PERRIN ET AL.: GROWTH AND REPRODUCTION OF THE EASTERN SPINNER DOLPHIN 



TABLE 11.— Calculation of estimates of gross annual reproductive rate of the eastern spinner dolphin, 1973-75. 

 Standard error follows estimate (see text). Sample sizes in parentheses. 



B 

 Proportion 

 of females 



Annual pregnancy rate 



A ' B > C 

 Gross annual reproductive rate 



rors (SE) are attached to the various estimates 

 where sample size 3^100, under the assumption 

 that the binomial distribution tends to normality 

 in large samples (Bailey 1959), allowing calcula- 

 tion of SE as: 



SE = N /p(l-p)/n. 



Although gross annual reproductive rate as cal- 

 culated in Table 11 is a product of three estimates, 

 it can be calculated directly from the total sample 

 (number of females pregnant -J- total number of 

 males and females), allowing estimation of the 

 variance by the above method. The effect on the 

 variance by the constant used to adjust the preg- 

 nancy rate to an annual rate was ignored because 

 the constant (11.5 mo gestation -j- 12 mo, or 0.958) 

 is close to unity. 



The only statistically significant differences 

 among the estimates year-to-year (at a = 0.05) are 

 between the Method 1 estimates for 1973 and 1974 

 of annual pregnancy rate and, as a result of that, 

 gross annual reproductive rate. This sharp and 

 real shift cannot be accounted for by a time- 

 sampling effect, because seasonal coverage in the 

 2 yr was approximately the same. Prompted by the 

 knowledge that areal variation may exist in the 

 timing of calving peaks and/or in the degree of 

 breeding synchrony (see Length of Gestation and 

 Fetal Growth), we divided the data for each of the 

 years into three geographical strata: an "inside" 

 sample, an "outside" sample, and a "southern 

 sample" (Figure 31). More of the 1973 sample was 

 taken from the outside area than from the inside 

 area (108 versus 28), and the reverse was true in 

 1974 (46 versus 106). The southern samples, 5 in 

 1973 and 14 in 1974, were too small for analysis. 

 Comparison of the distribution of reproductive 

 condition in inside and outside samples in 1973 

 and 1974, however, reveals very small areal dif- 

 ferences compared with those between years (Ta- 

 ble 12). It must be concluded that the sharp in- 



1 35° 130' 125' 120' IIS* IIP' 105' 100' 95' 90' 65* 80* 



Rt*i!iogig*do II 



OUTSIDE 



135* 130° 125* 120" 115° 110° 105* 100* 95' 90' 85* 60« 



FIGURE 31.— Areas used to stratify 1973-74 samples of distribu- 

 tion of reproductive condition in female eastern spinner dol- 

 phins. 



TABLE 12. — Distribution of reproductive 

 stratified samples of sexually adult female 

 phins in 1973 and 1974. 



condition in area- 

 eastern spinner dol- 



crease in percent pregnant and decrease in percent 

 lactating from 1973 to 1974 is not a seasonal or 

 areal effect. Several other possible explanations 

 exist, to wit: 



747 



