Marine mammal

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Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their reliance on marine environments for feeding and survival.

Marine mammal adaptation to an aquatic lifestyle varies considerably between species. Both

molting. In contrast, both otters and the polar bear are much less adapted to aquatic living. The diets of marine mammals vary considerably as well; some eat zooplankton
, others eat fish, squid, shellfish, or seagrass, and a few eat other mammals. While the number of marine mammals is small compared to those found on land, their roles in various ecosystems are large, especially concerning the maintenance of marine ecosystems, through processes including the regulation of prey populations. This role in maintaining ecosystems makes them of particular concern as 23% of marine mammal species are currently threatened.

Marine mammals were first hunted by

effects of global warming
degrade Arctic environments.


Marine mammals of varying sizes and shapes

Classification of extant species

Phylogeny of marine mammals





Dugongidae (dugongs)


Trichechus manatus (West Indian manatee)

Trichechus senegalensis (African manatee)

Trichechus inunguis (Amazonian manatee; freshwater species)




(baleen whales)

river dolphins










Ursus maritimus
(polar bear)

all other ursids


Enhydra lutris
(sea otter)

Lontra felina
(marine otter)

Neogale macrodon
(sea mink)

all other mustelids


(eared seals)

Odobenidae (walruses)

(earless seals)

The taxa in bold are marine. Taxa indicated with a † symbol are recently extinct.[1]

The term "marine mammal" encompasses all mammals whose survival depends entirely or almost entirely on the oceans, which have also evolved several specialized aquatic traits. In addition to the above, several other mammals have a great dependency on the sea without having become so anatomically specialized, otherwise known as "quasi-marine mammals". This term can include: the

gray wolf (Canis lupus) populations which predominantly eat salmon and marine carcasses, the North Ronaldsay sheep (Ovis aries) which normally eats seaweed outside the lambing season, the Eurasian otter (Lutra lutra) which is usually found in freshwater but can be found along coastal Scotland, and others.[3]


Marine mammals form a diverse group of 129 species that rely on the ocean for their existence.[4][5] They are an informal group unified only by their reliance on marine environments for feeding.[6] Despite the diversity in anatomy seen between groups, improved foraging efficiency has been the main driver in their evolution.[7][8] The level of dependence on the marine environment varies considerably with species. For example, dolphins and whales are completely dependent on the marine environment for all stages of their life; seals feed in the ocean but breed on land; and polar bears must feed on land.[6]

The cetaceans became aquatic around 50 million years ago (mya).

Artiodactyla (even-toed ungulates).[10][11] The term "Cetartiodactyla" reflects the idea that whales evolved within the ungulates. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. Under this definition, the closest living land relative of the whales and dolphins is thought to be the hippopotamuses.[12][13][14][15]

Sirenians, the sea cows, became aquatic around 40 million years ago. The first appearance of sirenians in the fossil record was during the early Eocene, and by the late Eocene, sirenians had significantly diversified. Inhabitants of rivers, estuaries, and nearshore marine waters, they were able to spread rapidly. The most primitive sirenian, †Prorastomus, was found in Jamaica,[8] unlike other marine mammals which originated from the Old World (such as cetaceans[16]). The first known quadrupedal sirenian was †Pezosiren from the early middle Eocene.[17] The earliest known sea cows, of the families †Prorastomidae and †Protosirenidae, were both confined to the Eocene, and were pig-sized, four-legged, amphibious creatures.[18] The first members of Dugongidae appeared by the middle Eocene.[19] At this point, sea cows were fully aquatic.[18]


Otariidae (eared seals) and Odobenidae (walrus) split almost 28 mya.[21] Phocids (earless seals) are known to have existed for at least 15 mya,[22] and molecular evidence supports a divergence of the Monachinae (monk seals) and Phocinae lineages 22 mya.[21]

Fossil evidence indicates the sea otter (Enhydra) lineage became isolated in the North Pacific approximately two mya, giving rise to the now-extinct †

Hokkaidō and Russia, and then spread east to the Aleutian Islands, mainland Alaska, and down the North American coast. In comparison to cetaceans, sirenians, and pinnipeds, which entered the water approximately 50, 40, and 20 mya, respectively, the sea otter is a relative newcomer to marine life. In some respects though, the sea otter is more fully adapted to water than pinnipeds, which must haul out on land or ice to give birth.[23]

Polar bears are thought to have diverged from a population of

In general, terrestrial amniote invasions of the sea have become more frequent in the Cenozoic than they were in the Mesozoic. Factors contributing to this trend include the increasing productivity of near-shore marine environments, and the role of endothermy in facilitating this transition.[29]

Distribution and habitat

Marine mammal species richness: A) All species (n = 115), B) toothed whales (n = 69), C) baleen whales (n = 14), D) seals (n = 32), based on data from 1990 to 1999[30]

Marine mammals are widely distributed throughout the globe, but their distribution is patchy and coincides with the productivity of the oceans.[31] Species richness peaks at around 40° latitude, both north and south. This corresponds to the highest levels of primary production around North and South America, Africa, Asia and Australia. Total species range is highly variable for marine mammal species. On average most marine mammals have ranges which are equivalent or smaller than one-fifth of the Indian Ocean.[32] The variation observed in range size is a result of the different ecological requirements of each species and their ability to cope with a broad range of environmental conditions. The high degree of overlap between marine mammal species richness and areas of human impact on the environment is of concern.[4]

Most marine mammals, such as seals and sea otters, inhabit the coast. Seals, however, also use a number of terrestrial habitats, both continental and island. In temperate and tropical areas, they

mud flats, tide pools and in sea caves. Some species also rest on man-made structures, like piers, jetties, buoys and oil platforms. Seals may move further inland and rest in sand dunes or vegetation, and may even climb cliffs.[33]: 96  Most cetaceans live in the open ocean, and species like the sperm whale may dive to depths of −1,000 to −2,500 feet (−300 to −760 m) in search of food.[34] Sirenians live in shallow coastal waters, usually living 30 feet (9.1 m) below sea level. However, they have been known to dive to −120 feet (−37 m) to forage deep-water seagrasses.[35] Sea otters live in protected areas, such as rocky shores, kelp forests, and barrier reefs,[36] although they may reside among drift ice or in sandy, muddy, or silty areas.[37]

Many marine mammals seasonally migrate. Annual ice contains areas of water that appear and disappear throughout the year as the weather changes, and seals migrate in response to these changes. In turn, polar bears must follow their prey. In

Baja Peninsula.[41] During the winter, manatees living at the northern end of their range migrate to warmer waters.[42]


Marine mammals have a number of

counter-current heat exchange); and reduced appendages, and large size to prevent heat loss.[31]

Marine mammals are able to dive for long periods. Both pinnipeds and cetaceans have large and complex

muscles and the spleen which all have the capacity to hold a high concentration of oxygen. They are also capable of bradycardia (reduced heart rate), and vasoconstriction (shunting most of the oxygen to vital organs such as the brain and heart) to allow extended diving times and cope with oxygen deprivation.[31] If oxygen is depleted (hypoxia), marine mammals can access substantial reservoirs of glycogen that support anaerobic glycolysis.[43][44][45]

Sound travels differently through water, and therefore marine mammals have developed adaptations to ensure effective communication, prey capture, and predator detection.[46] The most notable adaptation is the development of echolocation in whales and dolphins.[31] Toothed whales emit a focused beam of high-frequency clicks in the direction that their head is pointing. Sounds are generated by passing air from the bony nares through the phonic lips.[47]: p. 112 These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. The focused beam is modulated by a large fatty organ known as the 'melon'. This acts like an acoustic lens because it is composed of lipids of differing densities.[47]: 121 [48]

Marine mammals have evolved a wide variety of features for feeding, which are mainly seen in their dentition. For example, the cheek teeth of pinnipeds and odontocetes are specifically adapted to capture fish and squid. In contrast,

baleen plates to filter feed plankton and small fish from the water.[31]

Polar bears, otters, and

epidermis and a thickened fat layer (blubber) to prevent drag. Wading and bottom-feeding animals (such as manatees) need to be heavier than water in order to keep contact with the floor or to stay submerged. Surface-living animals (such as sea otters) need the opposite, and free-swimming animals living in open waters (such as dolphins) need to be neutrally buoyant in order to be able to swim up and down the water column. Typically, thick and dense bone is found in bottom feeders and low bone density is associated with mammals living in deep water. Some marine mammals, such as polar bears and otters, have retained four weight-bearing limbs and can walk on land like fully terrestrial animals.[49]



Weddel seal

All cetaceans are

gut flora similar to that of terrestrial herbivores, probably a remnant of their herbivorous ancestry.[55]

Baleen whales use their baleen plates to sieve plankton, among others, out of the water; there are two types of methods: lunge-feeding and gulp-feeding. Lunge-feeders expand the volume of their jaw to a volume bigger than the original volume of the whale itself by inflating their mouth. This causes grooves on their throat to expand, increasing the amount of water the mouth can store.[56][57] They ram a baitball at high speeds in order to feed, but this is only energy-effective when used against a large baitball.[58] Gulp-feeders swim with an open mouth, filling it with water and prey. Prey must occur in sufficient numbers to trigger the whale's interest, be within a certain size range so that the baleen plates can filter it, and be slow enough so that it cannot escape.[59]

Otters are the only marine animals that are capable of lifting and turning over rocks, which they often do with their front paws when searching for prey.[60] The sea otter may pluck snails and other organisms from kelp and dig deep into underwater mud for clams.[60] It is the only marine mammal that catches fish with its forepaws rather than with its teeth.[61] Under each foreleg, sea otters have a loose pouch of skin that extends across the chest which they use to store collected food to bring to the surface. This pouch also holds a rock that is used to break open shellfish and clams, an example of tool use.[62] The sea otters eat while floating on their backs, using their forepaws to tear food apart and bring to their mouths.[63][64] Marine otters mainly feed on crustaceans and fish.[65]

Pinnipeds mostly feed on fish and

hunt cooperatively in large groups, locating and herding their prey. Some species, such as California and South American sea lions, may forage with cetaceans and sea birds.[33]
: 168 

The polar bear is the most carnivorous species of bear, and its diet primarily consists of ringed (Pusa hispida) and bearded (Erignathus barbatus) seals.[67] Polar bears hunt primarily at the interface between ice, water, and air; they only rarely catch seals on land or in open water.[68] The polar bear's most common hunting method is still-hunting:[69] The bear locates a seal breathing hole using its sense of smell, and crouches nearby for a seal to appear. When the seal exhales, the bear smells its breath, reaches into the hole with a forepaw, and drags it out onto the ice. The polar bear also hunts by stalking seals resting on the ice. Upon spotting a seal, it walks to within 100 yards (90 m), and then crouches. If the seal does not notice, the bear creeps to within 30 to 40 feet (9 to 10 m) of the seal and then suddenly rushes to attack.[70] A third hunting method is to raid the birth lairs that female seals create in the snow.[69] They may also feed on fish.[71]

Sirenians are referred to as "sea cows" because their diet consists mainly of seagrass. When eating, they ingest the whole plant, including the roots, although when this is impossible they feed on just the leaves.[72] A wide variety of seagrass has been found in dugong stomach contents, and evidence exists they will eat algae when seagrass is scarce.[73] West Indian manatees eat up to 60 different species of plants, as well as fish and small invertebrates to a lesser extent.[74]

Keystone species

Sea otters are a classic example of a keystone species; their presence affects the ecosystem more profoundly than their size and numbers would suggest. They keep the population of certain


An apex predator affects prey population dynamics and defense tactics (such as camouflage).[77] The polar bear is the apex predator within its range.[78] Several animal species, particularly Arctic foxes (Vulpes lagopus) and glaucous gulls (Larus hyperboreus), routinely scavenge polar bear kills.[79] The relationship between ringed seals and polar bears is so close that the abundance of ringed seals in some areas appears to regulate the density of polar bears, while polar bear predation in turn regulates density and reproductive success of ringed seals.[80] The evolutionary pressure of polar bear predation on seals probably accounts for some significant differences between Arctic and Antarctic seals. Compared to the Antarctic, where there is no major surface predator, Arctic seals use more breathing holes per individual, appear more restless when hauled out on the ice, and rarely defecate on the ice.[79] The fur of Arctic pups is white, presumably to provide camouflage from predators, whereas Antarctic pups all have dark fur.[79]

Killer whales are apex predators throughout their global distribution, and can have a profound effect on the behavior and population of prey species. Their diet is very broad and they can feed on many vertebrates in the ocean including

white sharks),[82][83] large baleen whales,[84] and nearly 20 species of pinniped.[85] The predation of whale calves may be responsible for annual whale migrations to calving grounds in more tropical waters, where the population of killer whales is much lower than in polar waters. Prior to whaling, it is thought that great whales were a major food source; however, after their sharp decline, killer whales have since expanded their diet, leading to the decline of smaller marine mammals.[40] A decline in Aleutian Islands sea otter populations in the 1990s was controversially attributed by some scientists to killer whale predation, although with no direct evidence. The decline of sea otters followed a decline in harbor seal and Steller sea lion populations, the killer whale's preferred prey, which in turn may be substitutes for their original prey, now reduced by industrial whaling.[86][87][88]

Whale pump

Fish and phytoplankton bring nutrients to the seafloor in the form of detritus, and whales bring nutrients up to the surface also in the form of detritus.
"Whale pump" – the role played by whales in recycling ocean nutrients[89]

A 2010 study considered whales to be a positive influence to the productivity of ocean fisheries, in what has been termed a "whale pump". Whales carry nutrients such as nitrogen from the depths back to the surface. This functions as an upward biological pump, reversing an earlier presumption that whales accelerate the loss of nutrients to the bottom. This nitrogen input in the Gulf of Maine is more than the input of all rivers combined emptying into the gulf, some 25,000 short tons (23,000 t) each year.[89]

Whales defecate at the ocean's surface; their excrement is important for fisheries because it is rich in iron and nitrogen. The whale feces are liquid and instead of sinking, they stay at the surface where phytoplankton feed off it.[89][90]

Upon death, whale carcasses fall to the deep ocean and provide a substantial habitat for marine life. Evidence of whale falls in present-day and fossil records shows that deep-sea whale falls support a rich assemblage of creatures, with a global diversity of 407 species, comparable to other

chemoautotrophic organisms, which in turn, support other organisms such as mussels, clams, limpets, and sea snails. This stage may last for decades and supports a rich assemblage of species, averaging 185 species per site.[92]

Interactions with humans


Due to the difficulty to survey populations, 38% of marine mammals are data deficient, especially around the Antarctic Polar Front. In particular, declines in the populations of completely marine mammals tend to go unnoticed 70% of the time.[32]


Marine mammals were hunted by coastal aboriginal humans historically for food and other resources. These subsistence hunts still occur in Canada, Greenland, Indonesia, Russia, the United States, and several nations in the Caribbean. The effects of these are only localized, as hunting efforts were on a relatively small scale.[31] Commercial hunting took this to a much greater scale and marine mammals were heavily exploited. This led to the extinction of the Steller's sea cow (Hydrodamalis gigas), sea mink (Neogale macrodon), Japanese sea lion (Zalophus japonicus), and the Caribbean monk seal (Neomonachus tropicalis).[31] Today, populations of species that were historically hunted, such as blue whales (Balaenoptera musculus) and the North Pacific right whale (Eubalaena japonica), are much lower than their pre-whaling levels.[93] Because whales generally have slow growth rates, are slow to reach sexual maturity, and have a low reproductive output, population recovery has been very slow.[46]

A number of whales are still subject to direct hunting, despite the 1986

minke whales per year are set.[94][95] Japan also harvests several hundred Antarctic and North Pacific minke whales each year, ostensibly for scientific research in accordance with the moratorium.[93] However, the illegal trade of whale and dolphin meat is a significant market in Japan and some countries.[96]

The most profitable furs in the

Commercial sealing was historically just as important as the whaling industry. Exploited species included harp seals, hooded seals, Caspian seals, elephant seals, walruses and all species of fur seal.[99] The scale of seal harvesting decreased substantially after the 1960s,[100] after the Canadian government reduced the length of the hunting season and implemented measures to protect adult females.[101] Several species that were commercially exploited have rebounded in numbers; for example, Antarctic fur seals may be as numerous as they were prior to harvesting. The northern elephant seal was hunted to near extinction in the late 19th century, with only a small population remaining on Guadalupe Island. It has since recolonized much of its historic range, but has a population bottleneck.[99] Conversely, the Mediterranean monk seal was extirpated from much of its former range, which stretched from the Mediterranean to the Black Sea and northwest Africa, and remains only in the northeastern Mediterranean and some parts of northwest Africa.[102]

Polar bears can be hunted for sport in Canada with a special permit and accompaniment by a local guide. This can be an important source of income for small communities, as guided hunts bring in more income than selling the polar bear hide on markets. The United States, Russia, Norway, Greenland, and Canada allow subsistence hunting, and Canada distributes hunting permits to indigenous communities. The selling of these permits is a main source of income for many of these communities. Their hides can be used for subsistence purposes, kept as hunting trophies, or can be bought in markets.[103][104]

Ocean traffic and fisheries

A right whale sliced on both sides after colliding with a boat. A large amount of its flesh is visible as well as the intestines floating in the water
The remains of a North Atlantic right whale after it collided with a ship propeller

gill nets cause the highest mortality levels for both cetaceans and pinnipeds, however, entanglements in long lines, mid-water trawls, and both trap and pot lines are also common.[105] Tuna seines are particularly problematic for entanglement by dolphins.[106] By-catch affects all cetaceans, both small and big, in all habitat types. However, smaller cetaceans and pinnipeds are most vulnerable as their size means that escape once they are entangled is highly unlikely and they frequently drown.[93] While larger cetaceans are capable of dragging nets with them, the nets sometimes remain tightly attached to the individual and can impede the animal from feeding sometimes leading to starvation.[93] Abandoned or lost nets and lines cause mortality through ingestion or entanglement.[107] Marine mammals also get entangled in aquaculture nets, however, these are rare events and not prevalent enough to impact populations.[108]

Vessel strikes cause death for a number of marine mammals, especially whales.

dolphin watching can also negatively impact on marine mammals by interfering with their natural behavior.[110]

The fishery industry not only threatens marine mammals through by-catch, but also through competition for food. Large-scale fisheries have led to the depletion of fish stocks that are important prey species for marine mammals. Pinnipeds have been especially affected by the direct loss of food supplies and in some cases the harvesting of fish has led to food shortages or dietary deficiencies,[111] starvation of young, and reduced recruitment into the population.[112] As the fish stocks have been depleted, the competition between marine mammals and fisheries has sometimes led to conflict. Large-scale culling of populations of marine mammals by commercial fishers has been initiated in a number of areas in order to protect fish stocks for human consumption.[113]

Shellfish aquaculture takes up space so in effect creates competition for space. However, there is little direct competition for aquaculture shellfish

finfish from farms, which creates significant problems for marine farmers. While there are usually legal mechanisms designed to deter marine mammals, such as anti-predator nets or harassment devices, individuals are often illegally shot.[108]

Habitat loss and degradation

U.S. Geological Survey
shows projected changes in polar bear habitat from 2005 to 2095. Red areas indicate loss of optimal polar bear habitat; blue areas indicate gain.

hydroelectric projects, and aquaculture both degrade the environment and take up valuable habitat.[46] For example, extensive shellfish aquaculture takes up valuable space used by coastal marine mammals for important activities such as breeding, foraging and resting.[108]

plastic debris and sewage threaten the livelihood of marine mammals.[114]

acoustic deterrent devices used by aquaculture facilities to scare away marine mammals emit loud and noxious underwater sounds.[108]

Two changes to the global

global warming due to increased carbon dioxide levels in the atmosphere. Raised sea levels, rising sea temperatures and changed currents are expected to affect marine mammals by altering the distribution of important prey species, and changing the suitability of breeding sites and migratory routes.[117] The Arctic food chain would be disrupted by the near extinction or migration of polar bears. Arctic sea ice is the polar bear's habitat. It has been declining at a rate of 13% per decade because the temperature is rising at twice the rate of the rest of the world.[78][118] By the year 2050, up to two-thirds of the world's polar bears may vanish if the sea ice continues to melt at its current rate.[119]

A study by evolutionary biologists at the University of Pittsburgh showed that the ancestors of many marine mammals stopped producing a certain enzyme that today protects against some neurotoxic chemicals called organophosphates,[120] including those found in the widely used pesticides chlorpyrifos and diazinon.[121] Marine mammals may be increasingly exposed to these compounds due to agricultural runoff reaching the world's oceans.


Signatory countries of the International Whaling Commission


Marine Mammal Commission (MMC) was established to review existing policies and make recommendations to the Service and NOAA to better implement the MMPA. The Service is responsible for ensuring the protection of sea otters and marine otters, walruses, polar bears, the three species of manatees, and dugongs; and NOAA was given responsibility to conserve and manage pinnipeds (excluding walruses) and cetaceans.[123]
: 7 

The Act was updated on 1 January 2016 with a clause banning "the import of fish from fisheries that cannot prove they meet US standards for protecting marine mammals."[124] The requirement to show that protection standards are met is hoped to compel countries exporting fish to the US to more strictly control their fisheries that no protected marine mammals are adversely affected by fishing.[124]

The 1979

Wadden Sea Agreement.[127] In 1982, the United Nations Convention on the Law of the Sea (UNCLOS) adopted a pollution prevention approach to conservation, which many other conventions at the time also adopted.[128]

An adult and sub-adult Minke whale are dragged aboard the Nisshin Maru
, a Japanese whaling vessel

The Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and contiguous Atlantic area (ACCOBAMS), founded in 1996, specifically protects cetaceans in the Mediterranean area, and "maintains a favorable status", a direct action against whaling.[128] There are 23 member states.[129] The Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS) was adopted alongside ACCOBAMS to establish a special protection area for Europe's increasingly threatened cetaceans.

environmental agreement (a type of international law) the International Convention for the Regulation of Whaling which controlled commercial, scientific, and subsistence whaling.[131]

The Agreement on the Conservation of Seals in the Wadden Sea, enforced in 1991, prohibits the killing or harassment of seals in the Wadden Sea, specifically targeting the harbor seal population.[132]

The 1973 Agreement on the Conservation of Polar Bears between Canada, Denmark (Greenland), Norway (Svalbard), the United States, and the Soviet Union outlawed the unregulated hunting of polar bears from aircraft and icebreakers, as well as protecting migration, feeding, and hibernation sites.[133]


non-governmental organizations participate in marine conservation activism, wherein they draw attention to and aid in various problems in marine conservation, such as pollution, whaling, bycatch, and so forth. Notable organizations include the Greenpeace who focus on overfishing and whaling among other things, and Sea Shepherd Conservation Society who are known for taking direct-action tactics to expose illegal activity.[134]

As food

The whale meat is dark purple and shredded like jerky, the blubber is a pale-pink color and in slices, the dried fish is a light-brown color and ripped into slices, and the potatoes are light-yellow and cut into thin slices.
Pilot whale meat (bottom), blubber (middle) and dried fish (left) with potatoes, Faroe Islands

For thousands of years, indigenous peoples of the Arctic have depended on whale meat & seal meat. The meat is harvested from legal, non-commercial hunts that occur twice a year in the spring and autumn. The meat is stored and eaten throughout the winter.[135] The skin and blubber (muktuk) taken from the bowhead, beluga, or narwhal is also valued, and is eaten raw or cooked. Whaling has also been practiced in the Faroe Islands in the North Atlantic since about the time of the first Norse settlements on the islands. Around 1000 long-finned pilot whales are still killed annually, mainly during the summer.[136][137] Today, dolphin meat is consumed in a small number of countries worldwide, which include Japan[138][139] and Peru (where it is referred to as chancho marino, or "sea pork").[140] In some parts of the world, such as Taiji, Japan and the Faroe Islands, dolphins are traditionally considered food, and are killed in harpoon or drive hunts.[138]

There have been human health concerns associated with the consumption of dolphin meat in Japan after tests showed that dolphin meat contained high levels of methylmercury.[139][141] There are no known cases of mercury poisoning as a result of consuming dolphin meat, though the government continues to monitor people in areas where dolphin meat consumption is high. The Japanese government recommends that children and pregnant women avoid eating dolphin meat on a regular basis.[142] Similar concerns exist with the consumption of dolphin meat in the Faroe Islands, where prenatal exposure to methylmercury and PCBs primarily from the consumption of pilot whale meat has resulted in neuropsychological deficits amongst children.[141]

The Faroe Islands population was exposed to methylmercury largely from contaminated pilot whale meat, which contained very high levels of about 2 mg methylmercury/kg. However, the Faroe Islands populations also eat significant amounts of fish. The study of about 900 Faroese children showed that prenatal exposure to methylmercury resulted in neuropsychological deficits at 7 years of age

Ringed seals were once the main food staple for the

seal oil, which is marketed as a fish oil supplement. In 2001, two percent of Canada's raw seal oil was processed and sold in Canadian health stores.[145]

In captivity


A killer whale with a collapsed dorsal fin breaching out of a pool in front of an audience in stands
Performing killer whale at SeaWorld San Diego
, 2009

Various species of dolphins are kept in captivity. These small cetaceans are more often than not kept in theme parks and dolphinariums, such as SeaWorld. Bottlenose dolphins are the most common species of dolphin kept in dolphinariums as they are relatively easy to train and have a long lifespan in captivity. Hundreds of bottlenose dolphins live in captivity across the world, though exact numbers are hard to determine.[146] The dolphin "smile" makes them popular attractions, as this is a welcoming facial expression in humans; however, the smile is due to a lack of facial muscles and subsequent lack of facial expressions.[147]

Organizations such as

anthropomorphic behaviors, including waving and kissing—behaviors wild dolphins would rarely do.[151]


The large size and playfulness of pinnipeds make them popular attractions. Some exhibits have rocky backgrounds with artificial haul-out sites and a pool, while others have pens with small rocky, elevated shelters where the animals can dive into their pools. More elaborate exhibits contain deep pools that can be viewed underwater with rock-mimicking cement as haul-out areas. The most common pinniped species kept in captivity is the California sea lion as it is abundant and easy to train.

disneyfied", and consequently, people often anthropomorphize them with a curious, funny, or playful nature.[154]

Some organizations, such as the Humane Society of the United States and World Animal Protection, object to keeping pinnipeds and other marine mammals in captivity. They state that the exhibits could not be large enough to house animals that have evolved to be migratory, and a pool could never replace the size and biodiversity of the ocean. They also oppose using sea lions for entertainment, claiming the tricks performed are "exaggerated variations of their natural behaviors" and distract the audience from the animal's unnatural environment.[155]

Sea otter

Sea otters can do well in captivity, and are featured in over 40 public aquariums and zoos.[64] The Seattle Aquarium became the first institution to raise sea otters from conception to adulthood with the birth of Tichuk in 1979, followed by three more pups in the early 1980s.[156] In 2007, a YouTube video of two cute sea otters holding paws drew 1.5 million viewers in two weeks, and had over 20 million views as of January 2015.[157][158] Filmed five years previously at the Vancouver Aquarium, it was YouTube's most popular animal video at the time, although it has since been surpassed.[159] Otters are often viewed as having a "happy family life", but this is an anthropomorphism.[160]


The oldest manatee in captivity was

River Safari at Singapore features seven of them.[169]


A bottlenose dolphin jumping out of the water (the entire body is visible) in front of a trainer in camouflage. The dolphin is wearing a small, cylindrical camera on its right fin
A dolphin wearing a locating pinger, performing mine clearance work in the Iraq War

Bottlenose dolphins and California sea lions are used in the United States Navy Marine Mammal Program (NMMP) to detect mines, protect ships from enemy soldiers, and recover objects. The Navy has never trained attack dolphins, as they would not be able to discern allied soldiers from enemy soldiers. There were five marine mammal teams, each purposed for one of the three tasks: MK4 (dolphins), MK5 (sea lions), MK6 (dolphins and sea lions), MK7 (dolphins), and MK8 (dolphins); MK is short for mark. The dolphin teams were trained to detect and mark mines either attached to the seafloor or floating in the water column, because dolphins can use their echolocative abilities to detect mines. The sea lion team retrieved test equipment such as fake mines or bombs dropped from planes usually out of reach of divers who would have to make multiple dives. MK6 protects harbors and ships from enemy divers, and was operational in the Gulf War and Vietnam War. The dolphins would swim up behind enemy divers and attach a buoy to their air tank, so that they would float to the surface and alert nearby Navy personnel. Sea lions would hand-cuff the enemy, and try to outmaneuver their counter-attacks.[170][self-published source?][171]

The use of marine mammals by the Navy, even in accordance with the Navy's policy, continues to meet opposition. The Navy's policy says that only positive reinforcement is to be used while training the military dolphins, and that they be cared for in accordance with accepted standards in animal care. The inevitable stresses involved in training are topics of controversy, as their treatment is unlike the animals' natural lifestyle, especially towards their confined spaces when not training. There is also controversy over the use of

muzzles and other inhibitors, which prevent the dolphins from foraging for food while working. The Navy states that this is to prevent them from ingesting harmful objects, but conservation activists say this is done to reinforce the trainers' control over the dolphins, who hand out food rewards. The means of transportation is also an issue for conservation activists, since they are hauled in dry carriers, and switching tanks and introducing the dolphin to new dolphins is potentially dangerous as they are territorial.[170][171]

See also


  1. OCLC 30643250
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Further reading

External links