are mature also appears to be higher in the southern 

 stock than in the northern stock. Given the depen- 

 dence of this parameter on sampling conditions, lit- 

 tle confidence should be placed on this result. The 

 direction of the difference (more immatures in the 

 northern population) is consistent with a higher 

 population growth rate in the north. Work is in pro- 

 gress to determine whether this difference could be 

 due to differences in the age at sexual maturation 

 (Myrick^). 



No data exist yet on the mean age at sexual 

 maturation for females in the southern stock. As 

 mentioned in the Methods section, data do exist on 

 the "length at sexual maturation" for both the north- 

 ern and southern stocks. For the northern stock, this 

 length appears to have increased from 176 cm in 

 1974 to 181 cm in 1983. For the southern stock this 

 length was estimated as 175 cm from the pooled 

 1974-83 data. The length at sexual maturation is 

 greater for the northern stock, which is consistent 

 with the greater mean asymptotic length of the 

 northern specimens (Perrin et al. 1979). 



CONCLUSION 



My intent in writing this paper was to identify in- 

 dices that may be of value in monitoring the repro- 

 ductive health of spotted dolphins in the eastern 

 Pacific. Two of the indices that were examined (the 

 percentage of mature females that are pregnant and 

 the percentage of pregnant females that are lac- 

 tating) are likely to be useful for this purpose Both 

 are relatively insensitive to sampling biases, and both 

 measure important aspects of the female reproduc- 

 tive cycla Problems exist in measuring the fraction 

 of females that are mature This parameter is also 

 an important index of net reproduction in a popula- 

 tion. It is possible that a stable index of percent 

 mature could be obtained using some stratification 

 scheme A first approach might be to examine finer 

 scale geographic differences in percent mature Addi- 

 tional work is necessary before significance can be 

 ascribed to between-population differences in per- 

 cent mature 



ACKNOWLEDGMENTS 



Much credit is deserved by the many technicians 

 who gathered the data upon which this report is 

 based. Their labors were often under very difficult 

 working conditions. I also wish to thank the data 



^A. C. Myrick, NOAA, National Marine Fisheries Service, P.O. 

 Box 271, La Jolla, CA 92038, pers. commun. December 1984. 



FISHERY BULLETIN: VOL. 83, NO. 4 



editing and managing group and the graphics 

 department, SWFC. Ideas presented in this manu- 

 script were largely generated in discussions with A. 

 Hohn and W. Perrin. The current draft of this manu- 

 script benefited greatly from critical reviews by J. 

 4Bengtson, D. Chapman, D. DeMaster, F. Hester, A. 

 Hohn, J. Mean, A. Myrick, W. Perrin, S. Reilly and 

 G. Sakagawa. 



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