FISHERY BULLETIN: VOL. 83, NO. 4 



tions divided by lat. 1°S, based on a division selected 

 by Perrin et al. (1S79). Areas 1 and 2 in Figure 1 

 correspond to the region occupied by the southern 

 offshore spotted dolphin and areas 3-8 correspond 

 to the region occupied by the northern offshore spot- 

 ted dolphin. 



ESTIMATION OF LENGTH AT BIRTH 



An accurate estimate of length at birth is impor- 

 tant because it establishes a point through which any 

 growth curve should pass. This extra degree of 

 freedom allows greater accuracy in fitting growth 

 curves and estimating growth rates. Neither of our 

 methods of relating length to age, described below, 

 allows us to fix absolute age so it is essential here 

 for us to calculate an independent estimate of length 

 at birth. 



A commonly used method of estimating length at 



birth when a sufficient sample is available is to re- 

 gress the percent postnatal at each length interval 

 on length and to calculate the length at which 50% 

 of the specimens are predicted to be postnatal. This 

 method, using a linear model, has been employed to 

 estimate length at birth for spotted dolphins (Per- 

 rin et al. 1976), for striped dolphins, S. coeruleoalba, 

 (Miyazaki 1977), and for spinner dolphins, S. 

 longirostris, (Perrin et al. 1977), and using a 

 nonlinear model for spinner dolphins (Perrin and 

 Henderson 1984). Another method commonly 

 employed when a small sample is available is to 

 estimate the average length at birth as the mean 

 length of known neonates or the mean length of full- 

 term fetuses and small calves combined. This method 

 has been used to estimate length at birth for long- 

 finned pilot whales (Sergeant 1962), for spotted 

 dolphins (Kasuya et al. 1974), for bottlenose dolphins, 

 Tursiops truncatus, (Ross 1977; Hohn 1980), and for 



I'l l 



\tO lEE lED MS MO tSS 130 IZS IZO IIS 110 lOE 100 35 



Figure L— Areas inhabited by the offshore spotted dolphin. The numbered regions refer to strata investigated in analyses of monthly 

 distributions of length. In all analyses, the southern population is from areas 1 and 2, and the northern population is from areas 3 through 8. 



554 



