Fiedler and Reilly: Interannual variability of dolphin habitats and abundances 



459 



extent of the habitat of this stock (see Fig. 3). The 

 apparent westward extension of the habitat outside 

 the MOPS area is consistent with the recognition of 

 the "whitebelly" form as a hybrid/intergrade be- 

 tween eastern and pantropical subspecies of spin- 

 ner dolphins (Perrin, 1990). The partial separation 

 of eastern and whitebelly spinner dolphin habitats 

 defined by H is consistent with the management 

 boundary between the two forms proposed by Perrin 



Spotted Dolphin 



Figure 5 



Abundance-environment relationships 

 for spotted dolphins {Stenella attenuate) 

 in the MOPS area: (Topi Annual abun- 

 dance estimates (N t xl0~ 6 ) vs. annual 

 mean habitat quality (//,). (Bottom) 

 Interannual change in abundance esti- 

 mates (7V ( -iV M , x 10~ 6 ) vs. mean habitat 

 quality from June of year t-1 through 

 May of year t (H t _ 05 >. Solid line connects 

 lowess-smoothed values (Wilkinson, 

 1990). 



et al. (1991): eastern spinners north of 10°N and east 

 of 125°W, and whitebelly spinners south of 10°N or 

 west of 125°W. 



Reilly and Fiedler (1994) suggested that species 

 habitats defined by axis scores from CCA could be 

 used to improve the precision and accuracy of abun- 

 dance estimates from research vessel surveys. Pre- 

 cision could be increased by post-stratifying the 

 sighting data based on the spatial distribution of 

 axis scores. Bias could be reduced by using axis 

 scores to quantify the amount of habitat available 

 within a survey area. The present results suggest 

 that this approach could be extended by using spe- 

 cies habitat distributions incorporating environmen- 

 tal variability along more than one canonical axis. 

 For example, a large area of suitable spotted dolphin 

 habitat existed in equatorial water beyond the SOPS 

 population boundary during 1983, apparently caus- 

 ing a serious underestimate of abundance. Gerro- 

 dette et al. 4 suggested similar approaches for using 

 fields of H to adjust abundance estimates from 

 MOPS research vessel surveys. 



We utilized the results of a multi-species CCA for 

 this study. While this approach yields useful infor- 

 mation about community structure, as in the sepa- 

 ration of eastern and whitebelly spinner habitat, it 

 does not retain the maximum amount of informa- 

 tion about any single species for management ap- 

 plications. A similar type of analysis for each indi- 

 vidual species or stock might explain more of the 

 variability in abundance and improve the quantifi- 

 cation of habitat quality defined by Gaussian re- 

 sponses along dominant environmental gradients. In 

 addition, CCA could potentially be used to account 

 for environmental effects on school size and effec- 

 tive trackline width that cause error in dolphin 

 abundance estimates. However, preliminary results 

 of a CCA incorporating school size (Reilly and 

 Fiedler, 1994) showed no meaningful relation be- 

 tween school size and environmental variability. We 

 have only begun to exploit CCA in our work and 

 believe it is a powerful new technique with great 

 potential for quantitative ecological studies of popu- 

 lations of marine mammals and other organisms. 

 For example, environmental variability dominates 

 variations in recruitment of many fish stocks 

 (Longhurst, 1984; Hollowed et al., 1987). Although 

 our time series was not long enough to address popu- 

 lation change, this study demonstrates the potential 

 of CCA to detect environmental effects in multi-stock 

 fisheries studies. 



4 Gerrodette, T„ P. C. Fiedler, and S. B. Reilly. 1991. Including 

 habitat variability in line transect estimation of abundance and 

 trends. NMFS/SWFSC Admin. Rep. No. LJ-91-37. 



