128 



Fishery Bulletin 92(1). 1994 



Birth Months 

 Figure 6 



Cumulative distribution of back-projected striped dolphin, Stenella 

 coeruleoalba, birth dates (solid squares) and those predicted by a uni- 

 form distribution of births (open squares). 



D □ 



Length (cm) 



Figure 7 



Length-frequency distributions for specimens of striped dolphin 

 Stenuella coeruleoalba, (> 180 cm) taken incidentally in purse-seine 

 fishing in the eastern tropical Pacific and striped dolphins sampled 

 photogrammetrically that are > 180 cm. Samples from northern and 

 southern regions are combined in this figure. 



Discussion 



We found no significant differences in our length 

 samples of striped dolphins from the northern and 

 southern regions to support a recommendation that 

 they be managed as separate stocks. This must be 

 tempered by the fact that length differences of a 



scale not detectable in our 

 sample, i.e. < 4 cm, could exist. 

 The case for two stocks is also 

 weakened by the distribution of 

 sightings of this species from re- 

 cent research vessel surveys 

 (Wade and Gerrodette, in press). 

 These data indicate that, al- 

 though a hiatus in striped dolphin 

 distribution exists in the typically 

 tropical (high temperature, low 

 salinity) inshore habitat centered 

 around lat. 15° N, there appears 

 to be a broad avenue for movement 

 between the northern and southern 

 regions in the upwelling modified 

 habitat east of long. 110° West (Au 

 and Perryman, 1985; Reilly, 1990). 

 When we compared our sample 

 of lengths for "adult females" and 

 dolphins > 180 cm with data from 

 specimens killed incidentally in 

 purse-seine fishing, we found that 

 the means from the photogram- 

 metric sample were significantly 

 larger (by about 3-6 cm). This 

 does not seem unreasonable at 

 first glance because our measure- 

 ments to the trailing edge of the 

 flukes rather than to the fluke 

 notch introduces a positive bias in 

 the photogrammetric data of 

 about 2-2.5 cm. Also, the "adult 

 female" category probably in- 

 cludes only those females who 

 have carried and given birth to a 

 live calf, thus eliminating the 

 younger, presumably smaller, fe- 

 males who are physiologically 

 adult but have not yet had a suc- 

 cessful pregnancy. However, these 

 results for adult females are con- 

 trary to previous comparisons of 

 photographic and specimen data 

 for northern and central common 

 dolphins (Perryman and Lynn, 

 1993) and eastern spinner dol- 

 phins (Perryman, unpubl. data). 

 Since the photogrammetric data for all of these taxa 

 were collected in the same manner, it seems likely 

 that the difference between the two striped dolphin 

 samples reflects some form of selectivity in either 

 or both sampling systems. 



The schools of striped dolphins that we photo- 

 graphed showed a pattern of segregation by length 



