Marine Mammal Sci (1992)

DOI: 10.1111/j.1748-7692.1992.tb00059.x


(…) On 28 August 1989, we were informed by California State Park Ranger Robert Breen that a small porpoise had washed ashore at the James Fitzgerald Marine Reserve, San Mateo County, California (37°31’N, 122°31’W). Upon recovering the animal, we discovered that it was a 97-cm-long newborn (as evidenced by fetal folds on the flanks and flukes, and remnants of the umbilical cord) female Dall’s porpoise (Phocoenoides dalli) of the truei-type color pattern (defined by Houck 1976). Animals of this color type have the light flank patch extending forward to the level of the flipper, rather than only to the level of the anterior insertion of the dorsal fin, as is the case in the normally occurring dalli-type (Houck 1976, Kasuya 1978).

Truei-type animals were originally described as a separate species (Phocoenoides truei) by Andrews (1911). However, it is now thought that Dall’s porpoise is polymorphic, and dalli and truei forms merely represent color types of P. dalli (see Jefferson 1988 for discussion). The presence of a truei-type newborn among a dalli-type population supports the view of color types, rather than separate species or subspecies.

Although the truei-type is common off the Pacific coast of Japan, all other populations of Dall’s porpoise normally have the dalli-type color pattern (Kasuya 1978, 1982). Truei-type individuals, possibly strays from the Japanese coastal population have been sighted and collected as far east as the western Aleutian Islands (to about 180°E) (Miyazaki et al. 1984, Jones et al. 1987), but this record appears to be the first for the eastern North Pacific (east of 180°). (…)