FISHERY BULLETIN: VOL. 84, NO. 2 



that contradict earlier findings. Based on the more 

 reliable method of estimating age in spotted dol- 

 phins, we believe that our findings present a clearer 

 picture of the reproductive information than has 

 been reported previously. Our aged samples showed 

 that the youngest sexually mature female was 10 yr 

 old— the same age as the youngest pregnant and the 

 youngest lactating specimens. This suggests that 

 some females must become sexually mature before 

 the age of 10, even though mature specimens 

 younger than 10 were not found in our sample The 

 average age of a pregnant female in our sample was 

 about 18 yr, and some females of about 35 yr old 

 were pregnant or nursing. These values are substan- 

 tially higher than estimated previously for this stock 

 (Perrin et al. 1976), but they are similar to, though 

 still somewhat higher than, estimates for the western 

 Pacific stock (Kasuya 1976). 



The ASM estimate in this study (about 11.4 yr) is 

 higher than that estimated by Perrin et al. (1976). 

 Our calculations showed no significant difference 

 between the ASM calculated for the 1973-74 sam- 

 ple (taken during years of heavy fishing mortality) 

 and the ASM for the 1981 sample (taken after at 

 least 5 yr of reduced fishing mortality). 



An ASM of 11.4 yr means that the youngest 

 average age of first parturition would be 12.3 yr (11 

 mo later). Since not all females would conceive at 

 first ovulation, the actual average age would be 

 greater than this. The implication of this protracted 

 period before reproduction and a long (3.03 yr) 

 calving interval is that spotted dolphin survival rates 

 must be very high in order to maintain a stable 

 population level. 



There is a significant depression in the age struc- 

 ture of the 1973-78 and 1981 aged sample in the 6-12 

 yr age classes (Hohn and Myrick in prep. 2 ). Similar 

 age-structure patterns, interpreted as reflecting 

 some sort of schooling segregation, have been en- 

 countered in studies of other delphinids (see review 

 by Perrin and Reilly 1984). If animals at or near the 

 age of sexual maturity have been regularly under- 

 sampled because their schools were not targets of 

 purse seines (Hohn and Scott 1983), the ASMs 

 calculated for the aged samples could be upwardly 

 biased. However, there is no evidence that the 

 depression in the age structure represents missing 

 animals that were sexually mature 



The annual pregnancy rate averaged 0.33 from 



1973 through 1981. There were no sustained upward 

 or downward changes in age-specific pregnancy rates 

 with increased age A similar result was shown by 

 Kasuya (1976) for the western stock, although his 

 values were somewhat lower than the rates we have 

 estimated for the northern offshore stock. Our esti- 

 mates are different from those of Perrin et al. (1976) 

 who reported high pregnancy rates among younger 

 specimens and a decreasing rate with increased age 



The implications of an apparent progressive in- 

 crease in the lactation period are enigmatic It is 

 probable that the increase in lactation period reflects 

 the decrease in per capita mortality of calves due 

 to the more efficient releasing procedures employed 

 by the purse seine fleet from the mid-1970's onwards. 

 Decreased mortality of nursing calves would be 

 reflected by an apparent increase in the number of 

 lactating females because fewer nursing periods 

 were ended prematurely. 



Our study of postreproductive specimens suggests 

 that fertility diminishes as the complement of 

 follicles for a female becomes expended through 

 ovulation or atresia. Female spotted dolphins with 

 atrophic ovaries or with no macroscopic follicles are 

 reproductively senescent. Although the expenditure 

 of follicles progresses with age, reduction in fertility 

 is not strictly age related. The occurrence of repro- 

 ductive senescence in spotted dolphins in this study 

 was negligible and the number of specimens in this 

 state probably is of limited importance to estimates 

 of reproductive parameters. 



ACKNOWLEDGMENTS 



We thank D. DeMaster, W. F. Perrin, and S. Reilly 

 for their helpful comments and recommendations on 

 early drafts of the manuscript. We are grateful to 

 J. Bengtson, D. Chapman, F Hester, J. Mead, A. 

 York, and R. Wells for their very thorough reviews. 

 J. Walker and S. Chivers assisted in organizing and 

 accessing the life history data and S. Chivers helped 

 with the analyses. D. Stanley and M. Kimura 

 prepared the tooth sections for the aged subsamples. 

 Special thanks go to H. Orr who prepared the figures 

 and to H. Becker and S. Richardson and the SWFC 

 Technical Support Staff who typed parts of the 

 manuscript. D. DeMaster, N. Lo, and S. Reilly 

 assisted in statistical testing of some of the samples. 

 J. Michalski edited the final draft. 



2 Hohn, A. A., and A. C. Myrick, Jr. The age structure of north- 

 ern offshore dolphins, Stenella attenuata, from the eastern tropical 

 Pacific Manuscr. in prep. Southwest Fisheries Center La Jolla 

 Laboratory, National Marine Fisheries Service, NOAA, P.O. Box 

 271, La Jolla, CA 92038. 



LITERATURE CITED 



Barlow, J. 



1985. Variability, trends, and biases in reproductive rates of 



258 



