Ferrero and Walker: Growth and reproduction of Delphinus delphis 



487 



cause an empirical method for doing so is 

 lacking (Perrin and Reilly, 1984). 



We also used Perrin et al.'s ( 1977) regres- 

 sion equation: 



Log(y) = 0.1659 + 0.4586 Log(x), (2) 



substituting x with the length of our age-0 

 neonate. The D. delphis length at birth es- 

 timate of 82.0 cm approximated the gesta- 

 tion period at 11.1 months. 



Postnatal growth Growth curves were fit- 

 ted separately for males and females by 

 using a nonlinear least-squares method. 

 The Laird/Gompertz formula (Laird, 1969) 

 was used as a base model for both sexes: 



L(t) = L exp{a[l-exp(-a£)]}, 



(3) 



Female D Male 



I I 



M 



il 



1 



80 90 1 00 110 1 20 1 30 1 40 1 50 1 60 1 70 1 80 1 90 200 



Length (cm| 



Figure 3 



Length-frequency distribution of 93 male and 59 female common dol- 

 phin, Delphinus delphis, for which ages were determined. Samples 

 were obtained from Japanese, Korean, and Taiwanese drift nets, Feb- 

 ruary to November, 1990 and 1991, in the central North Pacific Ocean. 



where L(t) is the length at age t, L Q is the 

 length at birth, t is the age, a is the specific 

 rate of exponential growth, and a is the rate 

 of decay of exponential growth. 



For both sexes, we fit two Laird/Gompertz 

 curves, one for the sexually mature animals 

 and the other for the sexually immature, in 

 order to minimize the number of positive 

 residuals in the upper curve segment. The 

 low sample size prohibited attempts to it- 

 eratively fit the two curve segments and 

 locate the intersection point. 



Female growth through age 2 was rapid, 

 with a predicted length at age of 146.4 cm. 

 The predicted asymptotic length was 179.4 

 cm (Fig. 5). 



The male growth curve through age 2 was 

 slightly steeper than that portion of the fe- 

 male growth curve, reaching a predicted 

 length at age of 149.8 cm. The predicted 

 asymptotic length was 188.1 cm (Fig. 6). 



The mean length of females age 16 and 

 older (i.e. those animals likely to have reached maxi- 

 mum size based on the predicted length at age [16 

 yr] falling on the asymptotic portion of the upper 

 growth curve) was 179.8 cm (n=5, SE=6.76). The 

 mean length of males age 16 and older was 187.1 cm 

 (n=15, SE=5.57). The difference in mean lengths be- 

 tween sexes was significant U-test, one-tailed, 

 P=0.04). 



Reproduction 



Sex ratio Sex ratios were estimated for four sub- 

 sets of the aggregate sample of measured animals 



80 90 100 110 120 130 140 150 160 170 180 190 200 210 



Length (cm) 



Figure 4 



Length-frequency distribution of all common dolphins, Delphinus 

 delphis, examined at sea February-November 1990-91 (376 males 

 and 331 females). Samples were obtained from Japanese, Korean, and 

 Taiwanese drift nets, February to November, 1990 and 1991, in the 

 central North Pacific Ocean. 



(n=707) to represent progressively older age groups. 

 The predicted length at age from our postnatal 

 growth models (Figs. 5 and 6) were used to separate 

 the sample into four approximate age groups (<1 year, 

 1 to 10 years, 10 to 15 years, and >15 years) on the 

 basis of length. In addition, we calculated separate 

 sex ratios for the portions of the sample collected 

 north and south of 34°N latitude. The latitudinal 

 break corresponded to a break in the timing of col- 

 lections, splitting February, March, and April sam- 

 pling from the remainder of the year (Table 1). We 

 used the empirical logistic transform method (Cox 

 and Snell, 1989) to estimate sex ratio as 



