Harbor Porpoise

Description

The harbor porpoise is one of the smallest cetaceans in the world and the most commonly encountered porpoise species in the Northern Hemisphere. Adults measure only 4.5 to 6 feet in length and weigh between 100 and 170 pounds, making them among the smallest marine mammals in the waters off Southern California. Their body shape is stocky and rotund, lacking the streamlined elegance of dolphins. The coloration is dark gray to dark brown on the dorsal surface, fading to a lighter gray on the flanks with a white or pale belly. A faint gray stripe may extend from the corner of the mouth to the flipper. Unlike dolphins, harbor porpoises have a short, triangular dorsal fin with a broad base rather than the tall, curved fin typical of dolphin species. Their head is rounded with no discernible beak, and their mouth is small with 22 to 28 small, spade-shaped teeth on each side of both jaws. The spade-shaped teeth are a key distinguishing feature of porpoises versus dolphins, which have conical teeth. Harbor porpoises are generally inconspicuous at the surface, producing a low, barely visible blow.

Habitat & Range

Harbor porpoises are found in cool temperate and subarctic waters of the Northern Hemisphere, including the North Pacific, North Atlantic, and Black Sea. In the eastern North Pacific, they range from Point Barrow, Alaska, to Point Conception, California, with a small number occasionally venturing further south into the Southern California Bight. The Morro Bay stock, which ranges from Monterey Bay to an area near Point Conception, represents the southernmost regularly occurring population in the eastern North Pacific. Harbor porpoises are occasionally sighted off the coast of Orange County, but they are at the southern edge of their range and are far less common here than in waters further north. They are strongly coastal animals, preferring shallow waters less than 200 meters deep over the continental shelf, and are frequently found in harbors, bays, estuaries, and river mouths, which gives them their common name. They tend to avoid areas of strong currents and wave action, preferring calm, sheltered waters.

Diet

Harbor porpoises feed on a variety of small fish, cephalopods, and crustaceans found in shallow coastal waters. Their diet varies regionally but commonly includes herring, sardines, anchovies, capelin, whiting, pollock, and various species of smelt. They also consume small amounts of squid, octopus, and shrimp. Harbor porpoises are generally solitary foragers that hunt close to the bottom or in mid-water, using echolocation to detect prey. Their echolocation clicks are at very high frequencies (around 130 kHz), well above the hearing range of most potential predators such as orcas, which may be an adaptation to avoid detection while hunting. Harbor porpoises have a high metabolic rate relative to their body size and must eat frequently, consuming approximately 7 to 10% of their body weight daily, which translates to roughly 7 to 15 pounds of food. They typically make short, shallow dives of 1 to 5 minutes when foraging, rarely exceeding depths of 200 meters. Their small size and high energy demands mean they are particularly sensitive to disruptions in prey availability.

Behavior

Harbor porpoises are among the most shy and elusive of all cetaceans, a stark contrast to the bold and curious behavior displayed by many dolphin species. They typically avoid boats and rarely bow-ride, instead tending to move away from approaching vessels. They are usually observed alone or in small groups of two to five individuals, though loose aggregations of up to 20 or more may occur in productive feeding areas. Their surface behavior is subtle: they roll gently at the surface to breathe, creating little disturbance, and they almost never breach or engage in the acrobatic displays characteristic of dolphins. This unobtrusive behavior, combined with their small size and low blow, makes them difficult to spot and easy to overlook. Harbor porpoises are relatively slow swimmers, typically traveling at 2 to 5 miles per hour, though they can reach speeds of up to 14 miles per hour in short bursts. Females give birth to a single calf after a gestation period of approximately 10 to 11 months, usually in late spring or early summer. Calves are weaned at about 8 to 12 months. Harbor porpoises have a relatively short lifespan for a cetacean, typically living only 8 to 13 years, though some individuals have been documented at up to 24 years.

Conservation Status

The harbor porpoise is listed as Least Concern globally on the IUCN Red List, though some regional populations are of greater conservation concern. In the United States, they are protected under the Marine Mammal Protection Act. The main threat to harbor porpoises worldwide is entanglement and drowning in fishing gear, particularly bottom-set gillnets, which are set in the same shallow coastal waters where harbor porpoises feed. Bycatch in gillnets has been identified as the primary source of human-caused mortality for this species in many regions. In California, the set gillnet fishery has been subject to increasing restrictions, and gillnets have been largely banned in state waters, which has benefited harbor porpoises. Other threats include habitat degradation from coastal development, pollution, vessel disturbance and noise, prey depletion from overfishing, and the effects of climate change on nearshore marine ecosystems. Harbor porpoises are particularly sensitive to anthropogenic noise, which can displace them from important feeding and breeding areas. Given their position at the southern edge of their range off Southern California, warming ocean temperatures associated with climate change could further restrict their distribution in this region.