Chivers and Myrick Comparison of age at sexual maturity for Stenella attenuata 



615 



Table 2 



The mean proportion of females by repro- 

 ductive condition for the northern offshore 

 and southern offshore stocks of spotted dol- 

 phin: the proportion of females in the sample 

 that were sexually mature, and the propor- 

 tion of sexually mature females that were 

 pregnant or lactating, or simultaneously 

 pregnant and lactating. 



Stock 



Reproductive 

 condition 



northern 



southern 



Mature 

 Pregnant 

 Lactating 

 Pregnant and 

 lactating 



0.62 

 0.33 

 0.55 

 0.06 



0.69 

 0.40 

 0.47 

 0.06 



Discussion 



Published evidence of density dependence 

 in dolphin species is rare, and results are 

 inconsistent. For example, in Japan, the 

 striped dolphin, Stenella coeruleoalba, is 

 heavily exploited, and the length of the 

 lactation period is reported to have de- 

 clined (Kasuya and Miyazaki 2 ). For ex- 

 ploited spotted dolphins in the western 

 Pacific, a decline in the ASM from 10.1 

 years in the 1961-63 cohorts to 8.6 years 

 in the 1964-66 cohorts was reported, al- 

 though the regression coefficient was not 

 statistically significant (Kasuya, 1985). 

 The decline in ASM for female striped 

 dolphin from 9.7 years for the 1956-58 

 cohorts to 7.4 years in the 1968-70 co- 

 horts was statistically significant (Ka- 

 suya, 1985). On the otherhand, for the 

 spinner dolphin (S. longirostris) in the 

 ETP, the less exploited southern white- 

 belly stock has a shorter lactation period 

 than the more heavily exploited eastern 

 spinner dolphin, the opposite of what 

 would be expected for a compensatory re- 

 sponse. The annual pregnancy rate, how- 

 ever, is lowest for the southern whitebelly 

 stock and highest for the more heavily 



2 Kasuya, T., and N. Miyazaki. 1975. The stock of 

 Stenella coeruleoalba off the Pacific Coast of Ja- 

 pan. Pap. ACMRR/MM/SC-25 pres. at FAO Scien- 

 tific Consultation of Marine Mammals, Bergen, Feb- 

 ruary, 1976, 36 p. 



exploited eastern stock, as expected for density compensation (Perrin 

 and Henderson, 1984). Barlow (1985) also reported differences in 

 the proportion of mature female spotted dolphins (S. attenuata) that 

 were pregnant between the eastern and western Pacific. The lowest 

 proportion of mature females pregnant was observed in the more 

 heavily exploited population, again, contrary to predictions based on 

 the reported exploitation rates and estimates of population abun- 

 dance for the ETP spotted dolphin. Also, for ETP spotted dolphins, 

 the proportion of lactating mature females significantly increased 

 from 46% in 1973 to 69% in 1981 (P<0.05) presumably in response 

 to exploitation; no other statistically significant trends in reproduc- 

 tive parameters were found (Myrick et al., 1986). 



Comparisons with other studies are difficult because in most cases 

 the degree of exploitation relative to K has not been quantified, and 

 techniques to estimate age and ASM vary. In our study, statistically 

 significant temporal trends were not detected for ASM but were 

 detected for some of the reproductive parameters. The increase in 

 the proportion of mature females simultaneously pregnant and lac- 

 tating for the northern offshore stock and the decrease in the pro- 

 portion of mature females in the population for the southern stock 

 both suggest that the populations are growing. The fact that we did 

 not observe the predicted compensatory responses in ASM may be 

 due to one or more factors, including 1) too few data, 2) parameter 

 estimation or measurement error, 3) a time lag in the response, or 

 4) biological differences between the stocks. Too few data and envi- 

 ronmental periodicity (e.g., El Nino) may increase the variability in 

 annual estimates of parameters, and both factors would reduce the 

 ability to detect differences in potential compensatory responses 

 (Goodman, 1984). The order in which parameters respond to changes 

 in population density is not known, but time lags in responses may 

 be expected owing to the late age at which these animals reach 

 sexually maturity and the multi-year breeding cycle characteristic 

 of the spotted dolphin (Goodman, 1981). 



Data for the southern offshore stock of spotted dolphin, in par- 

 ticular, were limited and precluded testing for trends in ASM or the 

 selected reproductive parameters. Although we selected our sample 

 size, a maximum of 50 specimens per year for the northern offshore 

 stock, on the basis of earlier work describing the effect of sample 

 size on the variance for estimates of ASM (Hohn, 1989), our results 

 indicate that larger sample sizes would be required to reduce the 

 variance of the estimates in order to detect small, but biologically 

 significant, changes (e.g., a 1.5-year difference) in ASM. Sample 

 sizes of fewer than 50 per estimate of ASM resulted in too few 

 specimens in the indeterminate age classes (i.e., those age classes 

 with both sexually immature and mature animals) to estimate ASM 

 accurately. 



Another concern when analyzing data collected from a kill is 

 whether the age distribution is representative of the population. All 

 immature age classes appear to be underrepresented in both stocks, 

 especially age classes between 10 and 15 years of the northern 

 offshore stock (Fig. 3). This phenomenon was noted in a previous 

 aging study of ETP spotted dolphin but occurred about five age 

 classes earlier than the one we observed (Barlow and Hohn, 1984). 

 This difference may be a result of different readers estimating age. 



