Abstract. — Variation in color 

 pattern, dorsal fin shape, and body 

 length exhibit sharp north/south gra- 

 dients centered at about 5-10°N lati- 

 tude and east/west gradients at 

 about 120-125°W longitude. A con- 

 servation zone with boundaries in 

 these regions would provide protec- 

 tion for the morphologically unique 

 eastern spinner dolphin Stenetta longi- 

 rostris orientalis. A radial pattern of 

 geographic variation in the eastern 

 Pacific and a complex pattern of dis- 

 cordant variation outside the core 

 range of S. I. orientalis suggest that 

 the present separate management of 

 "whitebelly" spinner dolphins (which 

 comprise a broad zone of hybridiza- 

 tion/intergradation between S. I. ori- 

 entalis to the east and the pan trop- 

 ical spinner dolphin S. I. longirostris 

 to the west and southwest) north and 

 south of the Equator may not be 

 justified on the grounds of conserva- 

 tion of distinct populations. 



Geographic Variation in 

 External Morphology of the 

 Spinner Dolphin Stenella longirostris 

 in the Eastern Pacific and 

 Implications for Conservation 



William F. Perrin 



Priscilla A. Akin 



Jerry V. Kashiwada 



La Jolla Laboratory, Southwest Fisheries Science Center 

 National Marine Fisheries Service, NOAA 

 PO. Box 271, La Jolla, California 92038-0271 



Manuscript accepted 15 March 1991. 

 Fishery Bulletin, U.S. 89:411-428 (1991). 



Spinner dolphins in the eastern trop- 

 ical Pacific vary geographically in 

 color pattern (Perrin 1972), in exter- 

 nal and skeletal size and shape (Per- 

 rin 1975, Perrin et al. 1979a, Schnell 

 et al. 1982 and 1985, Douglas et al. 

 1986), and in reproductive seasonal- 

 ity (Barlow 1984). Perrin (1975) and 

 Perrin et al. (1979a) described sev- 

 eral forms that have served as stock 

 units for management of populations 

 of dolphins killed incidentally in the 

 international purse-seine fishery for 

 yellowfin tuna in the region (Perrin 

 et al. 1985). Kills in recent years have 

 been on the order of 100,000 annual- 

 ly. The management units have been 

 known as the "Costa Rican," "east- 

 ern," "northern whitebelly," and 

 "southern whitebelly" spinner dol- 

 phins. The geographic ranges of the 

 eastern and whitebelly forms overlap 

 broadly (Perrin et al. 1985). 



The model used in studies and, to 

 a lesser degree, management of the 

 species in the region has been one of 

 several discrete, more-or-less repro- 

 ductively isolated stocks with some 

 geographic overlap. However, as 

 more morphological data have ac- 

 cumulated, a different picture has 

 emerged. For example, while the 

 boundary between the present north- 

 ern and southern whitebelly manage- 

 ment stocks was drawn latitudinally, 



the overall pattern of variation in the 

 eastern Pacific now appears radial 

 (Schnell et al. 1982 and 1985, Perrin 

 et al. 1985). Further, the results of 

 a genetic study indicate substantial 

 exchange of at least mitochondrial 

 DNA between and among the east- 

 ern and whitebelly forms (Dizon et al. 

 1991). Most recently, Perrin (1990) 

 described two subspecies, Stenella 

 longirostris centroamericana and 

 S. I. orientalis, based on the Costa 

 Rican and eastern forms, respective- 

 ly, and concluded that the whitebelly 

 "forms" (Fig. 1) collectively comprise 

 a broad zone of intergradation or 

 hybridization between the eastern 

 subspecies and a third subspecies to 

 the west and southwest, the pan- 

 tropical spinner dolphin S. I. longi- 

 rostris, a form occurring in the cen- 

 tral and western Pacific, Indian and 

 Atlantic Oceans (Perrin et al. 1981; 

 Gilpatrick et al. 1987). 



In this context, the purpose of the 

 present study was to reexamine vari- 

 ation in external morphology on a 

 finer geographical scale than had 

 been used in earlier studies. Speci- 

 mens previously identified as "Costa 

 Rican," "eastern", or "whitebelly" 

 based on the modal appearance of 

 adult animals in the schools from 

 which they came were pooled in the 

 analyses. In this way we hoped to 



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