Ferrero and Walker: Growth and reproduction of Delphtnus delphis 



489 



youngest sexually mature male (10 years) was 370 

 g; the testis weight of the oldest immature ( 10 years) 

 was 34.6 g. Weights of mature testes ranged from 

 273.2 to 1,190 g. 



Females Only a rough approximation of female av- 

 erage age at sexual maturation was determined. The 

 oldest sexually immature female was age 7.2 years, 

 and the youngest sexually mature was 8.5 years. Of 

 the forty-three female reproductive tracts collected, 

 8 were from mature animals. The youngest sexually 

 mature female had three corpora (Fig. 8A). Of the 

 mature females, three were resting, two were preg- 

 nant, two were lactating and had stage-2 corpora, 

 and one was of unknown status (Table 3). 



Average length at sexual maturation Calculation 

 of average length at sexual maturation was limited 

 for the male sample because no indeterminant length 

 classes were represented. The female sample was 

 adequate to estimate the parameter. Two methods 

 were used: the DeMaster ( 1978) method, modified to 

 estimate the average length instead of average age, 

 and a logistic regression method. 



The modification of DeMaster's method, applied to 

 females, used lengths grouped into even intervals as 



" max 



LSM= ^hifi-fi-!), 



(8) 



'=*mi. 



where/ is the index ofthe size class with the small- 



min 



est mature animal, i max is the index ofthe size class 

 with the largest immature animal, l t is the lower limit 

 ofthe ith size class, and f l is the fraction mature in 

 the z'th size class. 



The variance estimate on female LSM was ob- 

 tained by modifying the formula of DeMaster (1978) 

 to account for the interval width (w) so that 



Table 2 



Sex ratios for four age groups of Delphinus delphis. Pre- 

 dicted lengths at age from the Laird/Gompertz growth 

 model were used to determine the upper limits for each 

 group (i.e. 125 cm, both sexes <1 year; 171 cm for males 

 and 169 cm for females aged 1 to 10 years; 187 cm for males 

 and 179 cm for females aged 10-15 years). Sex ratios sig- 

 nificantly different from 1.0 (o=0.05) are marked with an 

 asterisk. SE = Standard error. 



Area and 

 age groups 



No. of individuals 



Male Female Sex ratio SE 



All areas 



<1 

 1-10 

 10-15 

 >15 



South of 34°N latitude 

 <1 

 1-10 



10-15 

 >15 



North of 34°Nlat 



< 1 

 1-10 



10-15 

 >15 



var(LSM) = w^ 



N.-l 



(9) 



where N t is number of individuals in the ith size class 

 and the interval width (w) was constant. 



The logistic regression (Cox and Snell, 1989) fits a 

 logistic curve, (u(a)), the probability that a dolphin 



