Two species of porpoises frequent the waters near Kodiak Island: The harbor porpoise and Dall’s porpoise. Our guests often ask if the terms dolphin and porpoise can be used interchangeably, and the answer is no! Porpoises and dolphins are as distinct as cats and dogs, and they belong to different taxonomic families. Porpoises are smaller than dolphins, and they are stockier and lack the characteristic “beak” of a dolphin. Porpoises have spade-shaped teeth while dolphins have conical teeth. Porpoises grow faster and reach sexual maturity at a younger age than most dolphin species. Most porpoise species are less social than dolphins, and porpoises usually hang out alone or in small, fluid groups.
The harbor porpoise is one of the smallest oceanic cetaceans, and it is the smallest cetacean found in Alaska. The body of a harbor porpoise is stocky and rotund through the mid-section, tapering to a slender tail stock. An average harbor porpoise is five ft. (1.5 m) in length and weighs 130 lbs. (60 kg). The body is dark gray or dark brown on the back, fading to a lighter gray on the sides. The throat and belly are white, but there may be a streak of gray on the throat and a dark chin patch. The flippers are dark in color, and a dark stripe extends from the flipper to the eye
Harbor porpoises primarily eat fish, but they may also feed on squid, octopus, and crustaceans. In Alaska, they feed on fish such as cod, herring, and pollock, and it has been estimated that they eat approximately 10% of their body weight each day. They surface in a slow roll and rarely “porpoise” out of the water. They are shy and seldom approach vessels, and they never play in the bow wake like Dall’s porpoises do. Large sharks, dolphins, and killer whales all prey on harbor porpoises.
Dall’s porpoise is easily identified by its striking black and white coloration that resembles the markings of a killer whale and the characteristic rooster-tail splash it often makes when surfacing. A Dall’s porpoise averages six ft. (1.8 m) in length and weighs approximately 270 lbs. (123 kg). It has a stocky, muscular body and is particularly robust through the mid-section. It has a small, round head that slopes steeply to a short, poorly defined beak. It has small teeth shaped like grains of rice. The teeth are the smallest of any cetacean species, and they often do not rise above the surface of the gums. The color pattern of Dall’s porpoises varies between individuals, but most are black on the upper sections of the body, with large oval-shaped white sides and white bellies. A band of white usually borders the flukes and the dorsal fin.
Dall’s porpoises forage at night, and they feed on small fishes and cephalopods. In Alaska, they eat squid and small schooling fishes such as capelin, lantern fish, and herring. A Dall’s porpoise consumes approximately 28 to 30 lbs. (12.7-13.6 kg) of food each day.
Dall’s porpoises are the fastest of the small cetaceans, reaching speeds of 35 mph ( 56.3 km/hr), which is a tie with killer whales for the fastest marine mammals. You can often see them from a distance, slicing through the water and creating a V-shaped splash called a rooster-tail splash. This splash creates a hollow cone that allows the porpoise to breathe under the surface of the water. Dall’s porpoises rarely engage in acrobatic behavior such as breaching or leaping out of the water, but they will charge a rapidly moving boat to ride the bow or stern waves, and they may remain in the bow wave for half an hour or more, darting in and out of the wake and making steep-angled turns.
Killer whales and sharks may prey on Dall’s porpoises, but because of their speed, agility, and fairly large body size, they often can escape predators. About 30 Dall’s porpoises per year die as a result of being caught in fishing nets in Alaska. It is unclear why they get caught in salmon nets, since they don’t feed on salmon, but many of the deep-sea species they do feed on come to the surface at night when these porpoises feed, making it more likely they will run into nets.