REILLY and BARLOW: INCREASE IN DOLPHIN POPULATION 



from 11 to 13 yr causes only a 0.01 decrease in 

 ROI. 



DISCUSSION 



The ranges of rate of increase estimated here are 

 potentially useful in bracketing possible ROIs for 

 delphinids in general. For any particular population 

 it should be possible to further narrow the range of 

 likely values of ROI, given available estimates for 

 vital rates. For example, Tursiops truncatus from 

 the northeast coast of Florida reportedly attain sex- 

 ual maturity at 12 yr on the average (Sergeant et 

 al. 1973) and have a 12-mo gestation period (Essa- 

 pian 1963), giving an estimated age at first birth of 

 13 yr. Knowledge of this single parameter can nar- 



row consideration to Figure 4. Here the estimated 

 range in ROI is up to a maximum of 1.05, for the 

 extreme case of an average calving interval of 2 yr, 

 and noncalf survival >0.96. Additional knowledge 

 of, say, minimal calving interval for Tursiops could 

 further narrow consideration to one of the three 

 panels of Figure 4, and establish minimal survival 

 rates for positive growth rates, or the maximum rate 

 of increase possible, given the above constraints on 

 age at first birth and calving interval. 



We assume that the ranges defined here also en- 

 compass the limits within which vital rates for any 

 one dolphin species might change in response to 

 changes in population density. This obviously entails 

 making simplistic assumptions about density depen- 

 dence in vital rates, and therefore in rate of increase. 



of calf and noncalf survival rates, for the following combinations of calving interval and age at first reproduction: 



c 

 O 



O 

 Q. 

 O 



CD 



■♦-• 

 CO 



DC 



"cO 



> 



> 



CO 



"co 

 O 



0.95 0.97 •■£ 



0.95 0.97 



0.85 0.90 0.95 0.97 



Noncalf Survival Rate (proportion) 



Figure 3.— First reproduction of dolphin age class 11 yr: a) 2-yr 

 calving interval (upper panel); b) 3-yr calving interval (middle 

 panel); c) 4-yr calving interval (lower panel). 



C 



o 



o 

 a 

 o 



CD 

 CO 



DC 



"c0 



> 

 ■> 



Z5 



CO 



"cO 

 O 



0.95 0.97 



0.96 



0.85 



0.90 



0.95 0.97 



0.85 0.90 0.95 0.97 



Noncalf Survival Rate (proportion) 



Figure 4.— First reproduction of dolphin age class 13 yr: a) 2-yr 

 calving interval (upper panel); b) 3-yr calving interval (middle 

 panel); c) 4-yr calving interval (lower panel). 



531 



