(Phocoena phocoena)

Morphology

Of all the members of the family Phocoenidae, the Harbor Porpoise is the most well known, hence the nickname "Common Porpoise". Like most members of this family, the Harbor Porpoise's size is small in comparison to most other cetaceans. The rotund body ranges in length from 1.5 to 2.0 meters. This porpoise normally weighs between 45 to 65 kilograms. Like the body, the Harbor Porpoise head is also quite small. Both a forehead and beak are absent from the head, as is typical with most porpoises. The mouth is short and straight, though it does curve up slightly at the ends.

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Color

The dorsal side of the Harbor Porpoise is dark in color, often deep brown or grey. In contrast, the ventral side is much lighter, as the belly ranges from light grey to white. The lips and chin are black. Black lines extend from the jawline to the flippers. Although these color patterns are considered to be the most common for this porpoise, albinos and partial albinos have been reported.

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Fins

The dorsal fin in this species is triangular in shape and possesses a blunt tip. The trailing posterior edge is concave. The leading edge of the dorsal fin may be covered by circular bumps refered to as tubercles. The petoral fins, or flippers, are short and slightly rounded. The Harbor Porpoise exhibits a pronouned keel, or distinctive bulge on the tail stock near the flukes. A slight notch is present in the center of the fluke, and the trailing edge of the fluke is concave. The underside of the fluke is dark in color, unlike the ventral portion of the body, which is much lighter.

Behavior

Like many other porpoises, this species is quite secretive and seems to be weary of boats. Because of this shy personality, Harbor Porpoises are not often observed in the wild. In fact, the blow, a cloud of moisture rich air expelled through the blowhole, is rarely seen or photographed. The Harbor Porpoise blow can, however, be identified by the distinct sound it produces. This sound, which is a puffing noise resembling a sneeze, is often used by whael watchers to identify this species, since the porpoise itself is rarely observed.

These seemingly reserved creatures do not approach boats and are seldom seen bow-riding, like so many other other cetaceans. In fact, the Harbor Porpoise will often flee when approached by a vessel.

As may be expected of such a timid and elusive animal, this species is not considered to be gregarious, as the porpoise is usually observed alone or in pairs. When large numbers of this species are gathered together, they are usually one large group consisting of many individual pods. These pods have probably joined together for migration or to take advantage of rich feeding grounds.

Swimming Style

Very little of the Harbor Porpoise body is exposed wen surfacing to breath. They do on occasion, however, make arc-shaped leaps when chasing prey. These leaps are often difficult to spot, though, because the body leaves and returns to the water so quickly they are often overlooked.

This porpoise can reach speeds of 22km/hr. When swimming quickly, usually in the pursuit of prey, Harbor Porpoises surface four times in a row, in 10 to 20 second intervals, every 2 to 6 minutes. When traveling, they surface much more often. Usually, the porpoise will surface 8 times in a row in one minute intervals.

Life History

There is some controversy regarding the reproductive biology of the Harbor Porpoise. However, it is generally accepted that the mating season for this species occurs in the summer, from June to October. It is also believed that gestation in the Harbor Porpoise last 11 months. Age and length at sexual maturity is still widely disputed. The Harbor Porpoise calf is much duller in color than the adult form. Birth rings on the calf are useful in determining the age of the individual. The dorsal fin of the calf does not possess the tubercles observed on the adult. Life expectancy for this species is approximately 15 years.

Diet

The Harbor Porpoise diet is comprised mainly of cephlapods and fishes. These porpoises tend to prefer schooling non-spiny fishes, such as herrings, mackerels, and sardines.

Distribution/Population

The Harbor Porpoise is a nearshore, or coastal, cetacean. They are normally found in temperate and subarctic waters of the North Atlantic and Northern Pacific. This porpoise also frequents bays, rivers, estuaries, and tidal channels in Western Europe and both coasts of North America.

Studies of this species in Monterey Bay suggest a seasonal north to south migrational pattern along the coast of California. Since Monterey Bay is a suspected feeding ground for this porpoise, it is believed this movement follows seasonal availability of prey items. Although the wide range of distribution suggests a significant population for the Harbor Porpoise, specific numbers are unknown.

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Harms

Being a primarily inshore cetacean, Harbor Porpoises are often in direct conflict with man. These porpoises are threatened by man, as they are hunted for both oil and food. However, the greatest risk to this species is drowning due to accidental entanglement in fishing nets. For example, in Shetland Waters, which are a breeding and nursery ground for this species, Harbor Porpoises are in constant conflict with the local fishing industry. Likewise, between 1,200 and 2,900 porpoise were killed annually in the Gulf of Maine by bottom-set gill nets from 1990 to 1993.

Another harm to this species is pollution. Alarmingly high levels of organochlorine pesticides and heavy metals have been found in the blubber of deceased Harbor Porpoises. However, whether these toxins played a significant role in the death of these porpoises has not been determined.

Man is not the only factor which places this species at risk. Due to thier subarctic range, some Harbor Porpoises are killed when becoming trapped in frozen blocks of ice. Similarly, this species is known to have at least two natural predators, sharks and killer whales.

An additional cause of mortality for this species has recently been discovered by researchers in Scotland. From the years 1991 to 1993, 105 necropsies were performed on Harbor Porpoises which stranded along the Moray Firth in Scotland. Scientists from the Scotland Agricultural College of Veterinary Services and the University of Aberdeen's Department of Zoology determined that 63% of the porpoises died as a result of multiple skeletal fractures and damaged internal organs. Extensive bruising and hemorrhage in the subcutis and underlying musculature was also observed. In addition, teeth marks were occasionaly present on the skin of the porpoises, and the blubber layer was often torn at a depth which suggests it had been driven inward with great speed and force. In some cases, the blubber had separated completely from the underlying musculature. In their report, Wilson and Ross also noted the fracturing of rib cages in 38 individuals, as well as the dislocation of the thoracic intervertebral joint in 14 specimens. Similarly, a fatal rupture of the liver capsule was observed in 7 cases.

Since the parallel skin wounds observed on the Harbor Porpoises resembled tooth-rakes seen on other cetaceans, 8 porpoises were examined to determine the species responsible for the marks. The inner-tooth spacings of odontocetes present in the North Sea were measured, and only the spacing of the Bottlenosed Dolphin (Tursiops truncatus) matched the marks found on the porpoises. Furthermore, 95% of the strandings of Harbor Porpoises displayed skin lesions and internal injuries found within the known range of the Moray Firth Bottlenosed Dolphin population.

According to Wilson and Ross, violent interactions between Harbor Porpoises and Bottlenosed Dolphins were reported on four occasions in the Moray Firth. On two of this episodes, a solitary porpoise was viewed fleeing, and eventually escaping, a group of pursuing dolphins. On two other occasions, the interaction was recorded on video and lasted up to thirty minutes. In the recorded instances, a single porpoise was again pursued by a group of dolphins. The dolphins were observed ramming the porpoise with thier heads and snouts, sending the animal into the air clear of the surface of the water. The attck on the porpoise continued until the sluggish animal disappeared from view. The fate of this porpoise is unknown.

Because of the distribution of the Harbor Porpoise strandings, the nature of the wounds and internal injuries, and the violent interactions observed between the two species, researchers from the two institutions concluded Harbor Porpoises displaying the aforementioned skin lesions and internal injuries were killed by Bottlenosed Dolphins.

Although speculation, such as competition for food and practice-fighting, have been suggested as possible causes for this new source of mortality for the Harbor Porpoise, no definite conclusions have been drawn from this discovery. Nevertheless, though similar interactions occur between lions and hyenas, this type of behavior between other mammals is rare.