Cetaceans - completely aquatic marine mammals - fall into two categories: baleen whales such as humpbacks; and toothed whales such as sperm whales, dolphins and porpoises. Dolphins and porpoises are similar and very closely related, but there are some differences. Dolphins have cone-shaped teeth, a larger, more curved dorsal fin, and a more prominent beak and melon, the bulging organ on the dolphin's forehead used for echolocation.
Two kinds of dolphins and two porpoise species are regularly seen in Southeast Alaska's Inside Waters: killer whales (the largest member of the dolphin family), Dall's porpoises, harbor porpoises and Pacific white-sided dolphins.
Both Pacific white-sided dolphins and Dall's porpoises "bow ride," a behavior that is ideal for wildlife watching. The bow of a moving ship creates a pressure wave in the water, something akin to the blast of wind that follows a passing truck. Porpoises and dolphins move up next to the boat and swim just below the surface, riding in the pressure wave.
Another term frequently used with marine mammals is porpoising, a style of rapid swimming at the water's surface. Dolphins, sea lions, otters and porpoises all do this. Water is a dense medium compared to air, and much more difficult to move through. A porpoising animal breaks the surface in a series of partial leaps, allowing it to move more quickly and with less energy than simply swimming.
Dall's porpoise is the most commonly seen porpoise, usually traveling in groups of two to 20 animals. These porpoises rival killer whales as the fastest creatures in Alaska waters. Avid bow riders, they will readily approach and accompany boats, breaking the surface with a splash but rarely showing more than their backs. Their black back and white belly and flank resemble the markings of the killer whale, but they are much smaller, averaging about six feet in length and weighing about 300 pounds.
Harbor porpoises are dark gray or dark brown, with a noticeably smaller dorsal fin than Dall's porpoises. They are often described as shy, but it is more appropriate to say they are indifferent to boats and human activities. They do not bow ride. Fairly common in Southeast waters, they are most often seen feeding, their round backs gently breaking the surface with a rolling, dipping profile. Averaging about 120 pounds, the harbor porpoise is less than half the weight of the Dall's porpoise and is the smallest cetacean in Alaska.
Pacific white-sided dolphins may travel with Dall's porpoises. They are similar in size and appearance, but white-sided dolphins are light gray along the side of their heads, where Dall's heads are black. Pacifics bow ride and are more acrobatic than Dall's and often show their fins and flukes when swimming. They are rare north of Wrangell and are most often seen between Wrangell and Ketchikan or out in the open ocean.