Harbour porpoise: Difference between revisions

Although conjoined twins are rarely seen in wild mammals, the first known case of a two-headed harbour porpoise was documented in May 2017 when Dutch fishermen in the [[North Sea]] caught them by chance.<ref>[https://web.archive.org/web/20210709183356/https://www.nationalgeographic.com/animals/article/two-headed-porpoise-found-first Two-Headed Porpoise Found For First Time] - Retrieved from National Geographic website - June 14, 2017</ref> A study published by the online journal of the [[Natural History Museum Rotterdam]] points out that conjoined twins in whales and dolphins are extremely rare.<ref>[http://www.hetnatuurhistorisch.nl/fileadmin/user_upload/documents-nmr/Publicaties/Deinsea/Deinsea_17/Kompanje_Campuysen_Leopold_DEINSEA_17_1-5_2017_06_07.pdf The first case of conjoined twin harbour porpoises Phocoena phocoena]</ref>

The harbour porpoise has a global population of at least 700,000.<ref name=Bjorge2018/> In 2016, a comprehensive survey of the Atlantic region in Europe, from [[Gibraltar]] to [[Vestfjorden]] in Norway, found that the population was about 467,000 harbour porpoises, making it the most abundant cetacean in the region, together with the [[Short-beaked common dolphin|common dolphin]].<ref name=StAndrewsTotal>{{cite web| title=New study: 1.5 million whales, dolphins and porpoises in European Atlantic | url=https://www.st-andrews.ac.uk/news/archive/2017/title,1276565,en.php | publisher=University of St. Andrews | date=2 May 2017| access-date=1 June 2017}}</ref> Based on surveys in 1994, 2005 and 2016, the harbour porpoise population in this region is stable.<ref name=StAndrewsTotal/> The highest densities are in the southwestern [[North Sea]] and oceans of mainland [[Denmark]];<ref name=StAndrewsTotal/> the latter region alone is home to about 107,000-300,000 harbour porpoises.<ref name=FyensS>{{cite news| title=Marsvin og delfiner boltrer sig i danske farvande | url=http://www.fyens.dk/modules/mobile/article?articleid=3154309 | publisher=[[Fyens Stiftstidende]] | language=da | date=2 May 2017| access-date=1 June 2017}}</ref><ref>{{Cite web|title=Marsvin|url=https://www.naturhistoriskmuseum.dk/viden/naturlex/pattedyr/marsvin|access-date=2021-09-17|website=Naturhistorisk Museum}}</ref> The entire North Sea population is about 335,000.<ref>{{cite journal | last1 = Hammond | first1 = P. S. | display-authors = etal | year = 2002 | title = Abundance of harbour porpoise and other cetaceans in the North Sea and adjacent waters | journal = Journal of Applied Ecology | volume = 39 | issue = 2| pages = 361–376 | doi=10.1046/j.1365-2664.2002.00713.x| s2cid = 59474268 | doi-access = free | bibcode = 2002JApEc..39..361H }}</ref> In the Western Atlantic it is estimated that there are about 33,000 harbour porpoises along the mid-southwestern coast of Greenland (where increasing temperatures have aided them),<ref name=Bjorge2018/> 75,000 between the [[Gulf of Maine]] and [[Gulf of St. Lawrence]], and 27,000 in the Gulf of St. Lawrence.<ref name="iucn status 12 November 2021" /> The Pacific population off mainland United States is about 73,000 and off Alaska 89,000.<ref name="iucn status 12 November 2021" /> After sharp declines in the 20th century, populations have rebounded in the inland waters of [[Washington (state)|Washington state]].<ref>{{Cite news|url=http://archive.kitsapsun.com/news/local/sound-seeing-increase-in-harbor-porpoise-numbers-2e43d8a1-0cf7-65f1-e053-0100007f0c83-372508321.html|title=Sound seeing increase in harbor porpoise numbers|publisher=Kitsap Sun|date=2016-03-17|access-date=2018-05-29|language=en}}</ref> In contrast, some subpopulations are seriously threatened. For example, there are less than 12,000 in the Black Sea,<ref name="iucn status 12 November 2021" /> and only about 500 remaining in the Baltic Sea proper, representing a sharp decrease since the mid-1900s.<ref name=StAndrewsBaltic>{{cite web| title=St Andrews researchers help secure survival of the Baltic Sea harbour porpoise | url=https://www.st-andrews.ac.uk/news/archive/2017/title,1155472,en.php | publisher=University of St. Andrews | date=8 March 2017| access-date=1 June 2017}}</ref>

Harbour porpoises prefer temperate and subarctic waters.<ref name= "Audubon"/> They inhabit fjords, bays, estuaries and harbours, hence their name.<ref name= "Audubon"/> They feed mostly on small pelagic schooling fish, particularly [[herring]], [[Pollack (fish)|pollack]], [[hake]], [[sardine]], [[cod]],<ref>{{Cite web|url=https://animaldiversity.org/accounts/Phocoena_phocoena/|title = Phocoena phocoena (Harbor porpoise)| website=[[Animal Diversity Web]] }}</ref> [[capelin]], and [[sprat]].<ref name=Bjorge2018/> They will, however, eat squid and crustaceans in certain places.<ref name=Bjorge2018/> This species tends to feed close to the sea bottom, at least for waters less than {{convert|200|m|fathom}} deep.<ref name=Bjorge2018/> However, when hunting sprat, porpoise may stay closer to the surface.<ref name=Bjorge2018/> When in deeper waters, porpoises may forage for mid-water fish, such as [[pearlside]]s.<ref name=Bjorge2018/> A study published in 2016 showed that porpoises off the coast of Denmark were hunting 200 fish per hour during the day and up to 550 per hour at night, catching 90% of the fish they targeted.<ref name=sn>{{cite web|title=For harbor porpoises, the ocean is a 24-hour buffet|author=Zielinski, Sarah|date=June 12, 2016|publisher=Science News|url=https://www.sciencenews.org/blog/wild-things/harbor-porpoises-ocean-24-hour-buffet|access-date=2017-03-18}}</ref><ref name=diet>{{cite journal|title=Ultra-High Foraging Rates of Harbor Porpoises Make Them Vulnerable to Anthropogenic Disturbance|journal=Current Biology|author=Wisniewska, D.M.|display-authors=etal|url=http://www.cell.com/current-biology/abstract/S0960-9822(16)30314-1|volume=26|number=11|date=June 6, 2016|pages=1441–1446|access-date=2017-03-18|doi=10.1016/j.cub.2016.03.069|pmid=27238281|s2cid=3923189|doi-access=free|bibcode=2016CBio...26.1441W |hdl=10023/10866|hdl-access=free}}</ref> Almost all the fish they ate were very small, between {{convert|3|and(-)|10|cm|in|frac=2|abbr=on}} long.<ref name=sn/><ref name=diet/>

Harbour porpoises tend to be solitary foragers, but they do sometimes hunt in packs and herd fish together.<ref name=Bjorge2018/> Young porpoises need to consume about 7% to 8% of their body weight each day to survive, which is approximately 15 pounds or 7 kilograms of fish. Significant predators of harbour porpoises include [[great white shark|white shark]]s and [[killer whale]]s (orcas). Researchers at the [[University of Aberdeen]] in [[Scotland]] have also discovered that the local [[Common bottlenose dolphin|bottlenose dolphins]] attack and kill harbour porpoises without eating them due to competition for a decreasing food supply.<ref name=read>{{cite book|author=Read, Andrew|title=''Porpoises''|year=1999|publisher=Voyageur Press|location=Stillwater, MN, USA|isbn=978-0-89658-420-4|url-access=registration|url=https://archive.org/details/porpoises00read}}</ref> An alternative explanation is that the adult dolphins exhibit [[Infanticide (zoology)|infanticidal]] behaviour and mistake the porpoises for juvenile dolphins which they are believed to kill.<ref name=infanticide>{{cite journal|title=Evidence for infanticide in bottlenose dolphins: an explanation for violent interactions with harbour porpoises?| pmc=1689180 | pmid=9699310 | doi=10.1098/rspb.1998.0414| volume=265| issue=1402 | year=1998| journal=Proc Biol Sci| pages=1167–70 | last1 = Patterson | first1 = IA | last2 = Reid | first2 = RJ | last3 = Wilson | first3 = B | last4 = Grellier | first4 = K | last5 = Ross | first5 = HM | last6 = Thompson | first6 = PM}}</ref> [[Grey seals]] are also known to attack harbour porpoises by biting off chunks of fat as a high energy source.<ref>{{cite journal|title=Detection of grey seal Halichoerus grypus DNA in attack wounds on stranded harbour porpoises Phocoena phocoena|first1=Judith D. L. van|last1=Bleijswijk|first2=Lineke|last2=Begeman|first3=Harry J.|last3=Witte|first4=Lonneke L.|last4=IJsseldijk|first5=Sophie M. J. M.|last5=Brasseur|first6=Andrea|last6=Gröne|first7=Mardik F.|last7=Leopold|date=22 October 2014|journal=Marine Ecology Progress Series|volume=513|pages=277–281|doi=10.3354/meps11004|bibcode=2014MEPS..513..277V|doi-access=free}}</ref>

The social life of harbour porpoises is not well understood. They are generally seen as a solitary species.<ref name= "Audubon"/> Most of the time, porpoises are either alone or in groups of no more than five animals.<ref name= "Audubon"/> Porpoises mate promiscuously.<ref name=Bjorge2018/> Males produce large amounts of sperm, perhaps for [[sperm competition]].<ref name=Bjorge2018/> Females become sexually mature by their third or fourth year and can calve each year for several consecutive years, being pregnant and lactating at the same time. The gestation of the porpoise is typically 10–11 months.<ref name= "Audubon"/> Most births occur in late spring and summer.<ref name=Bjorge2018/> Calves are weaned after 8–12 months.<ref name= "Audubon"/> Their average life-span in the wild is 8–13 years, although exceptionally individuals have reached up to 20.,<ref name=Bjorge2018/><ref name=ADWHarbourPorpoise>{{cite web|url=http://animaldiversity.org/accounts/Phocoena_phocoena/|title=Phocoena phocoena, harbor porpoise|work=animaldiversity.org|access-date=3 July 2018}}</ref> and in captivity up to 28 years.<ref>{{cite news| title=Marsvinet Freja bliver optaget i Guinness rekordbog | url=https://avisendanmark.dk/indland/marsvinet-freja-bliver-optaget-i-guinness-rekordbog | date=1 August 2023 | publisher=avisendanmark.dk | access-date=21 August 2023 }}</ref> In a study of 239 dead harbour porpoises in the Gulf of Maine–Bay of Fundy, the vast majority were less than 12 years old and the oldest was 17.<ref>{{cite journal| author1=Read, A. | author2=Hohn, A.A. | year=1995 | title=Life in the fast lane: The life history of harbor propoises from the Gulf of Maine | journal=Marine Mammal Science | volume=11 | issue=4 | pages=423–440 | doi=10.1111/j.1748-7692.1995.tb00667.x | bibcode=1995MMamS..11..423R }}</ref>

Harbour porpoises were traditionally hunted for food, as well as for their blubber, which was used for lighting fuel. Among others, hunting occurred in the [[Black Sea]], off [[Normandy]], in the [[Bay of Biscay]], off [[Flanders]], in the [[Little Belt]] strait, off [[Iceland]], western [[Norway]], in [[Puget Sound]], Bay of Fundy and Gulf of Saint Lawrence.<ref name="iucn status 12 November 2021" /><ref>{{cite web |title=Marsvin i Danmark |publisher=Hvaler.dk |url=http://hvaler.dk/onewebmedia/Marsvin%20i%20Danmark.pdf | last1= Kinze| first1=Carl C. |access-date=22 October 2016 }}</ref><ref>{{cite book |title=Prehistoric Europe: The Economic Basis |url=https://archive.org/details/prehistoriceurop00clar |url-access=registration |year=1966 |last1=Clark |first1=John Grahame Douglas |pages=[https://archive.org/details/prehistoriceurop00clar/page/67 67–68] |publisher=Stanford University Press | isbn= 9780416832402 }}</ref> The drive hunt in the Little Belt strait is the best documented example. Thousands of porpoises were caught there until the end of the 19th century (it was banned in 1899), and again in smaller scale during the worldshortages warsthat occurred in World War I and World War II.<ref>Aage{{cite book |last1=Petersen (1969).|first1=Åge Porpoises|title=Marsvin andog porpoisemarsvinjægere hunters.|date=1969 |publisher=Middelfart By- og Egnshistoriske Museum | language=da }}</ref> A similar, Denmarkshort-lived re-emergence of hunting during the world wars happened (in Danish)Poland and the Baltic countries.</ref name=NAMMCO>{{cite Currently,web| however,title=Harbour thisporpoise species| isurl=https://nammco.no/harbour-porpoise/#1475844711542-eedf1c7b-5dde not| subjectpublisher=North toAtlantic commercialMarine huntingMammal Commission | access-date=21 August 2023 }}</ref> Currently, butthe itspecies is only hunted foras foodpart andof soldthe traditional [[Inuit]] hunt in the Arctic, locallynotably in Greenland.<ref name="iucn status 12 November 2021" /><ref name=NAMMCO/> In [[prehistoric]] times, thisharbour animalporpoises waswere also hunted in many areas, for example by the [[Alby People]] of the east coast of [[Öland]], [[Sweden]].

[[File:Harbor Porpoise Fjord Baelt Denmark.JPG|thumb|A harbour porpoise in captivity in Denmark. The individuals at the center were rescued<ref>{{cite web| title=Fjord & Bælt | url=http://www.cetabase.org/captive/cetacean/fjord-baelt-center | publisher=Ceta Base | date=4 July 2018| access-date=4 July 2018}}</ref> after being injured following entanglement in fishing gear, showing the danger nets can represent to the species<ref name=Teilmann2006>{{cite journal | author=Teilmann | author2=Tougaard | author3=Miller | author4=Kirketerp | author5=Hansen | author6=Brando | name-list-style=amp | year=2006 | title=Reactions of captive harbor porpoises (Phocoena phocoena) to pinger-like sounds | journal=Marine Mammal Science | volume=22 | issue=2 | pages=240–260 | doi=10.1111/j.1748-7692.2006.00031.x| bibcode=2006MMamS..22..240T }}</ref>]]

The main threat to porpoises is static fishing techniques such as gill and tangle nets. [[Bycatch]] in bottom-set [[gill nets]] is considered the main anthropogenic mortality factor for harbour porpoises worldwide. Several thousand die each year in incidental bycatch, which has been reported from the Black Sea, the Baltic Sea, the North Sea, off California, and along the east coast of the United States and Canada.<ref name="iucn status 12 November 2021" /> Bottom-set gill nets are anchored to the sea floor and are up to {{convert|12+1/2|nmi|km|0|abbr=off|order=flip}} in length. It is unknown why porpoises become entangled in gill nets, since several studies indicate they are able to detect these nets using their [[animal echolocation|echolocation]].<ref>{{cite journal| last1 = Kastelein | first1 = R. | last2 = Au | first2 = W. W. L. | year = 2000 | title = Detection distances of bottom-set gill nets by harbour porpoises (''Phocoena phocoena'') and bottlenose dolphins (''Tursiops truncatus'') | journal = Mar.Environm Environ. Res. | volume = 49 | issue = 4| pages = 359–375 | doi=10.1016/s0141-1136(99)00081-1| pmid = 11285736 | bibcode = 2000MarER..49..359K }}</ref><ref>{{cite journal | last1 = Villadsgaard | first1 = A. | last2 = Wahlberg | first2 = M. | last3 = Tougaard | first3 = J. | year = 2007 | title = Echolocation signals of free-ranging harbour porpoises, ''Phocoena phocoena'' | journal = J. Exp. Biol. | volume = 210 | issue = 1| pages = 56–64 | doi=10.1242/jeb.02618| pmid = 17170148 | s2cid = 6184532 | doi-access = free }}</ref> Porpoise-scaring devices, so-called pingers, have been developed to keep porpoises out of nets and numerous studies have demonstrated they are very effective at reducing entanglement.<ref>{{cite journal | last1 = Kraus | first1 = S. D. | last2 = Read | first2 = A. J. | last3 = Solow | first3 = A. | last4 = Baldwin | first4 = K. | last5 = Spradlin | first5 = T. | last6 = Anderson | first6 = E. | last7 = Williamson | first7 = J. | year = 1997 | title = Acoustic alarms reduce porpoise mortality | doi = 10.1038/41451 | journal = Nature | volume = 388 | issue = 6642| page = 525| bibcode = 1997Natur.388..525K | s2cid = 4412647 | doi-access = free }}</ref><ref>Larsen, F (1999). The effect of acoustic alarms on the by-catch of harbour porpoises in the Danish North Sea gill net fishery. Paper SC/51/SM41 presented to the IWC Scientific Committee</ref> However, concern has been raised over the noise pollution created by the pingers and whether their efficiency will diminish over time due to porpoises habituating to the sounds.<ref name=Teilmann2006/><ref>{{cite journal | last1 = Cox | first1 = T. M. | last2 = Read | first2 = A. J. | last3 = Solow | first3 = A. | last4 = Tregenza | first4 = N. | year = 2001 | title = Will harbour porpoises (''Phocoena phocoena'') habituate to pingers? | journal = J.Cetacean.Res.Manage | volume = 3 | issue = 1| pages = 81–86 | doi = 10.47536/jcrm.v3i1.904 | doi-access = free }}</ref>

[[Overfishing]] may reduce preferred prey availability for porpoises. Overfishing resulting in the collapse of herring in the North Sea caused porpoises to hunt for other prey species.<ref>{{cite journal |last1=Santos |first1=M. |last2=Pierce |first2=G. |title=The diet of harbour porpoise (Phocoena phocoena) in the Northeast Atlantic |journal=Oceanogr Mar Biol Annu Rev |date=1 January 2003 |volume=41 |pages=355–390}}</ref> Reduction of prey may result from climate change, or overfishing, or both.

[[Noise pollution|Noise]] from ship traffic and oil platforms is thought to affect the distribution of toothed whales, like the harbour porpoise, that use echolocation for communication and prey detection. Noise from shipping traffic, particularly busy sea lanes, appears to instigate evasive behavior, with predominantly lateral movements during the day and deeper dives during the night.<ref>{{cite journal|last1=Frankish|first1=C.K.|last2=von Benda-Beckmann|first2=A.M.|last3=Teilmann|first3=J.|last4=Tougaard|first4=J.|last5=Dietz|first5=R.|last6=Sveegaard|first6=S.|last7=Binnerts|first7=B.|last8=de Jong|first8=C.A.|last9=Nabe-Nielsen|first9=J.|year=2023|title=Ship noise causes tagged harbour porpoises to change direction or dive deeper|journal=Marine Pollution Bulletin|volume=197|pages=115755|doi=10.1016/j.marpolbul.2023.115755}}</ref> The construction of thousands of offshore wind turbines, planned in different areas of North Sea, is known to cause displacement of porpoises from the construction site,<ref>{{cite journal | last1 = Carstensen | first1 = J. | last2 = Henriksen | first2 = O. D. | last3 = Teilmann | first3 = J. | year = 2006 | title = Impacts on harbour porpoises from offshore wind farm construction: Acoustic monitoring of echolocation activity using porpoise detectors (T-PODs) | journal = Mar. Ecol. Prog. Ser. | volume = 321 | pages = 295–308 | doi=10.3354/meps321295| doi-access = free }}</ref> particularly if steel monopile foundations are installed by percussive piling, where reactions can occur at distances of more than {{convert|20|km|nmi|frac=2|abbr=on}}.<ref>{{cite journal | last1 = Tougaard | first1 = J. | last2 = Carstensen | first2 = J. | last3 = Teilmann | first3 = J. | last4 = Skov | first4 = H. | last5 = Rasmussen | first5 = P. | year = 2009 | title = Pile driving zone of responsiveness extends beyond 20 km for harbour porpoises (Phocoena phocoena, (L.)) | journal = J. Acoust. Soc. Am. | volume = 126 | issue = 1| pages = 11–14 | doi=10.1121/1.3132523| pmid = 19603857 | bibcode = 2009ASAJ..126...11T }}</ref> Noise levels from operating wind turbines are low and unlikely to affect porpoises, even at close range.<ref>{{cite journal | last1 = Madsen | first1 = P. T. | last2 = Wahlberg | first2 = M. | last3 = Tougaard | first3 = J. | last4 = Lucke | first4 = K. | last5 = Tyack | first5 = P. L. | year = 2006 | title = Wind turbine underwater noise and marine mammals: Implications of current knowledge and data needs | journal = Mar. Ecol. Prog. Ser. | volume = 309 | pages = 279–295 | doi=10.3354/meps309279| bibcode = 2006MEPS..309..279M | doi-access = free | hdl = 10023/3847 | hdl-access = free }}</ref><ref>{{cite journal | last1 = Tougaard | first1 = J. | last2 = Henriksen | first2 = O. D. | last3 = Miller | first3 = L. A. | year = 2009 | title = Underwater noise from three offshore wind turbines: estimation of impact zones for harbor porpoises and harbor seals | journal = J. Acoust. Soc. Am. | volume = 125 | issue = 6| pages = 3766–3773 | doi=10.1121/1.3117444| pmid = 19507958 }}</ref>