General update: January 2020
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 global population numbering at least 700,000 (Hammond et al. 2008).
The harbour porpoise is very abundant, with a global population numbering at least 700,000 (Hammond et al. 2008).
The species is primarily found in coastal areas.
Harbour porpoises have historically been harvested for meat and blubber. In the NAMMCO area it is currently hunted only in Greenland, with a yearly harvest of around 2,500.
Harvest numbers are reported annually by the Greenlandic government. In some of its major habitats, conservation measures have been implemented following concerns about levels of by-catch in commercial fisheries.
In the most recent global assessment, the species is listed as ‘Least Concern’ on the IUCN Red List (2008).
Scientific name: Phocoena phocoena
English: Harbour porpoise, Common porpoise
Less than 5% of the population live beyond 12 years, but the maximum longevity recorded is 24 years.
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.
Harbour porpoises are flexible and opportunistic in their feeding. Their 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.
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.
Appearance, size and longevity
The harbour porpoise has 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).
Life History and Ecology
At the surface
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 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 every 240 sightings) and tail slapping (once in 300 sightings).
Harbour porpoises are not deep divers and generally make short but 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, however, a large variability in diving activity observed, with porpoises making anywhere from 6 to 179 dives per hour.
Harbour porpoises are most often seen alone or in small groups of 2-3 animals. In the SCANS aerial surveys (studying the distribution and abundance of cetaceans in European Atlantic waters), the mean group size observed was 1.35, while a mean group size 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 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, with groups between 1-6 animals seen at any one time (Cucknell et al. 2017).
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.
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 1000m (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 a 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 that 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.
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.
Timing of strandings
Strandings occur year-round, though may be more common at certain times of the year in some areas. 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 little later, from July through September. Along the Dutch coast, strandings have been 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).
By-catch and entanglements
Interactions with fishing gear are a significant source of mortality for harbour porpoises throughout their range, and animals that 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).
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). Most female porpoises in the Gulf of Maine 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 by-caught 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).
Main prey species
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.
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 and the coast of Wales (Bouveroux et al. 2014, 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 infections 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, harbour porpoises are found along the United States coast as far south as northern Florida, but more commonly down 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 as far north as Novaya Zemlya and the Barents Sea. These porpoises 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 also inhabit the waters around the UK and Ireland, and the North Sea. The harbour porpoise is also the only cetacean currently 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 with an overall shift in harbour porpoise distribution from north to south and into the English Channel (Hammond et al. 2017).
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 to track harbour porpoise movements has been used in various studies throughout their range. 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 of more than 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 and in the Bay of Fundy (IJsseldijk et al. 2015, 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). Even though there is a clear distinction based on genetic analysis, the group structure within the Northwest and Northeast Atlantic is not 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 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 Northeast Atlantic (Alfonsi et al. 2012).
It is also not entirely clear whether the Baltic Sea harbour porpoises are a separate population. 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. Recent genetic analysis presented at the Joint IMR/NAMMCO International Workshop on the Status of Harbour Porpoises in the North Atlantic (NAMMCO and IMR 2019) suggested a clear distinction between North Sea and Baltic Sea populations, with a transition zone in the Kattegat. Within the Baltic Sea, a further subdivision was also seen between the western Belt Sea and an eastern inner Baltic sea sub-population, with a transition zone around the southern tip of Sweden. Given that the distinction was most prominent during times of reproduction, some seasonal migration across population boundaries may be occurring. Studies combining genetic analysis, morphometrics, telemetry and passive acoustic monitoring also suggest three distinct populations in the North Sea, the Belt Sea (and adjacent waters) and the Baltic proper (NAMMCO and IMR 2019) .
In 2018, a Joint Institute of Marine Research/NAMMCO International Workshop on the Status of Harbour Porpoises in the North Atlantic was held in Tromsø, Norway. This workshop was followed by a working group focusing on harbour porpoises in West Greenland. The reports from both these meetings are available here.
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. Furthermore, 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, however, been carried out in parts of their range and some abundance estimates made.
Altogether, it has been estimated that there are at least 700,000 harbour porpoises globally (Hammond et al. 2008).
UK waters and the North Sea
In the North Sea and waters around the UK, a series of surveys known as SCANS (Small Cetacean Abundance in European Atlantic Waters and the North Sea), have 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.
From the SCANS III surveys, carried out in July 2016, it was estimated that there were 466,569 harbour porpoises in the study area (95% CI: 345,306 -630,417) (Hammond et al. 2017). Results from SCANS II (2005) are 519,864 (95% CI: 343,521 – 786,730) and for the 1994 SCANS I survey 407,177 (95% CI: 288,920 – 573,838) (Hammond et al. 2017).
One of the first surveys of the North Sea done 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.
The Baltic Sea
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 (unpublished data reported in Hammond et al. 2002).
Harbour porpoises were first 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 at that time, 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 of 25,000 porpoises was made for the Norwegian coast between Stadt and Vestfjorden (NAMMCO 2017).
After 1995, Norway began a mosaic survey program and from 2003 this was combined with a joint Norwegian-Russian ecosystem survey in which the transits between stations in the joint survey were used as transects for marine mammal detection (NAMMCO 2018). These mosaic surveys cover southern Norway, the northern North Sea and the Barents Sea.
From the sighting surveys conducted during the period 2002-2007, harbour porpoise abundance was estimated to be 189,604 (95% CI: 129,437 – 277,738). Following the sighting surveys conducted during the period 2008-2013, abundance was estimated at 30,556 (95% CI: 10,178 – 76,120). From surveys conducted during 2014-2018, the estimated abundance was 255,929 animals (95% CI: 172,742 – 379,175). Descriptions of the calculation of each of these abundance estimates, as well as a discussion of the anomalously low 2008-2013 estimate can be found in the 2019 report of the NAMMCO Abundance Estimates Working Group (NAMMCO 2019).
Harbour porpoises have been included in cetacean aerial surveys around Iceland, although they are not generally the target species.
Pike et al. (2009) estimated harbour porpoise abundance around Iceland for all North Atlantic Sighting Surveys (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.
The most recent NASS survey was done in 2016. During the 2016 survey, block 5 of the area was not able to be surveyed. Total abundance of harbour porpoises from the 2016 survey was estimated to be 22,806 (95% CI: 9,166 – 56,746) (NAMMCO 2019). This estimate has been corrected for perception bias but not availability bias (i.e. animals that are missed because they are submerged and unavailable for counting) and it should be noted that this bias can be substantial in aerial surveys for this species.
In West Greenland, an aerial survey for three whale species, including harbour porpoises, 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. A more recent survey was conducted in 2015 in both West and East Greenlandic waters. The initial calculated total abundance estimate from the 2007 was considered to be an underestimate due to a low number of sightings at the surface and it was revised at the NAMMCO Harbour Porpoise Working Group meeting in November 2013 to include porpoises seen down to 1 m below the surface.
During a meeting of the Harbour Porpoise Working Group in 2019, the estimates for West Greenland from both the 2007 and 2015 surveys were corrected to take into account animals that may not have been present in the strata at the time of the survey. Satellite tracking data from 26 harbour porpoises in Greenland was used to estimate the proportion of porpoises expected to be outside the strata at the time of the survey. On this basis, the fully corrected abundance estimates were 69,595 (95% CI: 34,689 – 139,624) porpoises in 2007, and 106,822 (95% CI: 55,149 – 206,909) porpoises for 2015.
In the Northwest 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).
UK, the North Sea and European Waters
Perhaps the best indication of population status and trends can be obtained from the 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 porpoises and other cetaceans in European Atlantic waters and the North Sea.
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, however, is a shift in distribution of porpoises, from the northern to the southern parts of the North Sea, 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.
One area where a decline in the harbour porpoise population has been noted is in the Baltic Sea. Porpoises were formerly common in the Baltic, but over the past several decades the population has decreased in both distribution and abundance (Hammond et al. 2002). The reason for this is not known, and further research is needed to provide a more complete picture of the harbour porpoise population in the Baltic Sea.
The NAMMCO Harbour Porpoise Working Group carried out an assessment for West Greenland in 2019 (NAMMCO 2019b). Bayesian population models were run using the corrected abundance estimates from the 2007 and 2015 surveys (see Abundance section), together with catch and age-structure data. This modelling showed that the population in West Greenland is relatively stable. The report with the the full assessment is available here.
In other parts of their range, harbour porpoises have not been surveyed systematically enough to establish any population trends.
The harbour porpoise is a widespread and abundant species. It is listed in the International Union for the Conservation of Nature (IUCN) red list in the category of “Least Concern”, although the Baltic Sea sub-population 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 CMS, the Convention on the Conservation of Migratory Species of Wild Animals. 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 porpoises, 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. Further information also provided under Hunting and Utilisation.
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 (‘pingers’) on fishing gear. For more information see Other Human Impacts.
Hunting and utilisation
Harbour porpoises were historically harvested for their meat and blubber across many parts of their range. Large catches were made in the Gulf of Maine and Bay of Fundy in the 18th and 19th centuries. In Denmark, Poland and some of the Baltic countries, harbour porpoises were hunted up until World War II. There was also some harvest in the Faroe Islands, but historically this has been at quite low levels. During the late 1980s, between 10 and 20 harbour porpoises were harvested each year in the Faroe Islands (Larsen 1995 in Stenson 2003), and the most recent report is that 3 were harvested in 1996.
Harbour porpoises are currently only hunted in Greenland, where the total reported harvest from 1900 to 2013 was 75,180. The period from 1993 – 2013 saw a significant increase in the annual harbour porpoise harvest due to increased effort, with a total of 45,072 porpoises taken during that time. In this period, 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). The current harvest level is around 2,500 porpoises per year (NAMMCO 2015).
Reported catches in NAMMCO member countries
|Faroe Islands||2018||Faroe Islands||N/A|
|Faroe Islands||1997-2017||Faroe Islands||N/A|
|Faroe Islands||1996||Faroe Islands||3|
|Faroe Islands||1992-1995||Faroe Islands||N/A|
|Greenland||1992||Total||*No reported catches|
This database of reported catches is searchable and information can be filtered by country, species or area. It is also possible to sort the information by the different columns, in ascending or descending order, by clicking the column you want to sort by and the associated arrows for the order. By default, 30 entries are shown, but this can be changed in the drop-down menu to show up to 100 entries per page.
Carry-over from previous years are included in the quota numbers, where this is applicable.
You can find the full catch database with all species here.
You can find a complete file with all comments and explanations here, under Overview Documents.
For any questions regarding the catch database, please contact the Secretariat at firstname.lastname@example.org.
Other human impacts
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.
The various types of commercial fishing gear harbour porpoises get caught in include: trawls, longlines, weirs and seines, but most are caught in pelagic or bottom set gill nets (Stenson 2003).
Monitoring and mitigation
In the late 1990s, concern over harbour porpoise by-catch levels 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 in the North Atlantic. At the same time, many governments began to institute management measures to reduce by-catch.
The North Sea
The North Sea is an area where high levels of by-catch occur. Between 1987-2001, some 5,700 porpoises were caught each year in the Danish North Sea bottom-set gillnet fisheries (Vinther and Larsen 2004). 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. The Norwegian Institute of Marine Research is currently conducting research into the use of pingers, or Acoustic Deterrent Devices, to mitigate by-catch levels.
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).
The Northwest Atlantic
In the Northwest 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-catch to sustainable levels. These measures have been successful to the point where current levels of by-catch are not considered to pose 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 the use of gill nets, has likely led to a reduction in by-catch since that time, but this has not been well monitored.
Due to its near shore distribution, harbour porpoises are exposed to various types of human activity throughout their range. Noise is a particular concern since porpoises use sound extensively in their foraging activities. It has been shown that porpoises feed almost constantly on around three thousand small fish per day (Wisniewska et al., 2016; Wisniewska et al., 2018). This behaviour makes these animals vulnerable to any disturbance that may affect their foraging and thereby their long-term fitness.
Types of underwater noise & impacts
Underwater noise may be divided into ambient background noise and man-made anthropogenic noise. The first is caused by natural sources like waves, wind, earth quakes and animals, while the other is caused by sources such as ships, military operations, oil and gas exploitation and extraction, wind turbines, bridges and building of offshore constructions.
The effect of noise on marine mammals can be determined on several levels: Effects on individuals (from changes in behaviour to survival); Changes in relative densities on a local scale; Changes in distribution and abundance; Population level effects.
Construction of wind farms in the North Sea, which involves driving piles 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 a 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).
Studies looking at the effects from operating wind farms have shown mixed results. Negative long-term effects have been documented in the Baltic Sea (Teilmann & Carstensen, 2012). No effects were seen in the eastern North Sea (Tougaard et al., 2006), while positive effects were observed in the southern North Sea (Scheidat et al., 2011). These diverging results may be related to factors such as: i) an increase in noise level due to turbine noise, ii) high motivation to follow fish schools leading to ignoring any noise increase, or iii) an increase in fish abundance due to reduced fishing activities. This means that similar offshore constructions may have very different effects on harbour porpoises depending on the context.
Although offshore constructions may create noise, they may also create artificial reefs with increased productivity and biodiversity that can also actually benefit top predators like the harbour porpoise.
Another potential source of noise and disturbance is vessel traffic. Analysis of harbour porpoises tagged in the Danish Belt Seas has shown that these animals experience ship noise from 17-89% of the time (Wisniewska et al., 2018). The study also showed a significant reduction in foraging echolocation (buzzing) at levels of received ship noise that were not unusual in Danish waters.
In a recent study using play-back signals from a single 10 inch3 airgun every 2-3 sec for 1 minute results showed that some animals responded with changes in horizontal movements (faster, longer and more directed) and shorter and shallower dives for up to 8 hours, with an additional 24 hours taken to resume baseline behaviour, while others did not show a response (van Beest et al., 2018). This indicates that the response of the animals to similar stimuli may depend on individual robustness to disturbances.
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 as such will accumulate in the lipid-rich blubber of porpoises, leading 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 a higher level 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).
Effects of contaminants
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 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). Another concern is that some POPs disrupt the endocrine and reproductive systems of marine mammals, which could lead to reproductive difficulties or failure.
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 than previously seen, perhaps due to warming seas there. Changes in water temperature could also affect the distribution of the porpoises’ prey species.
Change in diet and increase in parasites
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 from 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 Tolley 2018). 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.
The 2019 meeting of the NAMMCO Harbour Porpoise Working Group, which produced an assessment of the population in West Greenland, noted that specific and systematic monitoring of all the different non-hunting related anthropogenic stressors on harbour porpoises and their potential to have cumulative impacts is not currently being systematically performed. It suggested that further research on life history parameters and dietary shifts from existing samples could be one relevant approach to investigate these impacts further. It was also noted that although these stressors may be having an impact on an individual level, they did not as yet appear to be having population level impacts in Greenland.
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. 30 harbour porpoises were thus caught and instrumented with satellite transmitters. These porpoises displayed long-range oceanic movements, in contrast to 71 porpoises tagged in Danish waters which did not leave the continental shelf (Nielsen et. al, 2018). 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. The Norwegian Institute for Marine Research (IMR) is currently running experiments with Acoustic Deterrent Devices (ADDs), called pingers, on large-mesh gillnets in the coastal zone. Thus far, the use of pingers has resulted in a significant reduction of net entanglements when compared to nets without pingers (National Progress Report Norway 2018). By-caught harbour porpoises are also collected by the IMR 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. Pingers were tested for the first time in the Icelandic cod gillnet fishery in April 2017, but their use showed no reduction in porpoise by-catch. A different type of porpoise deterrent, the Porpoise Alerting Device (PAL) was tested in 2018, with the same results. Further pinger trials were planned for April 2019 (National Progress Report Iceland 2018).
Genetic studies are ongoing in Iceland as well, in a collaboration between the MFRI and the University of Potsdam. 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). Continuous porpoise detectors (C-PODS) were also deployed in Skjálfandi Bay in 2018 for detection of harbour porpoises (National Progress Report Iceland 2018).
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