120 



Fishery Bulletin 88(1), 1990 



tally entangled in gill nets set for salmon south of the 

 Near Island group at the western end of the Aleutian 

 Islands, Alaska. The capture locations of these 

 specimens are plotted by 1-degree block in Figure 1. 

 All specimens were of the dalli-type color morph (see 

 Houck 1976) and were presumably from the north- 

 western North Pacific population that calves south of 

 the western Aleutians (Kasuya and Ogi 1987). In addi- 

 tion to these specimens, four calves collected as beach 

 strandings along the California coast in 1987 and 1988 

 were used in the analyses. Although these four speci- 

 mens were from a separate population (Kasuya 1982, 

 Kasuya and Ogi 1987), they were included because the 

 sample size of calves was small, and geographic varia- 

 tion at the calf stage was assumed to be insignificant. 

 Little work has been conducted on geographic varia- 

 tion in subadult animals. However, some differences 

 in fin shapes between calves of different stocks of spin- 

 ner dolphins Stenella longirostris have been identified, 

 although these are minor compared with differences 

 in adults (W.F. Perrin, Southwest Fish. Cent., Natl. 

 Mar. Fish. Serv., NOAA, La Jolla, CA 92038, pers. 

 commun., Aug. 1989). 



Each porpoise was weighed and sexed, a specimen 

 number was assigned, and black-and-white photo- 

 graphs were taken. A series of seven measurements 

 were obtained to the nearest 0.5 cm (Fig. 2, Table 1), 

 and the animal was dissected. The reproductive status 

 of females was assessed as pregnant, lactating, resting, 

 or immature, based on gross examination. The distribu- 

 tion of total lengths of the specimens is shown in Fig- 

 ure 3. 



Each specimen was placed into one of seven age/sex 

 classes (Table 2). Most of the classes were based on the 

 total length of the porpoise, generally following the 

 growth curve presented by Newby (1982). Because 

 there was only one neonate male, and only one feature 

 (Canting Index) was measured on it, it was pooled with 

 the females. The assumption was made that there are 

 no sexual differences at the neonate stage. The cutoff 

 between immature and mature males was based on 

 Jones et al. (1987), who gave 180 cm as the average 

 length at sexual maturity. Attainment of sexual matur- 

 ity in males is considered to be more strongly corre- 

 lated with length than with age, and all males in this 

 study over 180 cm had enlarged testes, indicative of 

 maturity (Kasuya and Shiraga 1985). Female sexual 

 maturity is not well correlated with length, so average 

 length at maturity was not used to separate immature 

 and mature females. If a female was pregnant, lac- 

 tating, or had apparent corpora upon gross examina- 

 tion of the ovaries, it was classified as mature. If it 

 fulfilled none of these criteria, but was ^151 cm, it was 

 placed into the immature class. Because over 95% of 

 all mature females in the study area are pregnant or 



530N 



520 



51° 



500 



171" 



172« 



173° 



1740 



49- 

 175"E 



Figure 1 



(-'apture locations of Dall's porpoise specimens from the northwestern 

 North Pacific. 



lactating during the fishing season, these are very good 

 indicators of sexual maturity for this population (Jones 

 et al. 1983). 



In order to test quantitatively for variation in dorsal 

 fin shape, a value similar to that used by Perrin (1975) 

 for spinner dolphins was computed as the ratio alb 

 (where a is measurement 7 and h is measurement fi in 

 Table 1). This quantity is termed the Canting Index, 

 and for Ball's porpoise will generally fall between 



