The harbor porpoise is a small, stocky animal with a blunt short-beaked head. Placed about midway along the back is a short triangular dorsal fin with a wide base, generally with small bumps on the leading edge. The flippers are small and rounded at the tips. The flukes have a concave trailing edge, divided by a prominent median notch, and the tips are rounded. The straight mouthline slopes upward towards the eye.
Countershading is apparent in the harbor porpoise’s color pattern; the animals are generally medium to dark gray on the back and white on the belly. The sides are intermediate, with the border area often splotched with shades of gray. The flippers and lips are dark and there is a thin, dark gray gape-to-flipper stripe. Nineteen to 28 small, blunt teeth line each tooth row.
Most adult harbor porpoises are less than 1.8 m long; maximum length is about 2 m. Females are slightly larger than males. Weights range from 45-70 kg for adults. Newborns are 70-90 cm long.
Can be Confused With
Harbor porpoises, if seen clearly, should not be confused with any of the various species of dolphins that share their range. The only other porpoise that overlaps in the North Pacific, Dall’s porpoise, can be confused with this species when backlit fins are seen at a distance. However, the black and white color pattern and slight difference in dorsal fin shape of Dall’s porpoise will be unmistakable when seen well. Also, the behavior of the two species tends to be different, with Dall’s either roostertailing or bringing the tail stock higher out of the water when rolling.
Harbor porpoises are found in cool temperate to subpolar waters of the Northern Hemisphere. They are usually found in shallow water, most often nearshore, although they occasionally travel over deeper offshore waters. In the North Pacific, they range from central California and northern Honshu, Japan, to the southern Beaufort and Chukchi seas. In the North Atlantic, they are found from the southeastern United States to southern Baffin Island (they apparently do not enter Hudson Bay) in the west and Senegal, West Africa, to Novaya Zemlya in the east. Major populations in the North Pacific and North Atlantic are isolated from each another, and many provisional stocks have been recognized.
Ecology and Behavior
Harbor porpoises inhabit coastal waters. Some harbor porpoise populations migrate, such as those on the eastern coasts of the United States and Canada. Other populations have more restricted movements, as do stocks on the western coast of the continental United States. Geographic ranges of distinct stocks are often finely scaled.
Most harbor porpoise groups are small, generally consisting of less than five or six individuals. They do, at times, aggregate into large, loose groups of 50 to several hundred animals, mostly for feeding or migration. Behavior tends to be inconspicuous, compared to most dolphins. Harbor porpoises rarely approach boats to ride bow waves, and often actively avoid vessels. When moving slowly they tend to surface in a slow gentle roll. When moving fast, they surface in a behavior often called pop-splashing. Breaches and other leaps are rarely seen. Harbor porpoises sometimes lie at the surface for brief periods between submergences, although it is not known why they do this.
Reproductive biology has been well-studied in some parts of the world. Sexual maturity is generally reached at 3-4 years of age, with geographic and density-dependent variation. In some areas of this species’ range females give birth every year, while in others harbor porpoises give birth every other year. Pregnancy lasts 10.6 months, most calves being born from spring through midsummer. Lactation is thought to last between 8 and 12 months. Females in some stocks are likely to be pregnant and lactating simultaneously, placing heavy energy requirements on these individuals. This is a relatively short-lived odontocete, in which specimens living past 20 years are rarely found.
Feeding and Prey
Harbor porpoises eat a wide variety of fish and cephalopods (fish > squid > invertebrates), and the main prey items appear to vary regionally. Small, non-spiny schooling fish (such as herring and mackerel) are the most common prey in many areas, and many prey species are benthic or demersal. Harbor porpoises feed by seizing prey.
Other Invertebrates: Nereis sp., Meganyctiphanes norvegica, Pandalus montagui
Threats and Status
Main threats to harbor porpoises include:
• Fisheries bycatch
• Entanglement in fishing gear
• Organochlorine contamination
The IUCN classifies harbor porpoises as a vulnerable species, although the United States government does not consider the species threatened or endangered in its waters. The harbor porpoise faces many threats at the hands of humans. The species has been hunted in many areas of its range, and the major kills once occurred in the Bay of Fundy, Danish Baltic Sea, Black Sea, and Greenland. Today, the most significant threat in most areas is incidental catches in fishing nets, primarily various types of gillnets. Kills of over 1,000 porpoises per year have been documented for the Gulf of Maine, West Greenland, North Sea, and Celtic Shelf, but smaller kills occur in many other areas. In addition to gillnets, harbor porpoises are also taken in trawls, Japanese set nets, herring weirs, pound nets, and cod traps. Finally, other types of threats include pollution, vessel traffic, noise, and overfishing. Environmental contaminants may also pose a threat in some heavily industrialized areas.
The United States National Marine Fisheries Service considers animals in United States waters to be members of several distinct stocks, and assesses them separately. Delineations between stocks are often difficult to determine, therefore assessments should be considered ongoing processes. Stocks are estimated as follows
Morro Bay stock – 932 (CV = 0.41) based on 1997-1999 aerial surveys
Monterey Bay stock – 1,603 (CV = 0.42) based on 1997-1999 aerial survey data
San Francisco-Russian River stock – 6,674 (CV = 0.39) based on 1997-1999 aerial survey data
Northern California/southern Oregon stock – 17,763 (CV=0.39) based on 1997-1999 aerial survey data
Oregon/Washington coast stock – 39,586 (CV = 0.384) based on 1997 aerial survey data
Washington Inland Waters stock – 3,509 (CV = 0.396) 1997 estimate based on aerial surveys conducted in 1996
Southeast Alaska stock – 10,508 (CV = 0.274) based 1997 aerial survey data
The Gulf of Alaska stock – 21,451 (CV = 0.309) based on 1998 aerial survey data
Bering Sea Stock – 10,946 (CV = 0.300) based on 1991 aerial survey
Gulf of Maine/Bay of Fundy stock (inhabiting both United States and Canadian waters) – 89,700 (CV = 0.22) based on 1999 survey data.
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