FISHERY BULLETIN: VOL. 75, NO. 4 



mate of average calving interval. For the Method 2 

 estimates, calving interval was calculated by 

 summing the phases, taking into consideration 

 overlapping cycles by adjusting the effective 

 length of lactation downward by a factor equal to 

 the percentage of lactating females also pregnant. 

 Lacking an independent estimate of the length of 

 the "resting" phase, the Method 1 estimate for 

 1973-75 was used as a reasonable approximation 

 in the Method 2 calculations of length of cycle and 

 calving interval. 



CHANGES IN REPRODUCTIVE RATES 

 WITH AGE. — Pregnancy rate in the sample de- 

 creases with age after about 12 layers (8.0, 11.5, or 

 10.2 yr, depending upon whether layer Hypothesis 

 I, II, or III is applied, respectively), while lactation 

 rate rises (Figure 30). Assuming that the samples 

 are representative of the population, this may 

 mean that 1) pregnancy rate decreases with age in 

 the individual, or 2) that older females belong to 

 older cohorts in which reproductive rates have 

 been lower than in younger cohorts since recruit- 

 ment to the breeding population. The former 

 seems most likely; it would appear that older 

 females have fewer calves and nurse them longer. 

 The same result was obtained for S. attenuata in 

 the eastern Pacific (Perrin et al. 1976). 



Sex Ratios 



Sex ratios are at or very near parity at birth and 

 overall in the samples (Table 10), but there were 

 slightly more females than males in adults in the 

 samples for each of the 3 yr 1973-75, a result 

 consistent with that encountered in S. attenuata 

 (Perrin et al. 1976) but less pronounced. 



r 



9 10 II 12 13 14 15 16 17 22 

 GROWTH LAYERS (no.) 



FIGURE 30. — Change in reproductive rates with age in the east- 

 ern spinner dolphin. Sample sizes in parentheses. 



Gross Annual Reproduction 



Estimates of gross annual reproductive rates 

 can be made based on 1973-75 samples, the 3 yr 

 for which the samples are large and nonselected 

 with respect to age and sex structures (Table 11). 

 It must be noted that if, as discussed above, small 

 calves are overrepresented in small samples 

 (which make up most of the aggregate sample), 

 then the proportion of total females which are 

 reproductive and pregnancy rate (for Method 1) 

 are underestimated and the proportion of total 

 sample female is overestimated, all to an un- 

 known, but probably small, degree. Standard er- 



TABLE 10. — Sex ratios in 126 fetuses and 2,261 neonatal-to-adult eastern spinner dolphins. Fetal samples 



limited to fetuses longer than 15 cm. 



'Includes six specimens for which length data not available. 



746 



