488 



Fishery Bulletin 93(3), 1995 



8 10 12 14 16 

 Age (years) 



18 20 22 24 26 28 



Figure 5 



Age at length of female common dolphin, Delphinus delphis, with 

 fitted growth curves (Laird/Gompertz model). The open circles repre- 

 sent mature individuals; the closed circles represent immature indi- 

 viduals. Samples were obtained from Japanese, Korean, and Taiwan- 

 ese drift nets, February to November, 1990 and 1991, in the central 

 North Pacific Ocean. 



2 4 6 8 10 12 14 16 18 20 22 24 26 28 

 Age (years) 



Figure 6 



Age at length of male common dolphin, Delphinus delphis, with fitted 

 growth curves (Laird/Gompertz model). The open circles are mature 

 individuals; the closed circles represent immature individuals. Samples 

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

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



males. Of the sex ratios calculated by using 

 the sample collected from all areas, only the 

 10 to 15 year-old group ratio (1.792) was 

 significantly different from 1.0 (Exact Bi- 

 nomial Test, P<0.01). When only the por- 

 tion of the sample collected south of 34°N 

 was considered, none of the ratios was sig- 

 nificantly different from 1.0 (P-values were 

 all >0.2). The sample collected north of 34°N 

 yielded a sex ratio significantly different 

 from 1.0 (Exact Binomial Test, P<0.01) in the 

 10 to 15 year-old group (2.641) (Table 2). 



Average age at sexual maturation Only 

 preliminary estimates of average age at 

 sexual maturation (ASM) could be calcu- 

 lated owing to the insufficient number of 

 indeterminant age classes represented in 

 the sample. No indeterminant age classes 

 were represented in the female sample. For 

 males we used the DeMaster ( 1978) method 

 which computes the mean age as 



ASM = £<*(/„-/;_!), 



(6) 



a=J 



where f is the fraction of mature animals 

 in the sample aged a, j is the age of the 

 youngest mature animal in the sample, and 

 k is the age of the oldest immature animal 

 in the sample. 



Maturity status overlapped one age class 

 in the male sample, where we estimated the 



variance as 



var(ASM) = ]£ 



a=J 



fad-fa) 

 (N n - 1) ' 



(7) 



where N 



c 



aged a. 



is the total number of animals 



m + 0.5 ... 



f + 0.5 



where r is the ratio of males to females, m is the 

 number of males, and f is the number of females. 

 The natural logarithm of r is normally distributed 

 with variance, 



(n + l)(n + 2) 



var[ln(r)] = 



(5) 



n(m+Mf+D 



where n is the total number of males and females, m 

 is the number of males, and f is the number of fe- 



Males The preliminary estimate of aver- 

 age age at onset of sexual maturation for 

 males was 10.5 years (SE=0.50). Only at age 

 10 did both sexually mature and immature specimens 

 appear in the sample. 



There was a significant linear correlation between 

 testis weight and age among the immature animals 

 (r 2 =0.83, P<0.01), but the overall increase in testis 

 weight was small (<50 g over 10 years). A linear re- 

 lationship between age and testis weight was not 

 apparent among mature animals (r 2 =0.15). Testis 

 weight dramatically increased after age 10 (Fig. 7A). 

 The sample contained 21 sexually mature and 50 

 sexually immature males. The testis weight of the 



