FISHERY BULLETIN: VOL. 84, NO. 2 



dolphins (Perrin et al. 1976). The variance in gesta- 

 tion time has not been calculated. We can, however, 

 reasonably estimate that 95% confidence limits 

 would span 0.1 yr. From this we estimate the var 

 (T G ) to be 0.000625. 



For the aged sample, 31.1% of the sexually mature 

 specimens (n = 542) were pregnant. For the overall 

 sample during the same years, 31.6% of the sexually 

 mature specimens (n = 2,458) were pregnant and 

 for all aged and imaged mature specimens from 1973 

 through 1981 inclusive (n = 2,979), 31.3% were preg- 

 nant (Table 1). By dividing the fraction of pregnant 

 females by the gestation period (0.958), annual 

 pregnancy rates of 0.325 and 0.330 were obtained 

 for the aged and overall samples, respectively. The 

 var (APR) for the overall sample is 0.0005. 



To determine whether pregnancy rates changed 

 with age, we estimated percent pregnant for four 

 age-class intervals using the 1973-78 and 1981 

 samples combined. Sample size was small for esti- 

 mating age-specific rates with much precision. 

 Nevertheless, we detected neither a sustained in- 

 crease nor a sustained decrease in the percent of 

 pregnant females with age (Fig. 6); the variability 

 in the percent of pregnant females with age can be 

 accounted for by random sampling (x| = 4.6, P > 

 0.50). This result differs from that of Perrin et al. 

 (1976) which indicated a significant reduction in 

 pregnancy rate with age. 



Calving Interval 



Calving interval is an estimate of the mean period 

 between births for mature females. Typically, it is 

 estimated as the inverse of the annual pregnancy 

 rate (Perrin and Reilly 1984). The principal require- 

 ments for calculating the calving interval are un- 

 biased estimates of gestation time and of the frac- 

 tion of mature females that are pregnant. The 

 standard error in an estimate of calving interval (CI) 

 by these methods is approximated by 



SE (CI) = (APR-4) var (APR) 



(Perrin and Reilly 1984). 



Given our calculated APR estimate of 0.330 for 

 the overall sample, the calving interval is 3.03 yr. The 

 standard error of this estimate is about 0.205. Al- 

 though it is difficult to prove that our estimates of 

 the percent of pregnant females are unbiased, sup- 

 port for such a position is given by Barlow's finding 

 that the percent of pregnant females varies little 

 with sampling conditions (including sampling season, 

 geographic area, dolphin school size, and dolphin kill- 

 per-set) (Barlow 1985). However, if annual variability 

 in the percent of pregnant females is important, 

 binomial sampling theory is likely to underestimate 

 our certainty in estimating the percent of pregnant 

 females, APR, and calving interval. Because no 

 significant trends were detected in the percentage 



0.70 r 



0.60 

 0.50 



2 0.40 



o 



§ 0.30 



E 

 O. 



0.20 



0.10 - 



Lactating 



Pregnant 



<15 16-19 20-23 ^24 

 ESTIMATED AGE (years) 



Figure 6— Proportion lactating and proportion pregnant as a function 

 of age for sexually mature female spotted dolphins, in 1973-78 and 1981. 

 Bars represent one standard error from the mean (n = 542). 



254 



