Perrin et aL Geographic variation in morphology of Stenella longirostns 



423 



150" 140* 130* 120* 



o* wo* 90* eo* ro* 



150" 1*0- 130* 120* 



100* 90* 80* 70* 



Figure 18 



Standard deviations (in cm) of body length for adult male and 

 female spinner dolphins, by area. 



more closely associated in average length with those 

 in the east than those in the far south (below 5°S). 

 Body length is more variable in the regions of greater 

 body length, in both males and females (compare 

 Figures 17 and 18; Table 2). 



Discussion 



Year-to-year variation and home range 



The patterns of distribution of the eastern spinner 

 and the "whitebelly" intergrades/hybrids appear to be 

 relatively stable year to year, although analysis of the 

 present data is complicated by the fact that they come 

 from incidental kills in a very mobile pelagic tuna 

 fishery. The cumulative maps of specimen localities 

 (Figs. 3, 4) show the locations where tuna boats made 

 purse-seine sets that killed spinner dolphins. It reflects 

 only co-occurrence of tuna boats, spinner dolphins, and 

 yellowfin tuna. In addition, the spinner dolphin is not 

 the main target dolphin species in the fishery; the pan- 

 tropical spotted dolphin Stenella attenuata is relative- 



ly more often found together with tuna. The sets on 

 spinner dolphins are largely sets on mixed schools of 

 spotted and spinner dolphins; in some areas, especial- 

 ly in the southern and southwestern regions of the 

 study area, spinner dolphins are not often associated 

 with spotted dolphins and seldom are with tuna (Au and 

 Perryman 1985). 



Not all regions represented by capture localities in 

 the cumulative maps (Fig. 3) are represented in the 

 maps for individual years (Fig. 4). Possible reasons for 

 this are (1) the dolphins moved around from year to 

 year, (2) the tuna vessels moved around, (3) the 

 yellowfin tuna moved around, and, most likely, (4) some 

 combination of (1), (2), and (3). For some years (e.g., 

 1985-87), the samples included no spinner dolphins 

 from south of the Equator, but dolphin research survey 

 cruises in that region in 1986 and 1987 made numerous 

 sightings (Holt and Jackson 1987, 1988). That par- 

 ticular hiatus in the maps of localities for at least some 

 years therefore represents the absence of tuna vessels 

 and possibly yellowfin tuna associated with dolphins, 

 not an absence of spinner dolphins. On the other hand, 

 the pattern of paired latitudinal bands of high density 

 of localities for "whitebelly" schools in the cumulative 

 map (Fig. 3) was not apparent in annual plots. It re- 

 sulted mainly from relatively large numbers of speci- 

 mens from the area of the upper band in 1975 and 

 1984-87 and from the lower band in 1977 and 1978. 

 "Eastern" schools were sampled from the region of the 

 northern band in all years. Again, research surveys in 

 1986 and 1987 sighted comparable numbers of "white- 

 belly" spinner schools in the two regions. In this case, 

 it is possible that the pattern or degree of association 

 of tuna with the two forms varied interannually. How- 

 ever, the numbers of sightings of spinner dolphins dur- 

 ing all the research cruises were relatively small, and 

 it remains possible that some portion of the pattern- 

 ing in the cumulative distribution represents inter- 



