< 



I— 

 < 



CO 



o 



CL 



100 



80 



60 



40 



20 







Average 



length at 

 birtti 



I I I I I 



j^ 



74 77 80 83 86 89 92 



(14) (9) (17) (II) (II) (17) (7) 



LENGTH (cm) 



Figure 4. — Linear regression analysis of percent postnatality 

 on body length for 86 fetuses and calves of Stenella attenuata 

 from the offshore eastern Pacific grouped in 3-cm intervals. 

 Sample size for each 3-cm interval in parentheses. 



ity is discussed below). Apparent progression of 

 the smaller mode in the January 1972 sample is 

 consistent with a gestation period of roughly 1 yr. 

 Sample sizes for the other apparent modes are not 

 large enough for similar analysis. Linear regres- 

 sion analysis of the modal lengths plotted on 

 month (Figure 6) yields an estimate of the slope to 

 use in Laws' equation: 



L = 8.283 {t - to), or 

 length at birth = 8.283 {t^ - to) 



with (tg -^(j) (using months of 30. 4 days) = 9.96 mo 

 or 303 days, where tg = total gestation period. 



Laws (1959) proposed that to for length data is 

 slightly less than for weight data and assumed 

 ^OLn = 0-9 ^0^^- Roughly interpolating between 

 Huggett and Widdas' values for tf^/tgOf 0.1 for 

 tg > 400 days and 0.2 for tg = 100 to 400 days 

 (using provisional tg = 330 days to enter the itera- 

 tion) (Figure 7) and applying Laws' correction, 

 ^OLn of - 0.135 tg is obtained. This value yields an 

 estimate of gestation time of 11.5 mo (349 days). 

 The estimate of ^^ (47 days) is crude, but the true 



FISHERY BULLETIN: VOL. 74, NO. 2 



AVERAGE LENGTH AT BIRTH 



20 



10 



20 



10 



ii* 







20 



-g 10 



I I I I '~i p 



JAN 

 1972 



FEB. 



0-*— ^ 



^ 20i 



CO 



ID 



-FF| n I fp 



MAR. 





 20 



10 



r-n 



APR. 



04 

 201 



10 



_a_ 







P , Fh n n 



■T 1 \ r- 



MAY 



OCT. 



20 40 60 80 100 120 140 160 



LENGTH (cm ) 



Figure 5. — Length-frequency distributions by month for 281 

 fetal (open) and young (hatched) postnatal specimens of 

 Stenella attenuata captured by tuna seiners in the offshore 

 eastern tropical Pacific in 1972. 



value probably lies between 0.12 tg and 0. 15 ^g . We 

 therefore estimate the gestation period to be 11.5 

 ± 0.2 mo (interval between estimates using ^o = 

 0.12 tg and 0.15 tg), or 11.3 to 11.7 mo. 



Postnatal Growth 



The same cohort used for analysis of fetal 

 growth can be followed through the samples until 

 approximate length of 125 cm (Figure 8) at the age 

 of approximately 8.5 mo. In order to optimize res- 

 olution, 4-cm intervals were used, and the sam- 

 ples for April, May, and June were combined. 

 Modes were estimated with NORMSEP. 



A linear regression line through the modal 

 lengths yields the postnatal growth equation 



236 



