Harbour porpoise: Difference between revisions

The vocalizations of the harbour porpoise is made up of short clicks from 0.5 to 5 milliseconds in bursts up to two seconds long. Each click has a frequency between 1000 and 2200 hertz. Aside from communication, the clicks are used for echolocation.<ref name="Schevill">{{cite journal |last1=Schevill |first1=William E. |last2=Watkins |first2=William A. |last3=Ray |first3=Carleton |title=Click Structure in the Porpoise, Phocoena phocoena |journal=Journal of Mammalogy |date=Nov 1969 |volume=50 |issue=4 |pages=721-728721–728 |doi=10.2307/1378247|jstor=1378247 }}</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/>

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>

[[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. 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 }}</ref>