Harbour porpoise

Updated: September 2018

The harbour porpoise is one of the smallest cetaceans in the world. It has a dark grey colour overall, with sides that are lighter grey. A characteristic feature of these porpoises is the presence of tubercles (small hard bumps) on the leading edge of the dorsal fin. Their bodies are robust, with a small triangular dorsal fin located in the middle of the back, and relatively small, pointed flippers. The head is rounded with a poorly demarcated beak. The harbour porpoise is found primarily in coastal areas, and it has a widespread distribution throughout the northern hemisphere. It is very abundant, with a current North Atlantic population numbering around 700,000 (Hammond et al. 2008).

Harbour porpoise sightings during North Atlantic Sightings Surveys for cetaceans.

ABUNDANCE

Very abundant, with a current North Atlantic population numbering around 700,000 (Hammond et al. 2008).

DISTRIBUTION

Found primarily in coastal areas.

RELATION TO HUMANS

Historically harvested for meat and blubber in many parts of its range. Currently hunted only in Greenland, with a yearly harvest of around 2,500.

CONSERVATION AND MANAGEMENT

Harvest numbers are reported and monitored annually by the Greenlandic government. In some of its major habitats, conservation measures have been implemented after concerns about by-catch levels.

Harbour porpoise in gillnet © Krzysztof Skóra-Hel Marine Station

© Pixabay

Latin:  Phocoena phocoena

Faroese: Nísa
Icelandic: Selhnisa
Greenlandic: Niisa
Norwegian: Nise
Danish: Marsvin
Swedish: Tumlare
English: Harbour porpoise, Common porpoise

LIFESPAN

Less than 5% of the population live beyond 12 years, but maximum longevity recorded is 24 years.

AVERAGE SIZE

Adults are on average 141-163 cm long and weigh 45-65 kg, with females being larger than males at all ages.

MIGRATION AND MOVEMENTS

While there are noticeable seasonal shifts in distribution in certain locations, these porpoises do not appear to undertake coordinated migrations. Tidal currents appear to influence movements in a number of locations.

FEEDING

Flexible and opportunistic in their feeding. Diet varies by season, year and location. They take a wide variety of prey species from both benthic and pelagic habitats, but porpoises in one area tend to feed primarily on two or three species of fish.

General characteristics

The harbour porpoise (Phocoena phocoena) is one of the smallest cetaceans in the world, with a widespread distribution throughout the northern hemisphere. It is very abundant, with a current global population numbering around 700,000 (Hammond et al. 2008).

As its name implies, this porpoise is found primarily in coastal areas, though they are sometimes found over deeper offshore waters. They also frequent river estuaries, occasionally moving up the rivers many miles from the sea. The harbour porpoise has an almost circumpolar distribution, with four subspecies recognized in different regions: P. p. phocoena in the North Atlantic, P. p. vomerina, in the eastern North Pacific, an un-named subspecies in the western North Pacific and P. p. relicta in the Black Sea (Hammond et al. 2008). This account will deal with the North Atlantic subspecies: P. p. phocoena.

The harbour porpoise is a dark grey colour overall, with sides that are lighter grey. The ventral side is light grey or white, and in most individuals there are grey stripes running from the mouth along the throat to the front of the flippers. The body is robust, with a small triangular dorsal fin located in the middle of the back and relatively small, pointed flippers. The head is rounded with a poorly demarcated beak. The skull contains from 22 to 28 pairs of small spade-shaped teeth in the upper jaw, and 21 to 25 pairs in the lower. A characteristic feature of these porpoises is the presence of tubercles (small hard bumps) on the leading edge of the dorsal fin. It is not known if these have a specific purpose (Bjørge and Tolley 2018).

In the North Atlantic, adults of both sexes attain lengths from 141 to 163 cm and body masses between 46-65 kg, with females being larger than males at all ages (Gaskin 1992, Lockyer 2003). At birth the harbour porpoise averages 65-75 cm long and weighs from 6 to 10 kg. The maximum longevity recorded is 24 years, however, less than 5% of the population live beyond 12 years (Lockyer 2003).

© Pixabay
© Pixabay

Life History and Ecology

Behaviour

Harbour porpoises are a difficult cetacean to detect visually. Their small size and barely visible blow, as well as the fact that they generally spend little time at the surface and do not often engage in visible behaviours such as breaching or tail slapping, make them hard to spot. One study of porpoises feeding off the coast of Wales observed breaching at a rate of approximately once in every 120 sightings (Pierpoint 2008). Other behaviours observed in this study were “tail flipping”, where a porpoise would pivot abruptly at the surface, throwing their flukes forwards through the air (once in every 240 sightings) and tail slapping (once in 300 sightings).

Harbour porpoises are not deep divers, generally making short and frequent dives from the surface. One study of tagged porpoises found a maximum dive depth of 34 m, and a maximum duration of 213 seconds (Linnenschmidt et al. 2013). There was large variability in diving activity observed, with porpoises making anywhere from 6 to as many as 179 dives per hour.

Harbour porpoises are most often seen alone or in small groups of 2 or 3 animals. In the SCANS aerial surveys, the mean group size observed was 1.35, while a mean group of 1.53 was seen from the ship survey (Hammond et al. 2017). A similar group size (mean of 1.3 animals) was found in the Marsdiep area of The Netherlands, with only a single porpoise seen in 72% of the cases (IJsseldijk  et al. 2014). In the central North Sea, a mean group size of 1.6 (SD 1.3) individuals was observed from a ship survey, and from one to six animals was seen at a time (Cucknell et al. 2017).

© Peter Duley, NOAA Fisheries

Slightly larger groups were observed in a feeding study off the coast of Wales. At one location, groups of up to eight porpoises were commonly seen swimming abreast in a closely-spaced line, surfacing simultaneously and repeatedly. It was not clear, however, whether they were cooperating to feed or had just converged on a school of prey (Pierpoint 2008). Cooperative foraging has not been commonly observed in this species.

Sounds

Like many other toothed whales, harbour porpoises produce short pulsed high frequency (110-180 kHz) “click” sounds, primarily for echolocation (Carlström  2005, Cucknell et al. 2017). The rate of production of these clicks appears to vary with time of day, but also between individuals. Three tagged porpoises in one study had levels of echolocation activity from fewer than 100 to over 50,000 clicks per hour (Linnenschmidt et al. 2013).

Harbour porpoise clicks are distinctive to the species, and do not carry very far under water, perhaps to a maximum of 1000 m (Clausen et al. 2011). Clicks may be emitted singly or in a series called “click trains”. When echolocating, the click interval gets shorter the closer the animal is to its target (Todd et al. 2009). The use of pulsed, short-range clicks rather than whistling or other vocalizations has been suggested as an adaptation by these porpoises to avoid detection by killer whales or other predators (Clausen et al. 2011, Kyhn et al. 2013).

Studies on tagged harbour porpoises in different parts of their range have found differences in day and night patterns in click activity, with most studies showing higher production of clicks at night (e.g. Carlström 2005, Todd et al. 2009, Linnenschmidt et al. 2013). The difference in click activity is thought to be linked to foraging behaviour, which is likely related to the availability of the porpoises’ preferred prey species.

Harbour porpoises also appear to use clicks to communicate with each other. In a study of captive porpoises, specific patterns of clicks were observed which were linked to specific behaviours (Clausen et al. 2011). Aggressive behaviour, for example, was noted in which a porpoise would direct a high rate and volume of clicks at another. Porpoises exposed to this would move away from the animal producing the sounds. The same study did not find any indication that porpoises produce individual “signature” clicks that would allow them to recognize each other.

Strandings

Harbour porpoises are commonly found stranded throughout their range. Along the German North Sea coast, for example, 996 harbour porpoises were found stranded in the period 1990 to 2001, and a further 229 porpoises were found along the German Baltic Sea coast (Siebert et al. 2006). Between 2003 and 2008, an average of just over one harbour porpoise per week (1.23) was found stranded along the Danish coast (Wright et al.  2013). In the northwest Atlantic, a total of 515 porpoises were found stranded on the Gulf of Maine and Bay of Fundy shorelines in the period 2009 to 2013 (Waring et al. 2016). Harbour porpoises typically strand one at a time, rather than the mass strandings seen in some other cetacean species.

Strandings occur year-round, though in some areas may be more common at certain times of the year than others. In the North Sea, most porpoises are found stranded during the summer months, with the highest numbers found in June, July and August (Hasselmeier et al. 2004). In the Baltic, most strandings are seen a bit later, from July through September. Along the Dutch coast, strandings were most frequent between January and July (Osinga et al. 2008). Along the northeast coast of the USA, more harbour porpoises are found stranded during the winter months than during the summer (Polacheck et al. 1995).

Strandings may occur when porpoises follow a high tide in to coastal waters and become trapped when the tide recedes. Other stranded animals show signs of predation or interactions with fishing gear. Around the Moray Firth, Scotland, 63% of the harbour porpoises found stranded appeared to have been attacked by bottlenose dolphins and died from trauma, specifically bone fractures and internal injuries (Ross and Wilson 1996).

Interactions with fishing gear are a significant source of mortality for harbour porpoises throughout their range, and animals which die in this way may be found along coastlines. Along the German Baltic Sea coast, by-catch was identified as the cause of death in 47–86% of the annual strandings between 2000 and 2009 (Culika et al. 2015), as well as for about one-third of the porpoises found dead in an “unusual mortality event” in which 85 animals stranded in a single week in Denmark (Wright et al. 2013).

Disease and/or starvation also appear to be a source of mortality for harbour porpoises. In the spring, many dead juvenile porpoises in poor condition are found stranded along the northeastern U.S. east coast, apparently starved to death (COSEWIC 2006).

Life History

Age at sexual maturity for harbour porpoises varies by region, but is generally between 3 and 4 years of age for both sexes. It may, however be as young as 2 or as old as 5 years (Lockyer 2003). In the western north Atlantic, sexual maturity for both sexes is attained between 3 and 4 years old (Gaskin 1992). In the Gulf of Maine, most female porpoises were found to be mature at age 3 (Read and Hohn 1995). In the eastern north Atlantic, maturity appears to occur later. Female porpoises from the German North Sea and Baltic Sea were found to be mature at an average age of 4.95 years (Kesselring et al. 2017), and in Scottish waters, age at sexual maturity was estimated at 4.35 years for females and 5.00 years for males (Learmonth et al. 2014).

The mating season is in the summer, with mating occurring shortly after birth. In the Bay of Fundy, births occurred from May to July, with most births in May (Gaskin 1992). A similar period (May to July) was observed for porpoises in Scottish waters (Learmonth et al. 2014).  In the German North Sea and Baltic seas, the birth period is from June through August (Kesselring et al. 2017). Elsewhere in the North Sea, a birth period of between 6 June and 16 July was determined from examinations of stranded animals, with a mean date of birth of 27 June (Hasselmeier et al. 2004). This is close to the mean birth date of 30 June previously calculated for harbour porpoises in Danish waters (Sørensen and Kinze 1994).

After mating, there appears to be delayed implantation of 6 or 7 weeks (Gaskin 1992), resulting in a gestation period of 10 to 11 months (Lockyer 2003, Learmonth et al. 2014). Most females produce a calf each year. The lactation period is 8 or 9 months, although the calves may start taking solid food at around 5 months old. Calves remain with their mother for a period of around 18 months (Gaskin 1992).

Food and feeding

Studies of harbour porpoise diet have shown that it varies by season and location, and that these porpoises take a wide variety of prey species from both benthic and pelagic habitats. In many areas, small pelagic schooling fish such as herring and mackerel are important in the diet, while in other locations demersal fish such as sandeels predominate. In a study of stomach contents from 247 porpoises bycaught or stranded in the waters of Denmark, Sweden and Norway, a minimum of 30 different species of fish were identified, representing 16 families (Aarefjord et al. 1995). In porpoises from Icelandic waters, more than 40 different fish and invertebrate prey taxa were identified (Víkingsson et al. 2003). Porpoises along the northeast Atlantic coast of France were found to take 13 different species of fish and 5 species of cephalopods (Spitz et al. 2006).

While they may take a wide variety of species, porpoises in any one area tend to feed primarily on two or three species of fish. In the Gulf of St. Lawrence, for example, capelin (Mallotus villosus), Atlantic herring (Clupea harengus), and redfish (Sebastes marinus) were the three most important species in the diet, with capelin and herring making up more than 80% of the mass and caloric contribution to the diet (Fontaine et al. 1994). Capelin was found to be an important prey species for porpoises off northern Norway, while herring (Clupea harengus) was the single most important species in the diet overall in Scandinavian waters (Aarefjord et al. 1995). Capelin was also found to be the predominant prey of porpoises in Icelandic waters (Víkingsson et al. 2003). In waters off Scotland, whiting (Merlangius merlangus) and sandeels (Ammodytidae) were the most important species in the diet, where they made up more than 84% of the prey mass (Santos and Peirce 2003, Santos et al. 2004).

Blue whiting (Micromesistius poutassou), was the most frequent prey found in porpoise stomachs from the northeast coast of France, with sardine (Sardina pilchardus), scads (Trachurus sp.) and whiting (Merlangius merlangus) also being important (Spitz et al. 2006). In the western Baltic Sea, Atlantic cod and herring were the main prey of adult porpoises, constituting on average 70% of the diet mass (Andreasen et al. 2017).

Harbour porpoises generally appear to be flexible and opportunistic in their feeding, with variation not only by location, but by year and season. Differences in diet have also been seen between porpoises of differing ages and sexes. These may be due to either selective predation by the porpoises, or changes in the distribution and abundance of the prey.

Predators

Direct evidence of predation on harbour porpoises has been found for the white shark (Carcharodon carcharias), and for killer whales (Orcinus orca) (Arnold 1972, Bjørge and Tolley 2018). Grey seals (Halichoerus grypus) have also been observed both preying on and scavenging carcasses of harbour porpoises off both the coast of France in the eastern English Channel (Bouveroux et al. 2014) and the coast of Wales (Stringell et al.  2015).

While perhaps not direct predation, violent and fatal attacks on harbour porpoises by bottlenose dolphins have been reported in several locations (Santos and Peirce 2003, Spitz et al. 2006). Around the Moray Firth, Scotland, 63% of harbour porpoises stranded were found to have died from trauma, particularly multiple bone fractures and damaged internal organs, and many of the dead porpoises had cuts on their skin corresponding to dolphin teeth marks (Ross and Wilson 1996). Violent interactions between dolphins and porpoises were also witnessed directly.

Health – diseases and parasites

Little is known about the role of disease and parasitism in the health and natural mortality of harbour porpoises.

Parasites in harbour porpoises have not been well studied. A Danish investigation of 70 stranded porpoises found 5 species of helminths in the digestive tracts: Anisakis simplex, Hysterothylacium aduncum, Pholeter gastrophilus, Bolbosoma sp. and Diphyllobothrium sp. (Herreras et al. 1997), relatively few species compared to other cetaceans. This may be because harbour porpoises tend to be found singly or in very small groups instead of large schools or pods, and because their life span is short relative to other toothed whales.

In another study, stranded porpoises found on the Danish coast had nematodes in the stomach and around the heart, 4 of 85 animals had trematodes in the liver, and 5 had mild to severe parasitic  infection in the ear sinuses (Wright  et al. 2013). These parasites were not identified to species. In West Greenland, 20 harbour porpoises were investigated for parasites. Protozoa (Sarcocystis sp.), Nematoda  (Halocercus invaginatus, Stenurus minor, Anisakis simplex sensu lato (s.l.), Crassicauda sp.), Trematoda (Campula oblonga) and Cestoda (Phyllobothrium delphini, Monorygma grimaldii) were found (Lehnert et al. 2014). The nematode H. invaginatus was the most common parasite, and was found in the lungs of 19 out of 20 porpoises, while Campula oblonga was found in the pancreas and liver of 18 animals. Health effects of these parasites on the porpoises were bronchopneumonia, gastritis, cholangitis, pancreatitis and panniculitis, though these were usually mild (Lehnert et al. 2014).

While not a disease, harbour porpoises are prone to starvation. Because of their small size and limited energy reserves, they must feed frequently in order to maintain good body condition. If their food source is unavailable, they may die. Juveniles may be especially prone to starvation. Each spring, many emaciated juvenile porpoises are found stranded along the northeastern U.S. east coast, apparently starved to death (COSEWIC 2006). Emaciation was also noted as a common cause of death in porpoises found stranded on the Dutch coast (Osinga et al. 2008).

Distribution and habitat

The harbour porpoise is found primarily in coastal areas, though they may sometimes travel over deeper offshore waters. They also frequent river estuaries, occasionally moving up the rivers many miles from the sea.

In the western north Atlantic, they are found along the United States coast as far south as northern Florida, but generally to Cape Hatteras, North Carolina (Donovan and Bjørge 1995), and north along the Newfoundland and Labrador coasts to southern Baffin Island.  In the eastern north Atlantic, harbour porpoises are common and widely distributed over the continental shelf, and are found primarily at depths of 20-200 m. They range as far south as Senegal in northwest Africa, and north to Novaya Zemlya and the Barents Sea. They also occur around Greenland, as far north as Upernavik on the west coast, as well as in the waters around Iceland and the Faroe Islands. They are found in waters around the UK and Ireland, and in the North Sea. The harbour porpoise is the only cetacean that currently is found in the Baltic Sea.

In the North Sea, there have been distinct shifts in harbour porpoise distribution over the past decades. After becoming “virtually extinct” in Dutch coastal waters during the 1960s (Camphuysen 2004), they are now present along that coast and throughout the southern North Sea in increasing numbers (Alfonsi  et al. 2012, Peschko et al. 2016). The multinational Small Cetacean Abundance in the North Sea and Adjacent waters (SCANS) surveys , which have been carried out in 1994, 2005 and again in 2016, documented this trend, finding an overall shift in harbour porpoise distribution from north to south and into the English Channel (Hammond  et al. 2017).

© NOAA Fisheries

Migrations and movements

While there are noticeable seasonal shifts in harbour porpoise distribution in certain locations, these porpoises do not appear to undertake coordinated migrations.

Satellite telemetry has been used in various studies throughout their range to track harbour porpoise movements. In the Bay of Fundy and Gulf of Maine, tagged porpoises were seen to remain in one location for periods of days to weeks before moving rapidly to another location (Read and Westgate 1997). In a long term study in the North Sea and western Baltic Sea, tagged harbour porpoises were found to congregate in nine high-density areas rather than being evenly distributed throughout the region (Sveegaard et al. 2011). The same study found that immature porpoises travelled over larger areas than mature porpoises did.

Two female porpoises, one adult and one subadult, were tagged in West Greenland in 2012. The tags provided positions for more than a year. Both animals made extensive movements around Greenland, the central North Atlantic, and into Canadian waters. One animal wintered in the central North Atlantic, the other in offshore waters between Greenland and Canada. Both animals returned to the tagging site in West Greenland the following summer. They travelled over 17,500 km and 10,000 km, respectively, and spent on average 83% of their time in offshore areas with depths >200 m (Nielsen et al. 2013). These findings suggest that harbour porpoises may utilize offshore waters to a greater extent than was previously thought.

Porpoises have been seen to make seasonal movements, for example moving out of the western part of the German Baltic Sea in wintertime (Verfuß et al. 2006), but these appeared to be more individual and gradual movements rather than a synchronized migration. In the southern parts of the North Sea, harbour porpoise densities in Dutch and German coastal waters peak in the winter and spring, and decline through the summer and fall, suggesting offshore movements during these seasons. A similar pattern is seen along the Belgian coast (Haelters et al. 2011). Observations along the Dutch coast suggest that porpoises are moving inshore and offshore, rather than in a north-south direction (Camphuysen 2004). In the central North Sea, however, porpoises were found to be present year-round (Cucknell et al. 2017). In the more northern parts of their range, harbour porpoises may move to offshore waters in the winter to avoid nearshore ice (Bjørge and Tolley 2018).

Tidal currents appear to influence porpoise movements in a number of locations. Off the coast of Wales, harbour porpoises were observed regularly arriving and departing from a feeding area by moving with the prevailing tide (Pierpoint 2008). Similar behaviour has been seen along the coast of The Netherlands (IJsseldijk et al. 2015) and in the Bay of Fundy (Johnston et al. 2005).

North Atlantic stocks

In the North Atlantic, 13 different populations or sub-populations of harbour porpoises have been proposed, with a 14th group inhabiting the Black Sea (Donovan and Bjørge 1995, Andersen 2003).  While there is a clear distinction, based on genetic analysis, between harbour porpoises in the northwestern and northeastern Atlantic, the group structure within each region is not as clear. Part of the reason for this is that not all areas have been equally studied, and sample sizes for some of the suggested sub-populations are low. Differences in behaviour between the sexes also make separating stocks difficult: female porpoises tend to return to the same breeding areas each year while males may travel between areas (Andersen et al. 1997). Recent shifts in overall harbour porpoise distribution, primarily in the North Sea and along the western coasts of Europe are also confusing the population structure in this region.

The further apart the populations are geographically, the greater the genetic differences. Adjacent groups, however, appear to be more similar. Genetic analysis of the population structure of harbour porpoises from West Greenland, the North Sea and inner Danish waters, for example, found three differentiated populations but also found that there was some degree of gene flow between them (Andersen et al. 1997). Along the coast of France, genetic investigation of porpoises stranded or by-caught between 2000 and 2010 found them to be admixed individuals from the two genetically distinct populations previously identified from the Iberian coasts and further north in the northeastern Atlantic (Alfonsi  et al. 2012).

There is also some question of whether the Baltic Sea harbour porpoises are a separate population or not. One study found that the degree of genetic divergence among harbour porpoises between the Baltic Sea and the Kattegat-Skagerrak waters of Denmark is low (Palme et al. 2008), suggesting a single population in the region instead of two.

Current abundance and trends

Harbour porpoises are one of the more difficult cetaceans to obtain population estimates for. Their small size and lack of highly visible behaviours at the surface make them difficult to spot, and so the standard methods of aerial or ship surveys are likely to underestimate harbour porpoise numbers unless the survey methods are optimized for this particular species. As well, in some areas such as Greenland and Newfoundland, they are found throughout many small bays and inlets, which makes surveying additionally difficult.  Some surveys have been carried out in parts of their range and some abundance estimates have been made.

In the North Sea and waters around the UK, a series of surveys, known as SCANS (Small Cetacean Abundance in the North Sea) has been carried out. The first SCANS survey was in 1994, followed by SCANS II in 2005 and SCANS III in 2016. These surveys are large-scale ship and aerial surveys to study the distribution and abundance of harbour porpoise and other cetaceans in not only the North Sea, but other European Atlantic waters.

From the SCANS III surveys, carried out in July 2016, it is estimated that there are 466,569 harbour porpoises in the study area (95% CI 345,306 to 630,417)(Hammond et al.  2017). Results from SCANS II (2005) are 519,8640 (CI 343,521 – 786,730) and for the 1994 SCANS I survey 407,177 (CI 288,920 – 573,838) (Hammond  et al. 2017).

One of the first surveys of the North Sea in winter found an estimated abundance of 62,265 (95% CI 26,114 – 149,455) harbour porpoises in the central North Sea (Cucknell et al. 2017). The density estimate of 0.63 individuals/km from this survey is very similar to that found in the same general region from the SCANS II summer survey (0.6 individuals/km), suggesting that harbour porpoises inhabit this area of the North Sea year-round.

Harbour porpoise sightings during the North Atlantic Sightings Surveys from 1987-2015. Not all areas were surveyed in each year.

 

In the Baltic Sea, a 1995 survey gave an abundance estimate of about 600 harbour porpoises (CV = 0·57), although the entire range in the Baltic was not covered (Hammond  et al. 2002).

Further north, harbour porpoises were surveyed in Norwegian waters in 1989. For southern Norway and the northern North Sea, an abundance estimate of 82,600 (95% CI 52,100-131,000) was made, and for northern Norwegian waters and the Barents Sea the estimate was 11,000 porpoises (95% CI 4,790-25,200)(Bjørge and Øien 1995). From the SCANS III survey, an estimate was made of 25,000 porpoises along the Norwegian coast between Stadt and Vestfjorden (NAMMCO 2017).

Around Iceland harbour porpoises have been included in cetacean aerial surveys, although they are not generally the target species. The most recent survey was done in 2016, as part of the North Atlantic Sighting Surveys (NASS), a series of surveys for various species of cetaceans which have been carried out since the late 1980s. Total abundance of harbour porpoises from the 2016 survey was 38,010 (95% CI 31,755 – 161,899)(Pike et al.2017).

In West Greenland, an aerial survey for three whale species including harbour porpoise was conducted in the fall of 2007 over the shelf area from 69°N to 59°N. Prior to this study only limited information was available regarding the distribution and abundance of harbour porpoises in Greenlandic waters. The calculated total abundance estimate from the 2007 survey was 33,271 (cv = 0.39). This number was considered to be an underestimate, due to a low number of sightings at the surface, and was revised at the NAMMCO Harbour Porpoise working group meeting in November 2013 to include porpoises seen down to 1 m below the surface. The revised estimate was 50,461 (cv=0.39, 95% CI 24,043-105,904)(Heide-Jørgensen 2013).

A more recent survey was conducted in 2015 in both West and East Greenlandic waters. Abundance estimates from that survey were 83,321 harbour porpoises (cv= 0.34; 95% CI=43,377-160,047) in West Greenland and 1,642 (cv= 1.00; 95% CI= 318-8,464) in East Greenland (NAMMCO 2016). This is an increase in West Greenland from the 2007 estimate.

In the northwestern Atlantic, an abundance estimate of 12,732 (CV=0.61) harbor porpoises on the Scotian Shelf and in the Gulf of St. Lawrence was generated from the Canadian Trans-North Atlantic Sighting Survey in summer 2007 (Lawson and Gosselin 2009). This aerial survey covered the Canadian coast from northern Labrador to the Scotian Shelf. The total estimate including porpoises from the Gulf of Maine and Bay of Fundy was 16,058 (CV=0.50)(NOAA 2016). A more recent survey in the Gulf of Maine and lower Bay of Fundy which utilized both aircraft and surface vessels was carried out between June and August 2011, and generated an abundance estimate of 79,883 (CV=0.32)(NOAA 2016).

Altogether, including populations in the Baltic and Black Seas, there are estimated to be at least 700,000 harbour porpoises across the entire north Atlantic range. (Hammond et al. 2008).

Stock status

Perhaps the best indication of population status and trends can be obtained from the Small Cetacean Abundance in the North Sea (SCANS) series of surveys which have been carried out in the North Sea and waters around the UK. The first SCANS survey was in 1994, followed by SCANS II in 2005 and SCANS III in 2016. These surveys are large-scale ship and aerial surveys to study the distribution and abundance of harbour porpoise and other cetaceans in not only the North Sea, but other European Atlantic waters.

The series of three SCANS surveys do not reveal any trend in the total harbour porpoise population since the mid 1990s (see Current Abundance section).  What has been noted is a shift in distribution of porpoises, from the northern North Sea to the southern, into the English Channel and along the coasts of Belgium and The Netherlands (Camphuysen 2004, Peschko et al. 2016). The reasons for this shift are unclear.

The North Atlantic Sighting Surveys (NASS) is another series of surveys which has been carried out since the late 1980s, and can give an indication of any trends in harbour porpoise populations. Pike et al. (2009) estimated harbour porpoise abundance around Iceland for all NASS aerial surveys up to 2001. Abundance was estimated to be 4,239 (95% CI 2,724 – 6,599) in 1986 and 5,156 (95% CI 3,027 – 8,783) in 1995, while sightings were too few in 1987 and 2001 to develop estimates. The authors acknowledged that abundance was likely substantially underestimated due to uncorrected perception and availability biases.

Gilles et al. (2011) used correction factors developed during the SCANS-II survey, in which the primary harbour porpoise observer had taken part, to develop a fully corrected estimate of 43,179 (95% CI 31,755 -161,899) for the area in 2007. To date this remains the most accurate estimate of the summer abundance of this species in Icelandic coastal waters. The most recent estimate from the 2016 survey of 38,010 (95% CI 31,755 – 161,899)(Pike et al.2017) is not significantly different from this.

One area where a decline in harbour porpoise population has been noted is in the Baltic Sea. Porpoises were formerly common in the Baltic, but over the past several decades have decreased in both their distribution and abundance (Hammond et al. 2002). The reason for this is not known, and further surveys are needed to provide a more complete picture of the harbour porpoise population in the Baltic Sea.

In other parts of their range, harbour porpoises have not been surveyed systematically enough to establish any population trends.

Management

The harbour porpoise is a widespread and abundant species, and is listed in the International Union for the Conservation of Nature (IUCN) red list in the category of “Least Concern”, although the Baltic Sea subpopulation is considered “Critically Endangered” due to low population numbers (Hammond et al. 2008).

While not considered endangered, harbour porpoises are listed in Appendix II of CITES, the Convention on International Trade in Endangered Species of Wild Flora and Fauna, as well as in Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). These conventions provide for international cooperation in the management of species that are widely distributed. A further agreement was entered into under the auspices of the CMS, the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS). The ASCOBANS agreement ensures international cooperation for the management of small cetaceans, such as harbour porpoise, in European waters.

Although there is some harvest of harbour porpoise, it remains a widespread and abundant species. Hunting is allowed in Greenland year-round, and while there are no quotas or catch limits, harvest numbers are reported and monitored annually by the Greenlandic government.

In some of the major habitats for harbour porpoises (particularly the shelf waters of the USA and Europe) concerns about levels of by-catch in commercial fisheries have led to conservation measures being implemented. Examples of such measures are limiting seasons or areas for commercial fishing, or the use of acoustic deterrent devices on fishing gear.

Hunting and utilisation

Recent catches

Harbour porpoise catches in Greenland between 1994 and 2013. Data from the National Progress Reports.

Harbour porpoises were historically harvested for their meat and blubber in many parts of their range. Large directed catches were made in the Gulf of Maine and Bay of Fundy in the 18th and 19th centuries, and in Denmark, Poland and some of the Baltic countries they were hunted up until the time of World War II. There was some harvest in the Faroe Islands, but historically at quite low levels. During the late 1980s, between 10 and 20 harbour porpoises were harvested each year (Larsen 1995 in Stenson 2003), and the most recent report is that 3 were harvested in 1996.

Harbour porpoises are currently hunted only in Greenland, where the total reported harvest from 1900 to 2013 was 75,180. The period from 1993 – 2013 had a significant increase in the annual harbour porpoise harvest due to increased effort, with a total of 45,072 porpoises taken during that time. Almost all the harvest occurred in West Greenland with an average annual harvest of 2,139, while only around 13 were taken annually in East Greenland (Nielsen and Heide-Jørgensen 2013). Current harvest is around 2,500 porpoises per year (NAMMCO 2015).

Other human impacts

By-catch

Harbour porpoise in gillnet © Krzysztof Skóra-Hel Marine Station

Harbour porpoises are subject to high levels of by-catch by commercial fisheries in most parts of their range, and in some areas it is considered to be a possible threat to porpoise populations.

Harbour porpoises get caught in various types of commercial fishing gear: trawls, longlines, weirs and seines, but most are caught in pelagic or bottom set gill nets (Stenson 2003).

In the late 1990s, concern over porpoise by-catch led several organizations such as the International Whaling Commission (IWC), the International Council for the Exploration of the Sea (ICES) and NAMMCO to begin documenting and monitoring by-catch levels in the north Atlantic. At the same time many governments began to institute management measures to reduce by-catch.

The North Sea is an area where large by-catches occur. Between 1987-2001, some 5,700 porpoises were caught each year in the Danish North Sea bottom-set gillnet fisheries (Vinther and Larsen2004). In the early 1990s, an estimated 2,200 harbour porpoises were by-caught each year in the English and Irish hake fisheries in the Celtic Sea (Hammond et al. 2013). Since that time, the EU has attempted to reduce by-catch with measures such as restricting Baltic Sea drift net fisheries, making acoustic deterrent devices or “pingers” mandatory in some gillnet fisheries in the North and Baltic Seas, and using onboard observers on larger vessels to monitor by-catch.

Northern Norway is another area where there are high levels of by-catch, with an estimated average of 2,900 porpoises caught annually between 2006 and 2014 (NAMMCO 2016), primarily in gillnet fisheries for cod and monkfish.

In the northeastern Atlantic, high rates of by-catch in the U.S. and Canadian ranges of harbour porpoise during the 1990s led to the implementation of several management initiatives, primarily in the U.S., to reduce by-catches to sustainable levels. These measures have been successful to the point where current levels of by-catch are not considered to post a threat to harbour porpoises in this region (COSEWIC 2006). There is less information available for waters further north, in the Gulf of St. Lawrence and around Newfoundland and Labrador. By-catch was estimated to be several thousand per year during the early 1990s (Stenson 2003).  Reduction in fishing effort, particularly with gill nets, has likely led to a reduction in by-catch since that time, but this has not been well monitored.

Around Iceland, by-catch has been reported to be around 200 animals per year (NAMMCO 2015). This is based solely on reports by fishermen, so is likely a minimum number. Harbour porpoises were previously by-caught in salmon drift gillnet fisheries on the west coast of Greenland, but this fishery closed in 1976 (Stenson 2003).

Noise/disturbance

Due to its near shore distribution, harbour porpoises are exposed to various types of human activity throughout their range. Noise particularly is a concern, since porpoises use sound extensively in their foraging activities. Construction of wind farms in the North Sea, which involve driving pilings into the sea floor, is a source of significant noise. Harbour porpoises have been observed to move away from sites where pile driving is occurring, generally to about 20 km distance (Kastelein et al. 2018), which may result in loss of foraging opportunities. In some cases noise reducing measures, such as “bubble curtains” have been used at these construction sites, which appear to greatly reduce the area of disturbance for harbour porpoises (Peschko et al. 2016).

Another potential source of noise and disturbance is vessel traffic, although the impact of this is not clear. In the first SCANS (Small Cetacean Abundance in the North Sea) survey conducted in1994, neither the extent nor the direction of responsive movement by harbour porpoises to the survey vessels could be determined (Hammond et al. 2002).

Contaminants

Like other marine mammals, harbour porpoises ingest persistent organic pollutants (POPs) in their diet, as they consume fish which have been exposed to these compounds from coastal runoff and human industrial activities. POPs are lipophilic compounds, and so will accumulate in the lipid-rich blubber of porpoises and lead to higher levels in the tissues as the porpoises get older. In females, however, some of their contaminant burden is passed on to the calf during gestation and lactation, which may give the calf higher levels of POPs than its mother (Imazaki et al. 2015).

Studies on harbour porpoises have shown that the animals in the North Sea,  inner Danish waters and the Baltic Sea have accumulated high levels of a wide range of organochlorine contaminants such as polychlorinated biphenyls (PCBs), dioxins, hexachlorocyclohexanes (HCHs) and DDT/DDE  (Strand et al. 2005). Contaminant levels vary by region: porpoises stranded on the Belgian North Sea coast had relatively low amounts of organochlorine pesticides (DDTs, hexachlorobenzene (HCB), and HCHs) in their tissues, but higher levels of PCBs (Covaci et al. 2002). High levels of butyltin and mercury, within the range of 68-4,605 mg BT/kg and 0.22-92 mg Hg/kg, were found in the liver of harbour porpoises stranded or by-caught in Danish waters, while lower levels were seen in porpoises from West Greenland (Strand et al. 2005).

The effects of the presence of these contaminants is hard to determine, especially since there may be synergistic effects occurring when more than one type of pollutant is present (Imazaki et al. 2015). One potential effect is suppression of the immune system which could lead porpoises to increased mortality from infectious diseases. Another is that PCBs, DDT, and DDE may interfere with thyroid function in porpoises, leading to severe interfollicular fibrosis (Das et al. 2006 in Imazaki et al. 2015). An additional concern is that some POPs disrupt the endocrine and reproductive systems of marine mammals, which could lead to reproductive difficulties or failure.

Climate change

Climate change could have a number of impacts on harbour porpoises either directly or indirectly. For example, if water temperatures increase, this could have an impact on the habitat available for harbour porpoises and consequently their distribution. Harbour porpoises are currently residing longer in west Greenland, perhaps due to warming seawaters there. Changes in water temperature could also affect the distribution of the porpoises’ prey species.

In Greenland, an increase in the severity of parasitic infections and the emergence of new parasite species has been observed compared to a previous study of porpoises from 1995 (Lehnert et al. 2014). This is probably occurring due to changes in the porpoises’ diet, which is affected by increasing sea temperatures and receding ice cover. Another study in Greenland found that porpoises sampled in 2009 had a more varied diet than those sampled in 1995, with 23 different major prey items found in the recent stomachs compared to 11 previously (Heide-Jørgensen  et al.  2011). New items were also found in the 2009 samples that were not present before, Atlantic cod (Gadus morhua) and Greenland cod (G. ogaq).  The presence of these fish is thought to be due to the warmer waters occurring off West Greenland as a result of climate change.

Another impact of climate change has been observed in Norway, where in some fjords porpoises become trapped when freshwater input from a river freezes and forms a layer of ice on top of the seawater (Bjørge and Tolley2018). With warmer air temperatures, there is increased runoff from melting glaciers, and so increased freshwater input into these fjords, with the result that these entrapments may become increasingly common.

Research in NAMMCO member countries

Research into the life history, ecology and population dynamics of harbour porpoises is ongoing in several NAMMCO member countries.

In Greenland, two harbour porpoises were live captured and outfitted with satellite tags to track their movements. The success of that study has led Pinngortitaleriffik/the Greenland Institute of Natural Resources to initiate a larger study to investigate the porpoises’ migration behaviour, habitat use around Greenland, and their role in the west Greenland marine food chain. Greenland also collects tissue samples from harvested porpoises for various analyses and monitors harvest numbers.

Research in Norway is primarily focused on methods of mitigating porpoise by-catch in commercial fisheries, especially in northern Norway. By-caught harbour porpoises are also collected by the Norwegian Institute for Marine Research for biological sampling, and a food-web model is being developed for the Vestfjord area near Lofoten to study the role of harbour porpoises in this region (NAMMCO 2017).

Iceland also conducts research into levels of harbour porpoise by-catch in commercial fisheries and methods of mitigating this. As well, genetic studies are ongoing. Over 1,300 Icelandic harbour porpoises have been genotyped at 11 microsatellite loci. A recent genetic study, which included samples from Iceland, has developed single nucleotide polymorphisms for porpoises; this makes it possible to use the microsatellite data in a relatedness study, as an alternative method of estimating abundance (NAMMCO 2016).

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