View source for Porpoise

===Threats===

====Hunting====
[[File:FMIB 35846 Porpoise Fishery.jpeg|thumb|Traditional catch of [[harbour porpoise]] by lancing in [[Bay of Fundy]]]]
Porpoises and other smaller cetaceans have traditionally been hunted in many areas, at least in Asia, Europe and North America, for their meat and blubber. A dominant hunting technique is drive hunting, where a pod of animals is driven together with boats and usually into a bay or onto a beach. Their escape is prevented by closing off the route to the ocean with other boats or nets. This type of fishery for [[harbour porpoise]]s is best documented from the [[Little Belt]] in the [[Danish Straits]], where hunting was regulated by a [[guild]] for 400 years, until it was no longer profitable by the end of the 19th century (the guild was liquified in 1898). However, the hunt was resumed briefly during [[World War I]] and [[World War II]] due to fuel shortage.<ref>{{cite book |last1=Petersen |first1=Åge |title=Marsvin og marsvinjægere |date=1969 |publisher=Middelfart By- og Egnshistoriske Museum |location=Middelfart, Denmark}}</ref> The [[Inuit]] in the Arctic hunt harbour porpoises by shooting and drive hunt for [[Dall's porpoise]] still takes place in Japan. The number of individuals taken each year is in the thousands, although a quota of around 17,000 per year is in effect today<ref>{{cite journal |last1=Jefferson |first1=TA |last2=Braulik |first2=G |title=''Phocoenoides dalli'' |url=https://www.iucnredlist.org/species/17032/50370912 |journal=IUCN Red List of Threatened Species|date=22 March 2017 }}</ref> making it the largest direct hunt of any [[cetacean]] species in the world<ref name="smm">{{cite web |title=''Phocoenoides dalli'' (Dall's porpoise, Dall porpoise) |url=https://www.marinemammalscience.org/facts/phocoenoides-dalli/ |website=The Society for Marine Mammalogy}}</ref> and the sustainability of the hunt has been questioned.<ref name="IWChunt">{{cite journal |title=Report of the Sub-Committee on Small Cetaceans |journal=International Whaling Commission |date=2008}}</ref><ref name="smm2">{{cite web |last1=Wells |first1=RS |title=Letter to Japanese Government Regarding Dolphin and Small Whale Hunts |url=https://www.marinemammalscience.org/letters/letter-to-japanese-government-regarding-dolphin-and-small-whale-hunts/ |website=The Society for Marine Mammalogy|date=29 May 2012 }}</ref>

====Fishing====
{{Main|Cetacean bycatch}}
[[File:Vaquita2 Olson NOAA.jpg|thumb|A [[vaquita]] swimming in the [[Gulf of California]].]]
Porpoises are highly affected by [[bycatch]]. Many porpoises, mainly the [[vaquita]], are subject to great mortality due to [[gillnetting]]. Although it is the world's most endangered marine cetacean, the vaquita continues to be caught in small-mesh gillnet fisheries throughout much of its range. Incidental mortality caused by the fleet of El Golfo de Santa Clara was estimated to be at around 39 vaquitas per year, which is over 17% of the population size.<ref>{{cite journal|last1=D'agrosa|first1=C.|last2=Lennert-Cody|first2=C. E.|last3=Vidal|first3=O.|year=2000|title=Vaquita Bycatch in Mexico's Artisanal Gillnet Fisheries: Driving a Small Population to Extinction|journal=Conservation Biology|volume=14|issue=4|doi=10.1046/j.1523-1739.2000.98191.x|pages=1110–1119|bibcode=2000ConBi..14.1110D |s2cid=44057831}}</ref> Harbour porpoises also suffer drowning by gillnetting, but on a less threatening scale due to their high population; their mortality rate per year increases a mere 5% due to this.<ref>{{cite journal|url=https://www.umb.edu/editor_uploads/images/Caswell_etal_EcolAppl1998.pdf|first1=Hal|last1=Caswell|first2=Solange|last2=Brault|first3=Andrew|last3=Read|first4=Tim|last4=Smith|title=Harbor Porpoise and Fisheries: an Uncertainty Analysis of Incidental Mortality|journal=Ecological Applications|volume=8|issue=4|year=1998|access-date=30 October 2015|pages=1226–1238|doi=10.1890/1051-0761(1998)008[1226:hpafau]2.0.co;2|archive-date=8 December 2015|archive-url=https://web.archive.org/web/20151208132734/https://www.umb.edu/editor_uploads/images/Caswell_etal_EcolAppl1998.pdf|url-status=dead}}</ref>

The fishing market, historically has always had a porpoise bycatch. Today, the [[Marine Mammal Protection Act of 1972]] has enforced the use of safer fishing equipment to reduce bycatch.<ref>{{cite newsgroup|title=America's Favorite Fish Tainted by Porpoise Deaths|last=Croft|first=Chris|url=https://www.latimes.com/archives/la-xpm-1988-04-17-op-1956-story.html|access-date=30 October 2015}}</ref>

====Environmental hazards====
{{See also|Marine mammals and sonar}}
Porpoises are very sensitive to [[anthropogenic effect|anthropogenic]] disturbances,<ref name=":0">{{cite journal |title=Harbour porpoises (''Phocoena phocoena'') stranded along the southern North Sea: an assessment through metallic contamination |last1=Mahfouz |first1=C |last2=Henry |first2=F |last3=Courcot |first3=L |last4=Pezeril |first4=S |last5=Bouveroux |first5=T |last6=Dabin |first6=W |last7=Jauniaux |first7=T |last8=Khalaf |first8=G |last9=Amara |first9=R |year=2014 |journal= Environmental Research|doi=10.1016/j.envres.2014.06.006 |pmid=24981825 |volume=133 |pages=266–73|bibcode=2014ER....133..266M |hdl=2268/169176 |url=http://orbi.ulg.ac.be/handle/2268/169176 |hdl-access=free }}</ref> and are [[keystone species]], which can indicate the overall health of the marine environment.<ref name=":0" /> Populations of harbor porpoises in the North and Baltic Seas are under increasing pressure from anthropogenic causes such as offshore construction, ship traffic, fishing, and military exercises.<ref>{{cite journal |last1=Siebert |first1=Ursula |last2=Pozniak |first2=Blazej |last3=Hansen |first3=Kirstin |last4=Anderson |last5=Nordstrom |first5=Gwyneth |last6=Teilmann |first6=Jonas |last7=Van Elk |first7=Niels |last8=Vossen |first8=Ardnt |last9=Dietz |first9=Rune |year=2011 |title=Investigations of Thyroid and Stress Hormones in Free-Ranging and Captive Harbor Porpoises (''Phocoena phocoena''): A Pilot Study |journal=Aquatic Mammals |volume=37 |issue=4 |pages=443–453 |doi=10.1578/am.37.4.2011.443 |bibcode=2011AqMam..37..443S }}</ref> Increasing pollution is a serious problem for marine mammals. Heavy metals and [[plastic pollution|plastic waste]] are not biodegradable, and sometimes cetaceans consume these hazardous materials, mistaking them for food items. As a result, the animals are more susceptible to diseases and have fewer offspring.<ref name=wdcs>{{cite book |author=Cara E. Miller |title=Current State of Knowledge of Cetacean Threats, Diversity, and Habitats in the Pacific Islands Region |publisher=Whale and Dolphin Conservation Society |year=2007 |access-date=5 September 2015 |isbn=978-0-646-47224-9 |url=http://uk.whales.org/sites/default/files/whales-and-dolphins-in-the-pacific-islands.pdf |archive-url=https://web.archive.org/web/20150908003502/http://uk.whales.org/sites/default/files/whales-and-dolphins-in-the-pacific-islands.pdf |archive-date=8 September 2015  }}</ref> Harbour porpoises from the [[English Channel]] were found to have accumulated heavy metals.<ref name=":0" />

The military and geologists employ strong [[sonar]] and produce an increases in noise in the oceans. [[Marine mammal]]s that make use of [[biosonar]] for orientation and communication are not only hindered by the extra noise, but may race to the surface in panic. This may lead to a bubbling out of blood gases, and the animal then dies because the blood vessels become blocked, so-called [[decompression sickness]].<ref>{{cite book |author1=M. Andre |author2=T. Johansson |author3=E. Delory |author4=M. van der Schaar |title=Europe Oceans 2005 |chapter=Cetacean biosonar and noise pollution |publisher=Oceans 2005–Europe |year=2005 |volume=2 |pages=1028–1032 Vol. 2 |doi=10.1109/OCEANSE.2005.1513199 |isbn=978-0-7803-9103-1 |s2cid=31676969 }}</ref> This effect, of course, only occurs in porpoises that dive to great depths, such as Dall's porpoise.<ref>{{cite journal |author=Kastelein, R. A. |author2=Schop, J. |author3=Gransier, R. |author4=Steen, N. |author5=Jennings, N. |title=Effect of Series of 1 to 2 kHz and 6 to 7 kHz Up-Sweeps and Down-Sweeps on the Behavior of a Harbor Porpoise (''Phocoena phocoena'') |doi=10.1578/AM.40.3.2014.232 | volume=40 |issue=3 |journal=Aquatic Mammals |pages=232–242|year=2014 |bibcode=2014AqMam..40..232K }}</ref>

Additionally, civilian vessels produce sonar waves to measure the depth of the body of water in which they are. Similar to the navy, some boats produce waves that attract porpoises, while others may repel them. The problem with the waves that attract is that the animal may be injured or even killed by being hit by the vessel or its propeller.<ref>{{cite journal |last1=Culik |first1=B. |last2=von Dorrien |first2=C. |last3=Muller |first3=V. |last4=Conrad |first4=M. |year=2015 |title=Bioacoustics |journal=The International Journal of Animal Sound and Its Recording |volume=24 |issue=3 |issn=0952-4622}}</ref>