Ferrero and Walker: Growth and reproduction of Delphinus delphis 



491 



where x is age and y is the day of the year 

 of collection. At age 0.0, the intercept corre- 

 sponds to a date in early June, suggesting 

 the calving mode. Both the slope and the 

 intercepts were significant (P<0.001 and 

 P<0.05, respectively). 



We also regressed the lengths of speci- 

 mens <115 cm from the overall sample 

 (ti=30) with day of the year. The 115-cm 

 length corresponded to ages slightly older 

 than 0.5 year for both sexes according to 

 the predicted length-at-age relationship 

 from the Laird/Gompertz growth model. We 

 obtained the equation 



y = -133.44 + 3.435*, 



(13) 



where x is the length andy is the day of the 

 year. Substituting 82 cm, our provisional 

 length at birth, for x, the line intercepts a 

 date in mid-May Both the age- and length- 

 based regressions suggest a seasonal peak in 

 mid-May or early June. The lengths and dates 

 of collection for the two fetuses also support 

 this modal peak. The 65-cm fetus collected 

 28 March 1991 and the 12.6-cm fetus collected 

 23 September 1990 were projected to reach 

 full-term in late May to early June. 



Discussion 



8 - 



7 I 

 6 



5 



o 3 

 o 

 2 



12 16 

 Age (years) 



20 



24 



28 



8 T 



7 

 6 



3 

 2 



115 



B 



125 



135 



145 155 165 

 Length (cm) 



175 



185 



195 



The structure of the D. delphis sample sug- 

 gested that it was suitable for preliminary 

 estimation of several basic growth and re- 

 production parameters, although our val- 

 ues would be improved with larger sample 

 sizes. With uncertain prospects for collect- 

 ing additional D. delphis samples, owing to 

 the discontinuation of the high seas squid driftnet 

 fisheries and thus any associated sampling programs, 

 we attempted to provide as much quantitative analy- 

 sis as possible; however, we must regard many of our 

 estimates as provisional. In particular, more reliable 

 estimates of ASM and LSM will require more samples 

 in the indeterminant age and length classes. Strong 

 evidence of seasonality in calving made this sample 

 unsuitable for calculation of calving interval, length 

 of lactation, age of weaning, length of resting period, 

 or reproductive rates. 



Our application of the double Laird/Gompertz 

 growth model is consistent with methods applied in 

 previous growth studies on small cetaceans (Perrin 

 et al., 1976, 1977), although our sample was not large 

 enough to iteratively fit an intersection point between 



Figure 8 



(A) Scatterplot of age (years) and total corpus count for 59 female, 

 common dolphin, Delphinus delphis. The open circles represent ma- 

 ture 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 Pa- 

 cific Ocean. (B) Scatterplot of length (cm) and total corpus count for 

 59 female 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. 



the upper and lower curves. While our application of 

 the two-stage model suggests a secondary growth 

 surge near the onset of sexual maturation, the mag- 

 nitude and timing of that event should not be con- 

 sidered reliable without further sampling. 



We attempted to fit a single Laird/Gompertz model 

 to both the male and female data sets, but in both 

 cases we encountered a preponderance of positive 

 residuals at predicted lengths at age >10 years. 

 Using the single curve, we predicted asymptotic 

 lengths for both sexes at least 10 cm lower than any 

 of the observed lengths for mature specimens. Our 

 results from the double Laird/Gompertz model pro- 

 vide predicted length-at-age relationships with com- 

 paratively smaller residuals, distributed both posi- 

 tively and negatively at older ages. 



