490 



Fishery Bulletin 93(3). 1995 



1200 



1000 - 



"5 



~ 800 



c 



a 



| 600 

 S3 



s 40 ° 



200 







_» M«l.f*««» 



12 16 



Age (years) 



20 



24 



28 



110 120 130 140 



1 50 1 60 1 70 



Length (cm} 



180 190 200 210 



Figure 7 



(A) Scatterplot of age (years) and testis weight (g) for 93 male com- 

 mon dolphin, Delphinus delphis. The open circles represent mature 

 individuals; the closed circles represent immature individuals. 

 Samples were obtained from Japanese, Korean, and Taiwanese drift 

 nets, February to November, 1990 and 1991, in the central North 

 Pacific Ocean. (B) Scatterplot of length (cm) and testis weight (g) for 

 93 male Delphinus delphis. The open circles represent mature indi- 

 viduals; the closed circles represent immature individuals. Samples 

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

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



Males Length at sexual maturation could 

 only be suggested by the largest immature 

 male, 179 cm, and the smallest mature 

 male, 182 cm. There was a significant lin- 

 ear correlation between length and indi- 

 vidual testis weight among immature ani- 

 mals (r 2 =0.61, P<0.0001). Testis weights 

 changed little with length up to the onset 

 of sexual maturity when weights increased 

 greatly (Fig. 7B). No correlation between 

 length of mature animals and testis weights 

 was detected (P>0.4). 



Females Female average length at sexual 

 maturation based on the DeMaster (1978) 

 method was 172.8 cm (SE=0.56). With the 

 logistic method, female LSM was 170.7 cm 

 (SE=2.74). The smallest sexually mature 

 female measured 170 cm; two corpora were 

 present on the left ovary. The largest sexu- 

 ally immature female was 178 cm (Fig. 8B). 



Ovulation rate Calculation of ovulation 

 rate followed methods used for Stenella 

 attenuata in Perrin et al. (1976). The aver- 

 age reproductive age ( A ) in interval p was 

 calculated as 



(11) 



of length / is mature, to the distribution of mature 

 and immature animals by age: 



,16+c 



(10) 



1 + e 



lb+c 



where, I is the length of the dolphin and b and c are 

 the slope and intercept of the regression. Average 

 length at sexual maturation is then estimated as the 

 age where \i(a) = 0.50 so that LSM = -c/b. 



The regression was done by using a maximum like- 

 lihood and iteratively reweighted least-squares al- 

 gorithm (Chambers and Hastie, 1992). The standard 

 error for Li was obtained by transforming the stan- 

 dard error of the linear fit. 



where a is the percent maturing in the j'th 

 interval, b is the average reproductive age 

 in interval p of females which matured in i, 

 and c is the percent mature in interval p. 

 The average corpus count in each age in- 

 terval was calculated by dividing the sum 

 of corpora counted in interval i by the number of ma- 

 ture females in interval i. We then regressed the aver- 

 age corpus count on the average reproductive age. 



Ovarian scars numbered from 1 to 8 corpora among 

 mature females. A linear model provided the best fit 

 to the corpus count and average reproductive age 

 data(r 2 =0.61,P<0.1). 



Seasonality Evidence of reproductive seasonality 

 was detected by correlating age and length on collec- 

 tion date. From the aged portion of the sample we 

 regressed ages of specimens <0.5 years (n=5) and the 

 day of the year of collection using a linear model that 

 resulted in the equation 



y= 158.76 + 265.14*, 



(12) 



