Dall’s Porpoise, Phocoenoides dalliz

Dall’s porpoise (Phocoenoides dalliz) can only be found in the North Pacific, with a range that includes the Sea of Japan and the Okhotsk and Bering Seas. This range extends to southern California in the east and to the southern waters of Japan in the west. When normal weather patterns change and waters become colder, this species can be found in in Baja, California, specifically in Scammon’s Lagoon, and strays can occasionally be found in the Chukchi Sea. It prefers to reside in cold waters, at depths of up to 590 feet. It is typically found in coastal areas but can be found in deeper areas in North American waters.

Although Dall’s porpoise had been previously discovered before the 1970’s, its existence was not well known until information was released to the public regarding its decreasing population numbers. At this time, it was found that many species of cetaceans, including Dall’s porpoise, were threatened by fishing trawls. It was reported that thousands of individuals were accidentally being caught in this traps each year. Dall’s porpoise was named after W. H. Dall, an American naturalist.

There are two distinct color morphs of Dall’s porpoise, known as the truei-type and the dalli-type. The dalli-type occurs throughout the species’ range, whereas the truei-type can only be found in the waters of the western Pacific and occasionally in the eastern Pacific. These types are numerous, with 100,000 occurring in United States waters and 554,000 occurring in the Okhotsk Sea. Some experts assert that these types are actually subspecies, but this has not been confirmed.

Dall’s porpoise can reach an average body length of 7.5 feet, with a weight between 290 and 490 pounds. Males grow slightly larger than females and also bear an enlarged hump behind the anus and a deeper caudal peduncle, displaying a sexual dimorphism. This species is unique in its appearance, making it easy to distinguish from other cetaceans. Its body is robust, while its head is small. Its coloration is similar to that of the orca, with dark gray to black on the majority of its body and white coloring on the flanks and underbelly. The dorsal fin and fluke hold white coloring on the edges. This fluke is another feature that allows this species to be distinguished from other porpoises.

Dall’s porpoise gathers in small groups comprised of two to twelve individuals. Groups numbering in the hundreds have been recorded while feeding. This species is active, moving about sporadically just below the surface in a zigzag pattern. This creates a type of spray known as a “rooster tail,” which makes the species seem as though it is surfacing and submersing very quickly. This species is the fastest of all cetaceans, reaching speeds of up to 34 miles per hour. It can often be seen swimming alongside boats, but it may not remain for very long if the boat is traveling slowly. It can also be seen swimming on waves made by the heads of large whales, an action that is known as “snout riding.” It rarely jumps out of the water and will often move slowly over the surface in a rolling manner.

The breeding season for Dall’s porpoise can occur once a year, if females are healthy. This species is polygynous, and males will guard a number of breeding females in his group. Males choose which females to mate with, and once chosen, the females have the protection of the males, which ensures breeding is successful. Males will often forego foraging in deeper waters in order to protect a female. After a pregnancy period of ten to eleven months, one baby porpoise is born, typically in the summer months. Weaning occurs around two months of age, when females stop lactating. It was found that Dall’s porpoise is capable of breeding with the harbor porpoise. A fetus was found in British Columbia that supported this, and it is thought that these hybrids may be common. These hybrids may represent the unusually colored individuals found off the coast of Vancouver Island, which are currently thought to be a color variation of Dall’s porpoise.

The diet of Dall’s porpoise consists of cephalopods and many types of small fish, including schooling fish like anchovies, mackerels, herrings, and hakes, among others. It may also consume krill, although krill do not compose a large portion of its diet. This species is hunted by orcas and white sharks, and is vulnerable to specific parasites like whale lice, and the trematode fluke Nastitrema, an internal parasite that easily kills or strands this species.

Dall’s porpoise is threatened by fishing gear, especially large fishing nets, and thousands are caught as by-catch each year. However, it is thought that hunting conducted by the Japanese may become this species’ main threat. This hunting increased in the 1980’s, after hunting of larger members of the Cetacea order was illegalized. In 1988, it was reported that more than 40,000 Dall’s porpoises were killed, but this number did not decrease until 1990, when the IWC resolved some issues regarding the hunting. After this, 15,000 porpoises were killed each year and the current quota allowed has risen to 16,000. Many groups, including the IWC, disagree with the hunting of this species. Ship strikes and accidental catching cause an unknown amount of damage to its population numbers each year, but it is thought to be resilient due to its commonality and stable reproductive rate.

Dall’s porpoise is found in Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), which grants it an unfavorable status that may require more research and conservation efforts. It appears on the IUCN Red List with a conservation status of “Least Concern.”

Image Caption: A wild Dall’s porpoise poses for her Wikipedia photo in the Shelikoff Strait off the Kodiak Archipelago. Credit: NancyHeise/Wikipedia