Pernn et al.: Geographic variation in morphology of Stenella longirosms 



425 



hypothesis of secondary contact after isolation (Perrin 

 et al. 1985) and consideration of the patterns of varia- 

 tion led to the formal recognition of the eastern form 

 as a subspecies, Stenella longirostris orientalis, and the 

 "whitebelly" form as an hybrid/intergrade between 

 that subspecies and the pantropical spinner dolphin S. I. 

 longirostris (Perrin 1990). The hypothesis of genetic 

 exchange between the eastern and "whitebelly" popu- 

 lations is also supported by results of analysis of mito- 

 chondrial DNA (Dizon et al. 1991); the presence of 

 unique haplotypes that would be expected in isolated 

 populations was not detected. Given this situation, a 

 relatively higher priority should probably be given to 

 protection of the eastern subspecies on the grounds 

 that it is a distinctly and locally adapted form vul- 

 nerable to depletion and to genetic "swamping" from 

 the larger and less-exploited population to the west. 

 In any case, the understanding presented here of the 

 complex pattern of variation within the "whitebelly" 

 range does not support the present division into north- 

 ern and southern stocks for management on grounds 

 of population distinctness. 



Conservation and management of 

 the eastern spinner dolphin 



There was generally good concordance between the 

 patterns in the individual morphological characters and 

 the relative distributions of the two kinds of schools 

 as identified by modal appearance (Fig. 3). One excep- 

 tion was the region west of the Bight of Panama; here 

 the very high frequency of morphologically interme- 

 diate dolphins and virtual absence of animals of the ex- 

 treme "eastern" appearance indicate that these schools 

 perhaps would have better been identified as "white- 

 belly" schools. Apart from this discrepancy, the con- 

 cordance between the character patterns and the field 

 identification of schools suggests that the modal- 

 appearance method of identification is largely effective. 

 The substantial number of "mixed schools" and uniden- 

 tified schools, however, indicates that the method can 

 still offer difficulties. In addition, it is sometimes dif- 

 ficult to identify a school of spinner dolphins to "stock" 

 (eastern or "whitebelly") before the seine is set. This 

 can be due to poor viewing conditions (time of day, 

 angle of sun, amount of wind, height of swell) or to the 

 behavior of the dolphins (e.g., "laying low," precluding 

 observation of ventral coloration). 



An alternative management scheme based on geo- 

 graphic location would avoid the difficulties of field 

 identification of schools. Although the working hypo- 

 thesis for management of the eastern spinner and the 

 "whitebelly" spinner (eastern/pantropical hybrids or 

 intergrades) has been one of broadly overlapping 

 ranges (e.g., Perrin et al. 1985), the actual transition 



20°- 



"WHITEBELLY 



Figure 19 



Localities of "eastern" and "whitebelly" schools of spinner 

 dolphins included in the present analyses in relation to an 

 example of a possible conservation zone for the eastern 

 subspecies, Stenella longirostris orientalis. 



between the two as indicated by gradients in the mor- 

 phological characters and the field identifications (Fig. 

 3) is quite sharp. An "eastern spinner conservation 

 zone" could be devised that would offer appropriate 

 and unequivocal protection to the unique and coherent 

 gene pool of the eastern subspecies. For example, such 

 a zone bounded by 10°N latitude and 125°W longitude, 

 the approximate latitude and longitude of the major 

 gradients in the morphological characters, would have 

 included 84% of the schools identified in the field as 

 "eastern" (excluding those in the Bight of Panama, 

 discussed above) and only about 5% of those identified 

 as "whitebelly" (Fig. 19). Implementation of an inter- 

 national quota or prohibition for such a zone would be 

 straightforward. The boundaries could be set based on 

 a balance of considerations of conservation and opera- 

 tional practicality and should take into account seasonal 

 and interannual variation in locations of concentrations 

 of schools of the eastern spinner. 



This scheme, of course, is only one of numerous pos- 

 sible alternatives to conservation and management of 



