(Linnaeus, 1758) - Harbor porpoise
The harbour porpoise is a chunky animal, with a blunt short-beaked head. Placed about midway along the back is a short, wide-based, triangular dorsal fin, with small bumps on the leading edge. The flippers are small and somewhat rounded at the tips. The flukes have a concave trailing edge, divided by a prominent median notch; the tips are rounded. The straight mouthline slopes upward towards the eye.
Countershading is apparent in the harbour porpoise's colour pattern; the animals are generally dark grey on the back and white on the belly. The sides are intermediate, with the border area often splotched with shades of grey. The flippers and lips are dark; there is a thin, dark grey gape-to-flipper stripe.
Nineteen to 28 small, spatulate, blunt teeth line each tooth row.
Can be confused with
Harbour porpoises, if seen clearly, should not be confused with any of the various species of dolphins that share their range. The 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 colour pattern and slight difference in dorsal-fin shape of Dall's porpoise will be distinguishable, when seen well.
Most adult harbour porpoises are less than 1.8 m long; maximum length is about 2 m. Females are slightly larger than males. Weights range from 45 to 70 kg for adults. Newborns are 70 to 90 cm long.
Harbour porpoises are found in cool temperate and 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 southern 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 other, and many provisional stocks have been recognized.
Biology and Behaviour
Most harbour porpoise groups are small, consisting of less than 8 individuals. They do, at times, aggregate into large, loose groups of 50 to several hundred animals, mostly for feeding or migration. Behaviour tends to be inconspicuous, compared to most dolphins, and harbour porpoises rarely approach boats to ride bow waves. When moving fast, they surface in a behaviour often called pop-splashing. Breaches and other leaps are rarely seen. Harbour porpoises sometimes lie at the surface for brief periods between submergences, although we do not know why they do this.
Reproductive biology has been well-studied in some parts of the world. Most calves are born from spring through mid-summer.
Harbour porpoises eat a wide variety of fish and cephalopods, 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.
Harbour porpoises produce an extreme large variety of acoustic signals, being echolocation pulses and social signals. These underwater sounds are highly variable with respect to pulse shape, frequency content, pulse repetition rate, etc. In fact in the frequency range from 0 to 160 kHz acoustic signals are emitted. Individual signals contain components which can be classified according to the following frequency ranges:
(i) one or two low-frequency components in the 1.4 kHz - 2.5 kHz range.
(ii) one or two components in the 110 kHz - 140 kHz range; these are the most dominant components,
(iii) low energy components at around 30 kHz and 60 kHz,
(iv) broadband component between 13 kHz and 100 kHz.
Because the low-frequency component (i) has a more omnidirectional beam pattern, it is assumed that this component is used for communication. The high-frequency component (ii) often appears to consist of two parts, at 120 and 140 kHz. The high frequency and consequently the narrow beam pattern does suggest that this signal is probably used for accurate bearing detection of objects. Signal type (iv) overlaps with the range of highest hearing sensitivity of Harbour porpoises, so these signals may be used for long-range scanning. In addition to these four signal types, continuous low-frequency sine signals are sometimes recorded, which may be described as whistles and will have a social function (Verboom and Kastelein, 1995).
A major human threat to harbour porpoises throughout their range is incidental capture in fisheries. Many thousands are taken each year in gillnets and in certain areas, incidental catches in herring weirs, cod and salmon traps, purse seines, trawl nets, and longlines also occur.
Directed fisheries have occurred in Puget Sound, the Bay of Fundy, Gulf of St. Lawrence, Labrador, Newfoundland, Greenland, Iceland, Black Sea, and the Baltic Sea. Many of these fisheries are now closed, but hunting of harbour porpoises still occurs in a few areas. Greenland and the Black Sea are the only areas where large direct catches have been reported recently.
Levels of pollutants in harbour porpoise tissues have been found to be high wherever studied, probably due to the species' coastal nature. Environmental contamination has been implicated, in part, for declines in harbour porpoise populations in Europe and some parts of North America.