454 



Fishery Bulletin 92(2). 1994 



Figure 1 



Ordination biplot of first two canonical axes from 

 CCA of species-environment data from 1986-90 

 MOPS surveys of the ETP. Points represent species 

 scores (optima) and vectors represent the regression 

 relationships of environmental variables with the 

 canonical axes. TEMP = surface temperature, Z20 

 = thermocline depth, ZD = thermocline thickness. 

 SPOT = spotted dolphin (Stenella attenuata), COM- 

 MON = common dolphin (Delphinus delphis), E. 

 SPIN = eastern spinner dolphin (S. longirostris 

 orientalis) , W. B. SPIN = whitebelly spinner dolphin 

 (S. longirostris), STRIPED = striped dolphin (S. 

 coeruleoalba). 



Table 2 



Fractions of individual and total species variances 

 explained by canonical correspondence analysis 

 (CCA). AX1 = canonical axis 1, AX2 = canonical 

 axis 2. Dolphin species: spotted = Stenella atten- 

 uata, common = Delphinus delphis, eastern spin- 

 ner = S. longirostris orientalis, whitebelly spin- 

 ner = S. longirostris, striped = S. coeruleoalba. 



AX1 



AX2 



AX1+AX2 



Spotted 0.191 0.000 0.191 



Common 0.321 0.004 0.325 



Eastern spinner 0.169 0.089 0.258 



Whitebelly spinner 0.014 0.118 0.132 



Striped 0.037 0.002 0.039 



Total 0.164 0.041 0.205 



fornia Currents) waters of the ETP and is also 

 present seasonally in the region of the Costa Rica 

 Dome at 10°N, 90° W (Fiedler, 1992). Positive scores 

 on canonical axis 2 indicate warm surface tempera- 

 ture and a shallow thermocline (Table 3). These are 

 characteristics of the "coastal tropical" habitat of 

 Reilly and Fiedler (1994). This habitat is centered 

 in the warmest tropical surface water of the ETP, 

 along the coast of Mexico south of Baja California. 

 Species responses along the canonical axes (Fig. 

 2) showed some separation of species habitats, as in 

 the complete CCA (Reilly and Fiedler, 1994). Axis 1 

 separated common dolphins from spotted and spin- 

 ner dolphins, while axis 2 separated eastern and 

 whitebelly spinner dolphins. The means (optima, 

 Table 4) of a species distribution on the two canoni- 

 cal axes in Figure 2 are equal to the species scores 

 plotted in the ordination biplot (Fig. 1). 



The validity of the species/environment relation- 

 ships calculated by CCA was confirmed in three 

 ways. First, distributions of climatological H, de- 

 rived from the CCA results and climatological val- 

 ues of environmental variables at each gridpoint in 

 the MOPS area (Fiedler, 1992), were consistent with 

 stock ranges indicated by the SOPS population 

 boundaries, with the exception of whitebelly spin- 

 ner dolphins (Fig. 3). Second, the distributions were 

 similar to patterns in maps of tuna and research 

 vessel sighting records (Perrin et al., 1985), although 

 such maps can give only a rough indication of habi- 

 tat distribution because the sighting or collection 

 frequencies are not standardized by effort. Third, 

 the distributions of H calculated for climatological 

 September-November environmental conditions 

 were significantly correlated with gridded fields of 

 mean (August-November, 1986-90) MOPS encoun- 

 ter rates as follows: spotted dolphin r = 0.52, com- 

 mon dolphin r = 0.45, eastern spinner dolphin r = 

 0.65, whitebelly spinner dolphin r = 0.36, striped 

 dolphin r = 0.41 (P<0.01 for all relationships). 



Table 3 



Regression/canonical coefficients for standardized 

 environmental variables on two environmental 

 axes (AX1 and AX2). TEMP = surface tempera- 

 ture, Z20 = thermocline (20°C isotherm) depth, 

 ZD = thermocline thickness (difference between 

 20°C and 15°C isotherm depths). 



TEMP 



Z20 



ZD 



AX1 

 AX2 



-0.501 

 f().439 



-0.326 

 -0.486 



+0.111 



