Chivers and Mynck: Comparison of age at sexual maturity for Stenella attenuata 



613 



Figure 1 



The collection location and sample size of female spotted dolphin specimens selected for this study summarized by 

 2°-degree square block. The sample of northern offshore spotted dolphins was selected from north of 1° south and 

 west of the Commission's Yellowfin tuna Regulatory Area I the dashed line), and the sample of southern offshore 

 spotted dolphins was selected from south 1° south. 



men. The associated life history data were also with- 

 held until each specimen was aged. 



ASM and its variance were calculated with the fol- 

 lowing formula (DeMaster, 1984b; Hohn, 1989). 



ASM=x + Xa 



s 2 = X {[(p ; * (1-pMnr-l)}, 



Other reproductive parameters examined include the 

 fraction of the females mature, and the fraction of ma- 

 ture females that were either pregnant or lactating, or 

 simultaneously pregnant and lactating. Trends in these 

 parameters between 1974 and 1988 were tested by 

 using a chi-square test for linear trends (Snedecor and 

 Cochran, 1973), and the mean estimates for each pa- 

 rameter, all years combined, were tested for statisti- 

 cally significant differences with a chi-square test. 



where, x = age of the youngest mature animal, 



j = age class of the oldest immature animal, 

 p, = proportion immature in age class i, 

 n, - sample size for age class i . 



Because population abundance estimates are not avail- 

 able for the geographic areas from which our samples 

 were collected, we substituted year as a correlate for 

 evaluating potential biological indices. We tested for 

 temporal trends in ASM with a linear regression to 

 determine whether the slope was statistically signifi- 

 cantly different from zero. In addition, data for each 

 stock were pooled for all years, and the null hypoth- 

 esis of no difference between estimates of ASM for the 

 two stocks was tested with a Student's ^-test. 



Results 



Average age at attainment of 

 sexual maturity 



ASM was estimated for each year with sufficient data 

 available (Table 1): 1976-1982 and 1984-1988 for the 

 northern offshore stock sample, and 1976, 1979, and 

 1982 for the southern offshore stock sample. The re- 

 gression line fit to the data for the northern offshore 

 stock was not statistically significantly different from 

 zero. We did not fit a regression line to the data from 

 the southern offshore stock because only three annual 

 estimates were available. The pooled estimate of ASM 

 for each stock was 11.1 years (SE=0.236; n=520) for 



