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Fishery Bulletin 88(1), 1990 



Figure 1 2 



An example of how the Canting Index is measured from a photo of a live [Jail's porpoise. In this case, the ratio equals O.liT, 

 indicating that this is a sexually mature male. 



proximity to an adult, but are rarely seen from boats 

 because cow-calf pairs do not often ride bow waves. 



Juveniles (Fig. 8b) are between 130 and 150 cm, and 

 from 4 months to 1 year old. They are significantly 

 lighter in color than adults, and the flank patch is still 

 light-gray. Frosting has begun to develop on the dor- 

 sal fin and flukes, but is not extensive and is usually 

 only a bit lighter than the rest of the body at this stage. 

 The Canting Index is between 0.25 and 0.55. A slight 

 to moderate postanal hump may be visible. These 

 animals are most likely weaned, and independent of 

 their mothers. 



Immature males and females (Fig. 8c, d) range from 

 about 150 to over 175 cm in females and to about 180 

 cm in males, but are nearly impossible to distinguish 

 in the field. Females are 1-3 or 4 years old, and males 

 are 1-5. Immatures are very difficult to distinguish 

 from adult females. The color pattern has mostly 

 reached black-and-white, although it may still be some- 

 what muted. There is usually prominent white to light- 

 gray frosting, and little obvious sexual dimorphism 

 (except that males may have a slighty more canted 

 dorsal fin and a larger postanal hump). The Canting 

 Index is between 0.25 and 0.55. Immatures appear to 

 be the most common bow-riders in many areas. 



Mature females (Fig. 8e) are from about 175 cm up 

 to 215 cm long, and more than 3 or 4 years old. The 



color pattern has reached the adult black-and-white 

 stage, with extensive light frosting on the flukes and 

 dorsal fin; but due to overlap adult females are difficult 

 to distinguish from immatures. The dorsal fin is not 

 strongly canted (Canting Index <0.55). There is usually 

 a small postanal hump. Adult feinales may be identified 

 if closely accompanied by a small calf. Lactating 

 females and those in late stages of pregnancy often 

 appear to be of extremely large girth. 



Adult males (Fig. 8f) are 180 to 225 cm long and 

 greater than 4 years old. The coloration is black-and- 

 white, and extensive white frosting is usually present 

 on the fin and flukes. The dorsal fin is strongly canted 

 (Canting Index >0.55). There is almost always a large 

 postanal hump (>1.2% of the total length), and the 

 peduncle is deepened (>15% of the total length). Often, 

 an enlargement of the thoracic epaxial muscle mass is 

 apparent as a hump between the blowhole and the dor- 

 sal fin. Many (possibly older) adult males appear to have 

 flukes with a convex rear border, and they are usually 

 very robust (the head may look disproportionately 

 small). 



The assumption is made that results from animals 

 of this northwestern North Pacific population are ap- 

 plicable to other Dall's porpoise populations as well. 

 Growth characteristics of reproductively isolated, small 

 cetacean populations are known to sometimes differ 



