PERRIN ET AL : GROWTH AND REPRODUCTION OF SPOTTED PORPOISE 



females are also greater in the Japanese popula- 

 tion, and in this case, all four of the estimates are 

 based on large and certainly adequate samples. 

 These differences all suggest that the Japanese 

 form is about 6 to 8 cm larger than the eastern 

 Pacific form. 



The estimates of Kasuya et al. (1974) of age at 

 attainment of sexual maturity are based on their 

 conclusion that one tooth layer corresponds to 1 yr 

 of growth. It appears from comparisons of their 

 first-year growth curve with ours (note rate in first 

 year and length at 1 yr) that our first two layers 

 correspond to their first layer. Kasuya (1972) in 

 his paper on growth of S. coeruleoalba mentioned 

 observing "one or two faint translucent layers in 

 the thick opaque layer accumulated just after the 

 birth" that were "not used for age determination 

 because it was not expected to show the annual 

 accumulation cycle," and Kasuya et al. (1974) 

 stated that the "dentinal growth layers of this 

 species [S. attenuata] does not differ so much from 

 that of S. coeruleoalba.'' After the first year, our 

 hypothesis 2 corresponds to the assumption of 

 Kasuya et al . of one layer per year, e.g., nine layers 

 of Perrin et al. (1973) = eight layers of Kasuya et 

 al. = 8 yr. 



The average length of calving interval in both 

 studies was estimated by several methods that 

 converged on the respective central estimates. 

 One minor difference between the two analyses is 

 that Kasuya et al. (1974) did not exclude postre- 

 productive females from the "resting/estrus" group. 

 Thus, their estimate of the average resting/estrus 

 period of 9.8 mo may be a slight overestimate. The 

 probable effect of this on the estimate of length of 

 total calving interval is very small, however, and 

 it therefore seems that the estimates are analyti- 

 cally comparable and that the difference between 

 them is real. Kasuya et al. estimated that indi- 

 vidual intervals in the Japanese population vary 

 from 23 to 60 mo, with modes at 28 to 30, 36 to 

 38, and 54 to 56 mo. The potential thus probably 

 exists for a shift in average length from 50 mo (4.17 

 yr) to 26 mo (2.17 yr) under exploitation. 



Kasuya et al. (1974) used the same methods 

 used here to estimate length of the lactation period 

 and arrived at a "best" estimate of 29.3 mo, some 

 18 mo longer than our estimate of 11.2 mo. They 

 found that the major shift from milk to solid food 

 occurs at body length of about 133 cm, about the 

 same as in our sample, but that some suckling and 

 lactation of the mother continues for an average 

 additional 20 mo. The prolonged suckling is prob- 



ably nutritionally a largely nonfunctional aspect 

 of general prolonged parental care. It has been 

 suggested on the basis of comparison of the life 

 histories and behavior of mysticetes and odonto- 

 cetes that this period in odontocetes may allow for 

 "sophisticated" communicational-navigational 

 training (Brodie 1969). Thus the apparent shorter 

 lactation period in the eastern Pacific, and the 

 concomitant shorter calving interval and higher 

 pregnancy rate, does not necessarily mean earlier 

 effective weaning, but may reflect a truncated pa- 

 rental care period. 



The apparent overall sex ratios are almost the 

 same for the two populations, but the proportion of 

 males was higher at birth and lower at maturity in 

 the Japanese samples than in the eastern Pacific 

 samples. A lower proportion of males at birth 

 could be a response to exploitation. Kasuya et al. 

 (1974) suggested that the very low proportions of 

 males in mature age-classes in the Japanese 

 catches could be partially caused by segregation of 

 adult males or by differential catchability but are 

 largely due to differential mortality rates. If the 

 decrease in proportion of males with age is caused 

 by differential mortality, the apparent faster de- 

 crease in the Japanese population must mean that 

 the disparity in mortality rates between the sexes 

 is greater there than in the eastern Pacific. 



In summary, the two sets of estimates differ in a 

 consistent way, and the differences are real. It 

 seems possible that the differences in some way 

 reflect exploitation in the eastern Pacific. 



ACKNOWLEDGMENTS 



This study would not have been possible without 

 the generous cooperation and assistance of the 

 owners, masters, and crews of the tuna seiners 

 Conte Bianco, Carol Virginia {now Carol S), Larry 

 Roe, Nautilus, Mary Antoinette, San Juan, Con- 

 cho, Kerri M, Queen Mary, Eastern Pacific, John F. 

 Kennedy, Sea Preme, Westport, Anne M, Pacific 

 Queen, J. M. Martinac, Lois Seaver, Marietta, 

 Independence, Sea Quest, Bold Contender, Jac- 

 queline A, Frances Ann, Elsie A, Sea Royal, Jac- 

 queline Marie, Trinidad, Mermaid, Bettie M, An- 

 tonina C, Day Island, Connie Jean, and Denise 

 Marie. Scientists and technicians who collected 

 data and specimens aboard the vessels include C. 

 E. Bowlby, R. W. Cunningham, W. E. Evans, R. S. 

 Garvie, J. M. Greene, D. B. Holts, J. La Grange, J. 

 S. Leatherwood, R. E. Loghry, R. L. McNeely, C. W. 

 Oliver, R. J. Olson, C. J. Orange, D. J. Otis, J. W. 



267 



