Dall’s Porpoise (Phocoenoides dalli)
The Difference Between the Subspecies

Kimberly C. Genther

Figure 1. Two Dall's porpoise, dalli type swimming (Bcadventure.com 1995).

Dall’s porpoise (Phocoenoides dalli) are widely geographically distributed throughout the northern Pacific Ocean. They are easily identified by their extremely robust black bodies with large white patches on their sides; pectoral fins that are small and pointed; an almost triangular dorsal fin with white trim, and relatively broad flukes with white trim. Dall’s porpoise’s heads are very small in proportion to their bodies and they lack a well-defined beak (Leatherwood et al.1983). Since Dall’s porpoise are broadly distributed throughout the north Pacific they have many common names from multiple countries identifying and naming the same species. These other common names include white-flank porpoise, spray porpoise, ishi-iruka (Japanese), rikuzen-iruka (Japanese), and belokrylaya morskaya svinya (Russian) (Houck and Jefferson 1999). Within this species P. dalli there is believed to be at least two subspecies (P. dalli dalli, and P. dalli truei) and several color-phases or morphs. Although it is still a debate whether there are different subspecies or color-phases, I have attempted to sort through the information provided on the species Phocoenoides dalli, and distinguish the differences between the types and their geographical distribution.

Phocoenoides dalli dalli

The dalli dalli subspecies is the most common type of Dall’s porpoise and can be found all across the northern Pacific Ocean. This subspecies differs most noticeably from dalli truei by the extent of its white patch anteriorly on the sides and belly. In dalli dalli this white area is much shorter in length. The patch tends to begin just prior to the beginning of the dorsal find, and continues on back ending just before the tailstock (Leatherwood et al. 1988). The geographic distribution of dalli dalli is much broader than that of the dalli truei. It is widely distributed in the northern Pacific ranging from 35° N in the Sea of Japan and Okhotsk Sea across the Pacific to the west coast of North America extending southward to 32° N in the summer (Ferrero and Walker 1999). The northern limit of dalli dalli seems to be somewhere in the sub-artic waters of the Chukchi Sea (Kasuya 1978; Houck and Jefferson 1999). In the central north Pacific this subspecies is known as far south as 39° N and 1100km from the closest land (Houck and Jefferson 1999). Within a dalli dalli population there is >99% dalli type and <1% truei type (Rice 1998). This variance in the populations may be genetically related.

Phocoenoides dalli truei

The subspecies dalli truei is the less common type of Dall’s porpoise and in most cases can be found in only a particular area of the northern Pacific Ocean (Houck and Jefferson 1999). In dalli truei the white patch on the sides and belly is much longer than the patch of the dalli dalli. The patch begins before the pectoral fin and proceeds back till it tapers off at the tailstock (Leatherwood et al. 1988). The geographic distribution of the dalli truei is more restricted than that of the dalli dalli. The known range of the dalli truei is from the Pacific coast off of northern Japan and Ostrova directly east to the Kuril Islands with its southern boundary limit at 35° N in the winter season (Rice 1998). In the summer season the distribution limits lie at 42° N to 54° N and extend from the east coast of the Kuril Islands to 168° E (Kasuya 1978). Due to the overlap of the range of distribution of the subspecies mixed and unmixed schools may occur, but within a dalli truei type population there is >95% truei type and <5% dalli type (Rice 1998; Houck and Jefferson 1999). This variance in the populations may be genetically related.

Phocoenoides dalli black, white, and gray types

There are several rare color phases of Dall’s porpoises that can occasionally be seen throughout all the ranges mentioned above. These rare color phases that have been seen are black, gray, or white. There also has been reported some porpoises with brown backgrounds rather than black backgrounds, and some with lateral white blazes marked with black speckling (Rice 1998; Leatherwood et al. 1988; Houck and Jefferson 1999). It is important to note though that the color pattern of Dall’s Porpoises changes with age. Newborns are dark and light gray with no trim, instead of black and white. As the animals become older, their coloration intensifies to black and white. The trim on their dorsal fins and flukes also develops and lightens (Houck and Jefferson 1999).

Conclusion

When distinguishing Dall’s porpoise subspecies and color-phases from each other two key aspects need to be taken into account in order to identify what type it is. The first aspect is the location in which you are at. Dall’s porpoises have not been seen below 35° N in the western Pacific and 32° N in the eastern Pacific. They also have not been observed to go any farther north than the sub-artic waters of the Chukchi Sea (Kasuya 1978; and Houck and Jefferson 1999). If you are in this broad region of the north Pacific, then there is a possibility of seeing a Dall’s porpoise. The second key aspect is distinguishing the difference between the dalli type and the truei type. The truei type of Dall’s porpoise geographical distribution does not extend past 168° E with the subspecies western boundaries lying at the Pacific coastlines of Ostrova and Japan. In the summer season the distribution limits of the truei are known to condense and extend from 42°N to54°N (Kasuya 1978). The major difference in identifying a dalli type or a truei type is the extent of its white side patch. On the truei type the patch extends past the pectoral fin and proceeds on back to the tailstock, but on the dalli type the patch begins just before the dorsal fin and then proceeds back to the tailstock (Leatherwood et al.).

Literature Cited

Bcadventure.com. http://www.bcadventure.com/adventure/wilderness/animals/dallporp.htm 1995.

Ferrero, R. C. and W. A. Walker. 1999. Age Growth and Reproduction Patterns of Dall’s Porpoise ( Phocoenoiced Dalli) in the Central North Pacific Ocean. Marine Mammal Science 15(2): 273-313.

Houck, W.J. and T.A. Jefferson. 1999. Handbook of Marine Mammals, Volume 6 The Second Book of Dolphins and the Porpoises. Academic Press. San Diego, CA. Pages 443-447.

Kasuya, T. 1978. The Life History of Dall’s Porpoise with Special Reference to the Stock off the Pacific Coast of Japan. The Scientific Reports of the Whales Research Institute. 30:1-63.

Leatherwood, S. and R. R. Reeves. 1983. The Sierra Club Handbook of Whales and Dolphins. Sierra Club Books, San Francisco, CA. Pages 273-276.

Leatherwood, S., R. R. Reeves, W. F Perrin, and W. E. Evans. 1988. Whales, Dolphins, and Porpoises of the Eastern North Pacific and the Adjacent Artic Waters, A Guide to their Identification. Dover Publications Inc. (Corrected and slightly revised republication of the work originally published by the National Marine Fisheries Service (National Oceanic and Atmospheric Administration, United States Department of Commerce) Seattle, Washington, July 1982 as NOAA Technical Report NMFS Circular 444), Mineola, NY. Pages 200-204.