Porpoises are often confused with being the same animals as dolphins, but the two groups of marine mammals maintain significant differences. Porpoises are shorter, stokier animals with flat faces, as opposed to the curved beak of dolphins. Porpoises have two blow holes, whereas dolphins have one. Porpoises have tiny, flat teeth, very different from the large, conical teeth found in dolphins. Porpoise and dolphin species differ genetically and do not interbreed.

Harbour Porpoises are the smallest cetacean on the Pacific coast, reaching 1.4 to 1.8m (4.5 to 5 feet) long, and weighing 45- 68kg (110- 150lbs). They are a mottled black and gray colour, sometimes having white undersides. Their dorsal fin is small and triangular. Harbour Porpoises are common from the Aleutian Islands south to California. In the winter much of the population heads to Prince William Sound, Alaska. They are most commonly seen around Victoria from May until July. They typically travel in small groups, but large gatherings are not uncommon. Harbour porpoises are shy and not known to ride the bows or wakes of of boats.

Harbour porpoises have a 10 to 11 month gestation period, giving birth to a single calf. When calves are born they will measure approximately 1m or 3 feet. Porpoises prefer smaller prey species such as fish and squid. The average lifespan of a Harbour Porpoise is 15 to 20 years. They share the same home range with Dallʼs Porpoises, and it is now evident that the two species interbreed. In 2011, a pregnant female hybrid was found washed up on San Juan Island, revealing that Harbour-Dallʼs hybrids are not sterile as previously assumed.

The population of Harbour Porpoise is declining worldwide. The greatest threats to these shy marine mammals are being caught in fishing nets and toxic contamination from their food caused by human produced pollution. Though predation does not threaten the population, Harbour Porpoises are a prey species of transient or mammal- hunting orca.