PERRIN ET AL.: GROWTH AND REPRODUCTION OF SPOTTED PORPOISE 



PREGNflNCr RATE 



LACTATION RATE 



II 12 13 



LAYERS (number) 



Figure 35. — Age-related changes in pregnancy (solid line) and 

 lactation rates (dashed line) in Stenella attenuata, based on 

 tooth layer data. Postreproductive females not included. 



PARITY 



OVERALL RATIO 



I I I I  I I I I ' I I I I I ' ' ' 



' I ' I ' I I 



100 150 200 250 



SAMPLE SIZE (no) 



300 



350 



Figure 36. — Scatterplot of sex ratio (percent males) on sam- 

 ple size in single-school samples of five or more specimens of 

 Stenella attenuata. Overall ratio (dashed line) from Table 12. 



Table lO. — Sex ratio, by color pattern stage, in 9,371 speci- 

 mens ot Stenella attenuata, 1971-73. 



43% of the adults examined were males. The 

 greatest change in ratio, from 48.0 to 43.5% male, 

 comes about during the transition to mottled col- 

 oration between 7 and 8 layers of age. Assuming 

 random sampling of the population, male and 



female mortality rates must diverge sharply at 

 this point. 



Gross Annual Production 



An estimate of average gross annual production 

 of calves for 1971 to 1973 was calculated based on 

 the estimate of annual pregnancy rate, the color 

 pattern phase data, and the proportions of mottled 

 and fused females found to be sexually mature 

 (Table 11). 



Seven hundred and twenty-nine of 9,371 ani- 

 mals were mottled females (7.8%) and 2,865 were 

 fused females (30.6%). Of 127 mottled and 1,141 

 fused females, 47.4 and 88.4% were sexually 

 mature, respectively (Table 11). Average preg- 

 nancy rate was 0.469. Production = [(0.078 x 

 0.474) + (0.306 X 0.884)] 0.469 = 0.144 of the popu- 

 lation per year. 



Table ll. — Sexual maturity (presence of ovarian corpora) in 

 mottled and fused females of Stenella attenuata, 1971-73. 



Schooling in Relation to Reproduction 



Kasuya (1972) reported changes in structure 

 and size of schools of .S. coeruleoalba correlated 

 with breeding condition and breeding activities. 

 Kasuya et al. (1974) proposed a complex hypothet- 

 ical system of school formation and breakdown 

 determined by reproductive activities in the 

 Japanese population of S. attenuata. They 

 suggested that juveniles of S. attenuata in Japa- 

 nese waters leave breeding schools and school sep- 

 arately, rejoining the breeding schools at puberty. 

 There is nothing to indicate that this happens 

 in the eastern Pacific. We examined the coloration 

 structure (= age structure) of single-school sam- 

 ples. Of 324 single-school samples of seven or more 

 animals, only 1 (of 17 animals) contained no adults 

 (or neonatal calves, which would indicate presence 

 of adult lactating females in the school). This sam- 

 ple (8 two-tone, 2 speckled, and 7 mottled) was 

 from a school of about 600 spotted porpoise, S. 

 attenuata, congregated with about 600 spinner 

 porpoise, iS. longirostris. Given that about half the 

 animals examined were adults, the probability of 



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