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Fishery Bulletin 89(3), 1991 



annual variation in distribution; i.e., the northern 

 boundary of the main "whitebelly" distribution may 

 fluctuate year to year by about 5 degrees of latitude. 

 This would be consistent with some of the results of 

 the present study; for example, the intermediate per- 

 centages of the extreme "eastern" and "whitebelly" , 

 character states in the band between 5° and 10° N 

 (Figs. 9-10, 12-15). 



Another possible explanation of the latitudinal pat- 

 tern has to do with distribution of tuna-fishing effort. 

 During the period 1976-79, the Inter-American Trop- 

 ical Tuna Commission set a regulatory demarcation line 

 at 5°N, between 95° and 110°W. During this period, 

 when an annual areal quota of yellowfin tuna for the 

 region north of the line was filled, the international 

 fleet was forced to fish south of the line or to the west 

 (Peterson and Bayliff 1985). A similar line was in force 

 in 1973-75 at 3°N. The subsidiary latitudinal band of 

 distribution records south of 5°N apparent in the spin- 

 ner dolphin data is also present in data for the pan- 

 tropical spotted dolphin S. attenuata, the principal 

 target species of the tuna fishery (Perrin et al. 1983). 

 Illustrations of the effect of the regulatory regime on 

 the distribution of fishing effort are given in Punsley 

 (1983); the effect was particularly pronounced in 1978. 



Some of the morphological results speak against the 

 hypothesis of large-scale latitudinal movements. Varia- 

 tion among sub-areas of the "whitebelly" range is 

 discordant. For example, for the character "ventral 

 field" (Figs. 9, 10), the far-western blocks, far-southern 

 blocks, and southwestern blocks are similar in having 

 relatively high frequencies of the Code-5 character 

 state (most "whitebelly-like"), while for the "cape" 

 (Fig. 11), the southwestern region differed from the 

 west and south in having low frequencies of observed 

 cape (they were most like the animals immediately to 

 the north). For male/female dorsal fin shape (Figs. 12, 

 13), the southern blocks had higher frequencies of the 

 falcate fin than did the western blocks. While in color 

 pattern and in cranial characters (Schnell et al. 1985), 

 the westernmost blocks were most like the southern 

 blocks, in body length and variability of body length 

 (Figs. 17, 18) they were most like the northern and cen- 

 tral blocks. The blocks just north of the Equator were 

 remarkable in having very high frequencies of females 

 intermediate between the most "whitebelly" state and 

 the most "eastern" state in combined character scores 

 (Fig. 15); the highest frequencies of these in males were 

 in the westernmost blocks. In body length in both males 

 and females (Fig. 17), the steepest gradient was at 

 about 5°N, whereas in combined ventral field, cape and 

 dorsal fin (Figs. 14, 15), it was at about 10°N. In cape 

 (Fig. 11) and body length (Fig. 17), the dolphins in the 

 Panama Bight and just to the south were most like 

 those in the core "eastern" region (lacking visible cape 



and small in body size), but in ventral field (Fig. 9) they 

 were intermediate between those to the north and 

 south, having approximately equal frequencies of Code 

 1, Code 5, and summed Codes 2-4. This complex patch- 

 work of variation suggests that there is not a large 

 amount of movement between the various regions and 

 that the year-to-year variation in the collection local- 

 ities (Figs. 3, 4) reflects mainly movements of the tuna 

 vessels and perhaps the tuna. 



The complex geographical pattern of variation in 

 the zone of intergradation/hybridization ("whitebelly 

 spinners") is consistent with the limited information 

 from tag returns (Perrin et al. 1979b), which suggests 

 a home range of a diameter of hundreds rather than 

 thousands of kilometers. This is also supported by 

 the stability of the core eastern region between years 

 and patterns of reproductive seasonality that suggest 

 areal structure within the eastern region (Barlow 

 1984). 



However, there may be mid-scale seasonal move- 

 ments within an overall pattern of interannual stabil- 

 ity, especially in the "whitebelly" areas outside the core 

 range of the eastern spinner. Based on census surveys 

 from dedicated research vessels, Reilly (1990) reported 

 increases in abundance west of 120°W along 10°N 

 coincident with seasonal shoaling of a thermocline 

 ridge. Because of interannual variation in distribution 

 of tuna fishing effort (discussed above), the coarse 

 geographic strata used, and the relatively small sam- 

 ple sizes for the outlying strata, the present analyses 

 probably could not be expected to detect such shifts, 

 although the shifts might be expected to blur the 

 perceived patterns of morphological gradients. 



Taxonomic status and management of 

 the "whitebelly" spinner dolphin 



The "whitebelly" spinner has been managed as two 

 "stocks" divided at the Equator, the northern white- 

 belly spinner and the southern whitebelly spinner, im- 

 plying effective reproductive isolation between them 

 and from the eastern spinner (e.g., Hall and Boyer 

 1990). The complex pattern of discordant geographic 

 variation and the relatively higher standard deviations 

 (of body length) in the range of the "whitebelly" spin- 

 ner are typical of a hybrid zone resulting from what 

 Mayr (1970) called allopatric hybridization, or "the for- 

 mation of a secondary zone of contact and of partial 

 interbreeding between two formerly isolated pop- 

 ulations that had failed to acquire complete reproduc- 

 tion isolation during the preceding period of geographic 

 isolation." The discordant pattern speaks against the 

 earlier hypothesis of a locally adapted "whitebelly" 

 population sharing features with populations to the east 

 and west and defined by morphological clines. An 



