989) and Anganuzzi et al. (1991) or, for striped 

 dolphins, as defined by Au et al. (Footnote 1.) 



mass of Wyrtki (1966). These three habitat centers 

 are occupied by the northern, central, and southern 

 stocks of common dolphins (Perrin et al., 1985). The 

 offshore H maximum along 10°N at 120-130°W does 

 not correspond to high encounter rates in the 

 MOPS data, but reflects a shoaling of the coun- 

 tercurrent thermocline ridge at that location 

 (Fiedler, 1992). 



Striped dolphins are the most widespread 

 and abundant of the target species. The high- 

 est H values tended to be in regions between or 

 offshore of the centers of spotted/eastern spinner 

 dolphin habitat in tropical surface water near the 

 coast of southern Mexico and northern, central, 

 and southern common dolphin habitats off Baja 

 California, near the Costa Rica Dome, and in 

 equatorial water (see also Reilly, 1990). 



Both seasonal and interannual variability 

 were evident in time series of mean seasonal 

 habitat quality, H (Fig. 4). The strongest 

 interannual signal for all species can be attrib- 

 uted to the El Nino events of 1982-83 and 

 1986-87. During both events, H increased for 

 spotted and spinner dolphins and decreased for 

 common and striped dolphins. Seasonal vari- 

 ability, indicated by the deviations of the sea- 

 sonal from the smoothed H values, was low for 

 species with large geographic ranges (e.g. striped 

 dolphin) and high for species with more restricted 

 ranges (e.g. eastern spinner dolphin). Seasonal 

 variability of// was as great as interannual vari- 

 ability for eastern spinner dolphins. 



Annual dolphin abundance estimates, N t , or 

 interannnual change in abundance estimates, 

 N l -N / _ 1 , were related to changes in the envi- 

 ronment, //, for spotted and eastern spinner 

 dolphins. Annual spotted dolphin abundance 

 was not significantly correlated with //, but 

 interannual change in abundance was nega- 

 tively correlated with H (Fig. 5, r=-0.65, 

 P=0.01). Calculating year-to-year changes in 

 abundance eliminates multi-year trends in the 

 time series. Buckland et al. (1992) found sig- 

 nificant trends in estimated spotted dolphin 

 abundance which might complicate the relation 

 between annual N and H values. An increase 

 in H for spotted dolphins indicates an expan- 

 sion of favorable habitat to the south of the SOPS 

 population boundary west of 100 W (Fig. 6A). 



Annual eastern spinner dolphin abundance 

 was negatively correlated with H (r=-0.49, 

 P=0.05). An increase in H for eastern spinner 

 dolphins indicates an expansion of favorable 

 habitat to the west and south of the SOPS 

 population boundary (Fig. 6B). 



