REILLY and BARLOW: INCREASE IN DOLPHIN POPULATION 



>0.97 are untenable as mean per-capita survival 

 rates. 



Ranges in Calf Survival Rate 



Again little information is available on calf sur- 

 vival for dolphins. Kasuya (1976) estimated a juven- 

 ile survival rate that was higher than that of adults, 

 based on a balance equation. His methods assume 

 that populations are neither growing nor declining, 

 and he did not show that this assumption was met. 

 Also his juvenile period included all sexually im- 

 mature age classes. The overwhelming body of 

 evidence from terrestrial mammals is that very early 

 juvenile mortality is higher than adult mortality 

 (Spinage 1972; Caughley 1977; Siler 1979). Even 

 human populations had a first year survival rate of 

 <0.88 prior to modern antibiotics (Fruehling 1982, 

 data for U.S. circa 1900). An upper limit on calf sur- 

 vival rates was generated by assuming a calf is ab- 

 solutely dependent on its mother for 1 yr. A calf has 

 the same risk of dying as an adult, plus the addi- 

 tional risk of dying of starvation if its mother dies 

 before completing 1 yr of lactation. The upper limit 

 on calf survival would thus equal the square of the 

 adult survival rate. The lower limit on calf survival 

 rates was chosen as 0.50, a value that seems typical 

 of pinnipeds (Smith and Polacheck 1981) and long- 

 lived terrestrial mammals (Spinage 1972). 



Fecundity-Related Rates 



Ranges in Calving Interval 



Observed calving intervals for dolphins general- 

 ly range from 2 to 4 yr (Perrin and Reilly 1984); con- 

 sequently, we have used this range in our computa- 

 tions. Intervals reported for killer whales (which are 

 also delphinids, but not "dolphins") are considerably 

 longer, up to 8 yr (e.g., Jonsgard and Lyshoel 

 1970). 



The literature includes reports of calving inter- 

 vals <2 yr for dolphins. These reports do not appear 

 to be valid. Reevaluation of data for three of these 

 reports 4 indicates that sampling was biased to- 

 ward pregnant females (Perrin and Reilly 1984), a 

 result of what may be a general tendency for 



Unpubl. manuscr. Southwest Fisheries Center La Jolla Labora- 

 tory, National Marine Fisheries Service, NOAA, 8604 La Jolla 

 Shores Drive, La Jolla, CA 92038. 



4 Three reported cases of dolphin calving intervals <2 yr, later 

 found to be biased due to age and sex segregation, are Black Sea 

 Delphinus delphis and Tursiops truncatus (KJeinenberg 1956) and 

 Western Pacific Stenella coeruleoalba (Miyazaki and Nishiwaki 

 1978). 



dolphins to segregate by age/sex groupings 5 . 



The remaining reports of calving intervals <2 yr 

 are from very small sample sizes. 6 Gestation periods 

 for dolphins are at minimum 10 mo, and intraspecific 

 variation is small. Reported lactation periods range 

 from 1 yr to over 2 yr (Perrin and Reilly 1984). Sum- 

 ming these two periods gives another indication that 

 dolphin calving intervals are not likely to be <2 yr. 



An exception to the 2-yr minimum calving inter- 

 val would possibly be in a population experiencing 

 very high calf mortality, causing premature cessa- 

 tion of lactation, and allowing females the opportun- 

 ity to begin a new calving cycle (assuming there was 

 no seasonality to breeding which could require a 

 resting period before the next breeding season). To 

 include consideration of this case we would need to 

 devise an arbitrary function relating low calf sur- 

 vival to short calving intervals. The net result would 

 again be low rates of increase. To avoid such com- 

 plications we have simply used 2 yr as the minimum 

 average calving interval. 



Ranges in Age at First Birth 



The available data suggest a range in age at at- 

 tainment of sexual maturity of 6 to 12 yr for dolphins 

 (Perrin and Reilly 1984). Early reports of Black Sea 

 common dolphins, Delphinus delphis, attaining sex- 

 ual maturity at an average of 3 yr (Kleinenberg 1956) 

 are almost certainly due to faulty age determina- 

 tion 7 . Because of the recent findings for S. attenuata 

 from the ETP (Myrick et al. 1986), we considered 

 the ages at first birth up to 15 yr. In our formula- 

 tion of the Leslie model, if females mature and first 

 conceive at an average age of 10 yr, the first nonzero 

 fecundity would be in age class 11 (Table 1). 



6 Hohn, A. A., and M. D. Scott. 1983. Segregation by age in 

 schools of spotted dolphins in the eastern tropical Pacific. Fifth 

 Biennial Conf. Biol. Mar. Mammals, Abstr., p. 47. 



6 Henderson, J. R., W. F. Perrin, and R. B. Miller. 1980. Rate 

 of gross annual reproduction in dolphin populations (Stenella spp. 

 and Delphinus delphis) in the eastern tropical Pacific, 1973-78. 

 Southwest Fisheries Center, La Jolla, California, Admin. Rep. 

 LJ-80-02, 51 p. 



7 Myrick, A. C. Jr., Southwest Fisheries Center La Jolla Labor- 

 atory, National Marine Fisheries Service, NOAA, 8604 La Jolla 

 Shores Drive, La Jolla, CA 92038, pers. commun. June 1984. 



Table 1 . — Parameters used and values included in the computa- 

 tion of rates of increase in dolphin population size. 



Parameter 



Values 



Calving interval 

 Age at first birth 

 Calf survival rate 

 Noncalf survival rate (Sa) 



2 yr 3 yr 4 yr 



7 yr 9 yr 11 yr 13 yr 15 yr 



0.50 0.52 0.54 . . . (Sa) 2 



0.850 0.855 0.860 0.865 . . . 0.970 



529 



