OCEAN CHRONICLES

SPECIES IN THE SPOTLIGHT

Dall's porpoise swimming in the emerald green waters of Southeast Alaska © Josh McInnes

DALL'S PORPOISE Phocoenoides dalli

JOSH MCINNES

The sun glinted off the emerald green water as we slowly traveled north in Frederick Sound, Alaska. We were on our final day of a weeklong marine mammal survey aboard the motor vessel David B, in the heart of Southeast Alaska. Up ahead a group of seven Dall’s porpoise charged head on towards our vessel, a v-shaped spray of water called a rooster tail emanated ahead of their torpedo shaped bodies. The porpoises began riding the bow wave created by the David B, using the wave’s momentum to push their bodies steadily forward. Each porpoise crisscrossed in a competitive dance as they vied for each wave the vessel created. This extraordinary behaviour lasted for over an hour before the Dall’s porpoises departed. An encounter with a group of Dall’s porpoise is truly a breathtaking experience, and mariners traversing the coastal waters of the Inside Passage commonly encounter these small but playful cetaceans.

The Dall’s porpoise was first described by the American naturalist and paleontologist William Healey Dall (18451927) during an expedition aboard the USS Nightingale in British Columbia, Alaska, and the Russian Far East in 1865-1867. His distinct notes on the species morphology, and the collection of a specimen, allowed his colleague Frederick W. True to classify it as a distinct species, which was subsequently named after him. The Dall’s porpoise is one of six species of porpoise belonging to the family Phocoenidae. They are endemic, but widely distributed in offshore and coastal waters of the cool temperate Pacific Ocean. Two sub-species known as Phocenoides dalli dalli and Phocenoides dalli truei have been distinguished based on differences in morphological and genetic characteristics. Phocenoides dalli dalli is primarily encountered in the Northeastern Pacific, and is distributed from the Bering Sea south to Southern California. It has been estimated that the total abundance of Dall’s porpoises in the North Pacific ranges 1 to 1.2 million. This subspecies is easily recognizable by their robust body, short-beaked head, and black and white coloration A similar pattern found in killer whales (Orcinus orca). Their dorsal fins are rectangular

A mixed aggregation of Dall's porpoise and harbour porpoise in Juan de Fuca Strait © Chelsea Mathieson

and pointed forward, with white or grey frosting on the upper portion. Large white patches run horizontally on both sides of the porpoise where they terminate central to the dorsal fin. Adult Dall’s porpoises can be up to 2 m long and weigh close to 200 kg. Older males have a pronounced hump that forms posteriorly of the dorsal fin. Caves are a pale grey color when born, and reach a length of approximately 1 m long. In the Northwestern Pacific Phocenoides dalli truei extends westwards towards the waters of Japan. The white flank patches extend forward and terminate near the pectoral fins.

In the coastal waters of the Pacific Northwest, it is not uncommon to see Dall’s porpoises in association with harbour porpoises. Even more surprising is that both species are known to interbreed, forming hybrids. The hybrids are fertile, as numerous photographs have identified them in association with calves. The hybrid porpoises share characteristics common to both species, but tend to lack the contrasting black patterns typical of Dall’s porpoises. It is believed that hybrid porpoises make up to two percent of the overall population.

Dall’s porpoises are primarily encountered in groups of 2 to 12, but larger aggregations of >20 have been sighted. These larger groups likely form in response to prey concentrated areas. In the coastal waters of the Salish Sea, Dall’s porpoises primarily forage on schooling fish such as Pacific herring (Clupea pallasii), Pacific hake (Merluccius productus), and northern anchovy (Engraulis mordax). Other potentially important prey are mesopelagic species such as lantern fish and cephalopods. The Dall’s porpoise is one of the fastest species of cetacean (whales, dolphins, and porpoises), and can reach short duration speeds of up to 50 km per hour. Their ability to maneuver at such high speeds may have evolved as an antipredator behaviour for evading Bigg’s (transient) killer whales. Observations of Bigg’s killer whales hunting Dall’s porpoises are commonly seen. However during hunts, Dall’s porpoises tend to out maneuver the killer whales, and most hunts are unsuccessful.

As a species that is commonly sighted in coastal water, Dall’s porpoises are subject to a number of anthropogenic threats. Interactions with local fisheries have resulted in the rare occurrence of entanglements resulting in death. Dall’s porpoises are also subject to pollution, such as bio accumulating chemicals.

Underwater view of a Dall's porpoise © Josh McInnes

Josh McInnes is a cetacean ecologist and a Research Scientist with Juan de Fuca Marine Research. He taught ocean sciences at the Bamfield Marine Science Centre from 2014-2016. He lives in Victoria, BC.