Perryman and Lynn: Stock and school structure of Stenella coeruleoalba 



125 



act date of birth for each dolphin but rather to exam- 

 ine the distribution of birth dates, based on the same 

 assumptions, from the two regions. We used 

 Kupier's modification of Kolmogorov's test for com- 

 parisons of circular distributions (Batschelet, 1965) 

 to compare the calculated distribution of birth dates 

 with a uniform distribution. 



Comparisons with specimen data 



We conducted four tests to compare the sample of 

 photogrammetric lengths with data collected from 

 striped dolphins killed incidentally in purse-seine 

 fishing in the ETP (Perrin et al., 1976). The data 

 from specimens included the information published 

 by Perrin et al. ( 1985) and a small set of data from 

 dolphins killed since 1985. T-tests were used to com- 

 pare the mean length of "adult females" with the 

 mean length of adult female specimens and with the 

 mean length of lactating adult female specimens. We 

 also compared the mean (f-test) and shape 

 (Kolmogorov-Smirnov test) of the photogrammet- 

 rically determined length distribution of striped 

 dolphins > 180 cm with data from specimens > 180 

 cm in length. 



School structure 



Examination of the structure of schools of striped 

 dolphins captured in the drive fishery in Japan has 

 revealed a distinct pattern of segregation based on 

 sex, maturity, and length (Miyazaki, 1977, 1984; 

 Miyazaki and Nishiwaki, 1978). Researchers have 

 categorized these schools as adult, juvenile, or mixed 

 depending on the proportion of juvenile dolphins 

 (excluding calves) captured. In these studies, length 

 (<174 cm) or age (<1.5 years) was used as the crite- 

 rion for eliminating nursing calves from the sample; 

 the remainder of the dolphins was determined to be 

 juvenile or adult by direct examination of the go- 

 nads. 



We examined the length distributions for the pho- 

 tographed schools to see if an analogous pattern of 

 segregation in schools from the eastern Pacific was 

 detectable. We divided our samples into two length 

 categories which we labeled juvenile or adult. The 

 minimum length for the juvenile category was set 

 at 165 cm to eliminate nursing calves as described 

 above. We selected this minimum value because 1) 

 length at birth for striped dolphins from the ETP is 

 apparently about 10 cm shorter than that reported 

 from the western Pacific (Miyazaki, 1977; Gurevich 

 and Stewart, 1979 3 ), and we assumed that the dif- 

 ference in the average length at weaning was ap- 

 proximately the same; 2) dolphins larger than 165- 



170 cm in length were very rarely found swimming 

 in the characteristic cow/calf configuration we see 

 in our photographs. 



We selected 195 cm as the upper bound for the 

 juvenile category because this appears to be about 

 the minimum size for adult male striped dolphins 

 that have been killed in the ETP tuna purse-seine 

 fishery (Perrin et al., 1985). This value was keyed 

 to male length data because the studies of school 

 structure from Japan indicated that a disproportion- 

 ate number of the dolphins captured in juvenile 

 schools were males (Miyazaki and Nishiwaki, 1978). 

 Thus dolphins in each school were categorized as 

 juvenile if they were between 165 and 195 cm in 

 length and as adult if they were > 195 cm in length. 

 The goal in this classification scheme was to create 

 one category that would be composed of mostly ju- 

 venile and young adult dolphins and another that 

 would include mostly adult animals. 



We used chi-square analysis to test the hypoth- 

 esis that the number of dolphins in the two catego- 

 ries in our schools was independent of school. For 

 this analysis, we eliminated schools from which we 

 had measured less than 20% of the school or fewer 

 than 17 dolphins. The second criterion was estab- 

 lished to minimize the number of predicted values 

 in the chi-square analysis that were less than five. 

 Application of these criteria reduced our sample to 

 21 schools for this test. Because the selection of 195 

 cm for the cut-off between the two size categories 

 probably includes more adult females in the juve- 

 nile category than males, we decreased the limit to 

 190 cm and repeated the chi-square test. We also 

 conducted a regression analysis to determine 

 whether the proportion of the measured sample in 

 the juvenile category was related to school size. 



With the exception of the power analyses and 

 birth date comparison which were done by hand, all 

 tests presented in this report were performed with 

 the program StatView developed by Abacus Con- 

 cepts (Berkeley, CA). Unless noted otherwise, tests 

 were considered significant for P values < 0.05. 



Results 



Regional comparisons 



We compared the average length of striped dolphins 

 from the northern and southern regions and found 

 no significant differences between the samples 

 (Table 1; Fig. 3). In tests for differences in mean 

 lengths of "adult females" (Fig. 4), no differences 

 were found between the regions. Although none of 

 the differences was significant, means of the 



