PERRIN ET AL.: GROWTH AND REPRODUCTION OF THE EASTERN SPINNER DOLPHIN 



TABLE 8. — Estimated lengths of postreproductive phases, under two methods of es- 

 timating length of lactation, of the eastern spinner dolphin based on single-year sam- 

 ples, 1971-75, and on pooled samples for all years, with estimates of pregnancy rate and 

 calving interval based on lactation estimate 1. 



TABLE 9.— Estimates of length of lactation in the eastern spinner dolphin based on the 

 "cumulative calf length/lactating females" method (see text), for 6 single-month samples 

 and for 1973-75 by year and pooled. 



1 1ncludes mature females (5=165 cm) without lactation data prorated to lactating and nonlactatmg based on 

 proportions in sample with lactation data. 



accompanying calves are more likely to be cap- 

 tured and killed in the net because of limitations 

 imposed on endurance of the mother by that of the 

 calf, certainly lower than adult endurance. The 

 second method could yield erroneous estimates if 

 1) nursing calves were overrepresented in the 

 samples, or, conversely, 2) lactating females were 

 underrepresented. Recent data for S. attenuata (J. 

 E. Powers pers. commun.) indicate that small 

 calves are probably overrepresented in small 

 single-set samples of that species. This may be 

 caused by the above-mentioned lesser stamina of 

 calves in the energetically stressful purse seine 

 chase, capture, and release sequence. The lesser 

 year-to-year variation in the estimates yielded by 

 Method 2 also supports the idea that these may be 



better estimates. If neonates are overrepresented 

 in the samples, however, then the percent preg- 

 nant must be underestimated to some unknown, 

 but small, degree. In view of these considerations, 

 both the proportion-in-phase estimate and the 

 cumulative calf length/lactating female estimate 

 are used below as alternatives in estimating preg- 

 nancy rate, calving interval, and gross annual re- 

 production, and we conclude that the true length 

 of lactation in an unperturbed birth-to-weaning 

 period can be assumed to lie somewhere between 

 the estimates yielded by the two methods. 



Annual pregnancy rate by Method 1 was calcu- 

 lated by division of the proportion pregnant (Fig- 

 ure 29) by the length of gestation 1 0.875 yr). The 

 reciprocal of annual pregnancy rate is the esti- 



745 



