Abstract.- The spinner dolphin 

 Stmella langirostris is widely distrib- 

 uted in the eastern tropical Pacific 

 Ocean. Geographic patterns in 30 cra- 

 nial features were determined from 

 246 museum specimens grouped in- 

 to 25 5° latitude-longitude blocks. 

 Statistically significant sexual dimor- 

 phism was demonstrated for one-half 

 of the cranial characters, with males 

 generally being larger. ANOVAs, as 

 well as principal components, canon- 

 ical variates, and cluster (UPGMA 

 and function-point) analyses demon- 

 strated geographic variation in all 

 characters. Patterns of geographic 

 variation in morphology were evalu- 

 ated for all S. longirostris specimens 

 using Mantel tests and matrix corre- 

 lations; 20 of 30 characters showed 

 significant "regional patterning," 

 while most (25 of 30) exhibited "local" 

 patterning. The latitude-longitude 

 block with specimens of S. I. centro- 

 americana was distinctive in a num- 

 ber of features. Also, eastern spin- 

 ner dolphins (S. /. orientalis) were 

 smaller than spinners found to the 

 south, southwest, or west. Many of 

 the cranial characters exhibited a 

 concentric pattern of geographic 

 variation similar to that found by 

 previous investigators for several ex- 

 ternal characters. Hawaiian speci- 

 mens are the largest incorporated 

 into this study and, typically, are 

 more like those from southern local- 

 ities than animals from geographical- 

 ly closer blocks. The association be- 

 tween morphological characters and 

 13 environmental measures was as- 

 sessed with Mantel tests and product- 

 moment correlations, revealing sta- 

 tistical concordance of morphological 

 patterns for a number of cranial char- 

 acters with those for water depth, 

 sea surface temperature in January 

 and July, surface salinity, thermo- 

 cline depth, and surface dissolved 

 oxygen. Several of these environ- 

 mental variables manifest the same 

 distributional pattern found in many 

 of the cranial features. 



Geographic variation in 

 cranial morphology of spinner 

 dolphins Stenella longirostris in 

 the eastern tropical Pacific Ocean 



Michael E. Douglas 



Oklahoma Biological Sun/ey and Department of Zoology 

 University of Oklahoma, Norman. Oklahoma 73019 

 Present address: Department of Zoology and Museum 

 Arizona State University, Tempe, Arizona 85287 



Gary D. Schnell 

 Daniel J. Hough 



Oklahoma Biological Survey and Department of Biology 

 University of Oklahoma, Norman, Oklahoma 73019 



William F. Perrin 



Southwest Fisheries Science Center, National Marine Fisheries Service, NOAA 

 PO Box 271, La Jolla, California 92038 



Manuscript accepted 9 December 1991. 

 Fishery Bulletin, U.S. 90:54-76 (1992). 



Information on geographic variation 

 of dolphins in the eastern tropical 

 Pacific is of intrinsic scientific inter- 

 est, but also has practical implica- 

 tions because fishermen in the region 

 kill dolphins in the course of purse- 

 seining for yellowfin tuna (Allen 

 1985). Tuna in the region associate 

 with schools of dolphins, primarily 

 Stenella spp. and Delphinus delphis, 

 and the fishermen set their nets on 

 the schools to capture the tuna below 

 them. In the process, many dolphins 

 die, as many as 80,000-125,000 aimu- 

 ally in recent years (Hall and Boyer 

 1988, 1989, 1990). The U.S. Govern- 

 ment has used a series of manage- 

 ment units, or stocks, in regulating 

 this exploitation of the dolphins by 

 U.S. vessels. For the spinner dolphin, 

 these have been the eastern spinner, 

 Costa Rican spinner, northern white- 

 belly spinner, and southern white- 

 belly spinner stocks (Perrin et al. 

 1985). These divisions are based on 

 morphology, including body length 

 and shape, color pattern, shape of the 

 dorsal fin, and cranial characters. 

 The Costa Rican form occurs close to 



the coast of Central America and is 

 relatively large, with relatively long 

 beak, erect to forward-canted dorsal 

 fin, and monotonic gray coloration. 

 The eastern form is smaller, with 

 shorter beak; it also has the erect or 

 canted fin and is gray overall, but 

 with light patches in the axillary and 

 genital areas. The whitebelly forms 

 have a tripartite color pattern of dark 

 gray, light gray, and (ventrally) white, 

 and the dorsal fin is highly variable, 

 ranging in adults from falcate to 

 erect. The northern and southern 

 stocks were divided based on modal 

 differences in cranial measurements; 

 the boundary is at the Equator. The 

 eastern spinner and northern white- 

 belly spinner stocks overlap broadly; 

 overlap between the eastern spinner 

 and southern whitebelly spinner is 

 very slight (Perrin et al. 1985). Dol- 

 phins killed in the fishery are iden- 

 tified to stock based on the modal ap- 

 pearance of adults in the school and, 

 in the case of the two whitebelly 

 stocks, location. 



Most recently, Perrin (1990) de- 

 scribed three subspecies of Stenella 



54 



